ALBERT

Description: Magnetic field and dynamic pressure ULF fluctuations in coronal-mass-ejection-driven sheath regions Annales Geophysicae, 31, 1559-1567, 2013 Author(s): E. K. J. Kilpua, H. Hietala, H. E. J. Koskinen, D. Fontaine, and L. Turc Compressed sheath regions form ahead of interplanetary coronal mass ejections (ICMEs) that are sufficiently faster than the preceding solar wind. The turbulent sheath regions are important drivers of magnetospheric activity, but due to their complex internal structure, relatively little is known on the distribution of the magnetic field and plasma variations in them. In this paper we investigate ultra low frequency (ULF) fluctuations in the interplanetary magnetic field (IMF) and in dynamic pressure ( P dyn ) using a superposed epoch analysis of 41 sheath regions observed during solar cycle 23. We find strongest fluctuation power near the shock and in the vicinity of the ICME leading edge. The IMF and P dyn ULF power have different profiles within the sheath; the former is enhanced in the leading part of the sheath, while the latter is increased in the trailing part of the sheath. We also find that the ICME properties affect the level and distribution of the ULF power in sheath regions. For example, sheath regions associated with strong or fast ICMEs, or those that are crossed at intermediate distances from the center, have strongest ULF power and large variation in the power throughout the sheath region. The weaker or slower ICMEs, or those that are crossed centrally, have in general considerably weaker ULF power with relatively smooth profiles. The strong and abrupt decrease of the IMF ULF power at the ICME leading edge could be used to distinguish the ICME from the preceding sheath plasma.

Description: Cluster observation of few-hour-scale evolution of structured plasma in the inner magnetosphere Annales Geophysicae, 31, 1569-1578, 2013 Author(s): M. Yamauchi, I. Dandouras, H. Rème, R. Lundin, and L. M. Kistler Using Cluster Ion Spectrometry (CIS) data from the spacecraft-4 perigee traversals during the 2001–2006 period (nearly 500 traversals after removing those that are highly contaminated by radiation belt particles), we statistically examined the local time distribution of structured trapped ions at sub- to few-keV range as well as inbound–outbound differences of these ion signatures in intensities and energy–latitude dispersion directions. Since the Cluster orbit during this period was almost constant and approximately north–south symmetric at nearly constant local time near the perigee, inbound–outbound differences are attributed to temporal developments in a 1–2 h timescale. Three types of structured ions at sub- to few keV range that are commonly found in the inner magnetosphere are examined: – Energy–latitude dispersed structured ions at less than a few keV, – Short-lived dispersionless ion stripes at wide energy range extending 0.1–10 keV, – Short-lived low-energy ion bursts at less than a few hundred eV. The statistics revealed that the wedge-like dispersed ions are most often observed in the dawn sector (60% of traversals), and a large portion of them show significant enhancement during the traversals at all local times. The short-lived ion stripes are predominantly found near midnight, where most stripes are significantly enhanced during the traversals and are associated with substorm activities with geomagnetic AL 〈 −300 nT. The low-energy bursts are observed at all local times and under all geomagnetic conditions, with moderate peak of the occurrence rate in the afternoon sector. A large portion of them again show significant enhancement or decay during the traversals.

Description: Influence of the terrestrial magnetic field geometry on the cutoff rigidity of cosmic ray particles Annales Geophysicae, 31, 1637-1643, 2013 Author(s): K. Herbst, A. Kopp, and B. Heber Studies of the propagation of charged energetic particles in the Earth's magnetic field go back to Carl Størmer. In the end, his investigations finally lead to the definition of the so-called cutoff rigidity R C ; that is, the minimum momentum per charge a particle must have in order to reach a certain geographical location. Employing Monte Carlo simulations with the PLANETOCOSMICS code we investigate the correlation between the geomagnetic field structure and the cutoff rigidity. We show that the geometry of the magnetic field has a considerable influence on the resulting cutoff rigidity distribution. Furthermore, we will present a simple geometry-based parameter, δ B , which is able to reflect the location-dependent cutoff rigidity. We show that this correlation is also visible in the temporal evolution of the Earth's magnetic field, at least over the last 100 yr. Using latitude scans with neutron monitors, changes of the relative counting rates at different positions are calculated, showing small variations for, e.g., Kiel and Moscow, while large ones occur at Mexico City as well as on the British Virgin Islands.

Description: Electron acceleration at Jupiter: input from cyclotron-resonant interaction with whistler-mode chorus waves Annales Geophysicae, 31, 1619-1630, 2013 Author(s): E. E. Woodfield, R. B. Horne, S. A. Glauert, J. D. Menietti, and Y. Y. Shprits Jupiter has the most intense radiation belts of all the outer planets. It is not yet known how electrons can be accelerated to energies of 10 MeV or more. It has been suggested that cyclotron-resonant wave-particle interactions by chorus waves could accelerate electrons to a few MeV near the orbit of Io. Here we use the chorus wave intensities observed by the Galileo spacecraft to calculate the changes in electron flux as a result of pitch angle and energy diffusion. We show that, when the bandwidth of the waves and its variation with L are taken into account, pitch angle and energy diffusion due to chorus waves is a factor of 8 larger at L -shells greater than 10 than previously shown. We have used the latitudinal wave intensity profile from Galileo data to model the time evolution of the electron flux using the British Antarctic Survey Radiation Belt (BAS) model. This profile confines intense chorus waves near the magnetic equator with a peak intensity at ∼5° latitude. Electron fluxes in the BAS model increase by an order of magnitude for energies around 3 MeV. Extending our results to L = 14 shows that cyclotron-resonant interactions with chorus waves are equally important for electron acceleration beyond L = 10. These results suggest that there is significant electron acceleration by cyclotron-resonant interactions at Jupiter contributing to the creation of Jupiter's radiation belts and also increasing the range of L -shells over which this mechanism should be considered.

Description: The coupling between the solar wind and proton fluxes at GEO Annales Geophysicae, 31, 1631-1636, 2013 Author(s): R. J. Boynton, S. A. Billings, O. A. Amariutei, and I. Moiseenko The relationship between the solar wind and the proton flux at geosynchronous Earth orbit (GEO) is investigated using the error reduction ratio (ERR) analysis. The ERR analysis is able to search for the most appropriate inputs that control the evolution of the system. This approach is a black box method and is able to derive a mathematical model of a system from input-output data. This method is used to analyse eight energy ranges of the proton flux at GEO from 80 keV to 14.5 MeV. The inputs to the algorithm were solar wind velocity, density and pressure; the Dst index; the solar energetic proton (SEP) flux; and a function of the interplanetary magnetic field (IMF) tangential magnitude and clock angle. The results show that for lowest five energy channels (80 to 800 keV) the GEO proton fluxes are controlled by the solar wind velocity with a lag of two to three days. However, above 350 keV, the SEP fluxes, accounts for a significant portion of the GEO proton flux variance. For the highest three energy channels (0.74 to 14.5 MeV), the SEPs account for the majority of the ERR. The results also show an anisotropy of protons with gyrocenters inside GEO and outside GEO, where the protons inside GEO are controlled partly by the Dst index and also an IMF-clock angle function.

Description: A statistical study on O + flux in the dayside magnetosheath Annales Geophysicae, 31, 1005-1010, 2013 Author(s): R. Slapak, H. Nilsson, and L. G. Westerberg Studies on terrestrial oxygen ion (O + ) escape into the interplanetary space have considered a number of different escape paths. Recent observations however suggest a yet insufficiently investigated additional escape route for hot O + : along open magnetic field lines in the high altitude cusp and mantle. Here we present a statistical study on O + flux in the high-latitude dayside magnetosheath. The O + is generally seen relatively close to the magnetopause, consistent with observations of O + flowing primarily tangentially to the magnetopause. We estimate the total escape flux in this region to be ~ 7 × 10 24 s −1 , implying this escape route to significantly contribute to the overall total O + loss into interplanetary space.

Description: Simulation of the influence of historical land cover changes on the global climate Annales Geophysicae, 31, 995-1004, 2013 Author(s): Y. Wang, X. Yan, and Z. Wang In order to estimate biogeophysical effects of historical land cover change on climate during last three centuries, a set of experiments with a climate system model of intermediate complexity (MPM-2) is performed. In response to historical deforestation, the model simulates a decrease in annual mean global temperature in the range of 0.07–0.14 °C based on different grassland albedos. The effect of land cover changes is most pronounced in the middle northern latitudes with maximum cooling reaching approximately 0.6 °C during northern summer. The cooling reaches 0.57 °C during northern spring owing to the large effects of land surface albedo. These results suggest that land cover forcing is important for study on historical climate change and that more research is necessary in the assessment of land management options for climate change mitigation.

Description: IMF effect on the polar cap contraction and expansion during a period of substorms Annales Geophysicae, 31, 1021-1034, 2013 Author(s): A. T. Aikio, T. Pitkänen, I. Honkonen, M. Palmroth, and O. Amm The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase. In addition, we suggest that the IMF B z component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.

Description: A comparison of bow shock models with Cluster observations during low Alfvén Mach number magnetic clouds Annales Geophysicae, 31, 1011-1019, 2013 Author(s): L. Turc, D. Fontaine, P. Savoini, H. Hietala, and E. K. J. Kilpua Magnetic clouds (MCs) are very geoeffective solar wind structures. Their properties in the interplanetary medium have been extensively studied, yet little is known about their characteristics in the Earth's magnetosheath. The Cluster spacecraft offer the opportunity to observe MCs in the magnetosheath, but before MCs reach the magnetosphere, their structure is altered when they interact with the terrestrial bow shock (BS). The physics taking place at the BS strongly depends on Θ Bn , the angle between the shock normal and the interplanetary magnetic field. However, in situ observations of the BS during an MC's crossing are seldom available. In order to relate magnetosheath observations to solar wind conditions, we need to rely on a model to determine the shock's position and normal direction. Yet during MCs, the models tend to be less accurate, because the Alfvén Mach number ( M A ) is often significantly lower than in regular solar wind. On the contrary, the models are generally optimised for high M A conditions. In this study, we compare the predictions of four widely used models available in the literature (Wu et al., 2000; Chapman and Cairns, 2003; Jeřáb et al., 2005; Měrka et al., 2005b) to Cluster's dayside BS crossings observed during five MC events. Our analysis shows that the Θ Bn angle is well predicted by all four models. On the other hand, the Jeřáb et al. (2005) model yields the best estimates of the BS position during low M A MCs. The other models locate the BS either too far from or too close to Earth. The results of this paper can be directly used to estimate the BS parameters in all studies of MC interaction with Earth's magnetosphere.

Description: Investigation of radiative effects of the optically thick dust layer over the Indian tropical region Annales Geophysicae, 31, 647-663, 2013 Author(s): S. K. Das, J.-P. Chen, M. Venkat Ratnam, and A. Jayaraman Optical and physical properties of aerosols derived from multi-satellite observations (MODIS-Aqua, OMI-Aura, MISR-Terra, CALIOP-CALIPSO) have been used to estimate radiative effects of the dust layer over southern India. The vertical distribution of aerosol radiative forcing and heating rates are calculated with 100 m resolution in the lower atmosphere, using temperature and relative humidity data from balloon-borne radiosonde observations. The present study investigates the optically thick dust layer of optical thickness 0.18 ± 0.06 at an altitude of 2.5 ± 0.7 km over Gadanki, transported from the Thar Desert, producing radiative forcing and heating rate of 11.5 ± 3.3 W m −2 and 0.6 ± 0.26 K day −1 , respectively, with a forcing efficiency of 43 W m −2 and an effective heating rate of 4 K day −1 per unit dust optical depth. Presence of the dust layer increases radiative forcing by 60% and heating rate by 60 times at that altitude compared to non-dusty cloud-free days. Calculation shows that the radiative effects of the dust layer strongly depend on the boundary layer aerosol type and mass loading. An increase of 25% of heating by the dust layer is found over relatively cleaner regions than urban regions in southern India and further 15% of heating increases over the marine region. Such heating differences in free troposphere may have significant consequences in the atmospheric circulation and hydrological cycle over the tropical Indian region.

Description: Low level jet intensification by mineral dust aerosols Annales Geophysicae, 31, 625-632, 2013 Author(s): O. Alizadeh Choobari, P. Zawar-Reza, and A. Sturman Modification of the intensity of a low level jet (LLJ) and near-surface wind speed by mineral dust is important as it has implications for dust emission and its long-range transport. Using the Weather Research and Forecasting with Chemistry (WRF/Chem) regional model, it is shown that direct radiative forcing by mineral dust reduces temperature in the lower atmosphere, but increases it in the layers aloft. The surface cooling is shown to be associated with a reduction of turbulent kinetic energy (TKE) and hence vertical mixing of horizontal momentum. Changes in the vertical profile of temperature over the regions that are under the influence of a LLJ are shown to result in an intensification of the LLJ and near-surface wind speed, but a decrease of winds aloft. These changes in the wind speed profile differ from results of previous research which suggested a decrease of wind speed in the lower atmosphere and its increase in the upper boundary layer.

Description: Parametric validations of analytical lifetime estimates for radiation belt electron diffusion by whistler waves Annales Geophysicae, 31, 599-624, 2013 Author(s): A. V. Artemyev, D. Mourenas, O. V. Agapitov, and V. V. Krasnoselskikh The lifetimes of electrons trapped in Earth's radiation belts can be calculated from quasi-linear pitch-angle diffusion by whistler-mode waves, provided that their frequency spectrum is broad enough and/or their average amplitude is not too large. Extensive comparisons between improved analytical lifetime estimates and full numerical calculations have been performed in a broad parameter range representative of a large part of the magnetosphere from L ~ 2 to 6. The effects of observed very oblique whistler waves are taken into account in both numerical and analytical calculations. Analytical lifetimes (and pitch-angle diffusion coefficients) are found to be in good agreement with full numerical calculations based on CRRES and Cluster hiss and lightning-generated wave measurements inside the plasmasphere and Cluster lower-band chorus waves measurements in the outer belt for electron energies ranging from 100 keV to 5 MeV. Comparisons with lifetimes recently obtained from electron flux measurements on SAMPEX, SCATHA, SAC-C and DEMETER also show reasonable agreement.

Description: Coherent amplitude modulation of electron-beam-driven Langmuir waves Annales Geophysicae, 31, 633-638, 2013 Author(s): K. Baumgärtel A linear approach to the phenomenon of irregular amplitude modulation of beam-driven Langmuir waves, developed in a previous paper, is extended to explain periodic modulation as well. It comes about by beating of the fastest growing mode of the instability with beam-aligned plasma oscillations. They are naturally generated in a uniform domain of beam–plasma interaction prior to the onset of the instability. Particle-in-cell (PIC) simulations support the results of the linear analysis.

Description: Seasonal variation of the ion upflow in the topside ionosphere during SAPS (subauroral polarization stream) periods Annales Geophysicae, 31, 1521-1534, 2013 Author(s): H. Wang and H. Lühr A statistical study has been performed by using two years of DMSP (Defense Meteorological Satellite Program) plasma observations to investigate the seasonal effect of SAPS (subauroral polarization stream) on the ion upflow in the duskside ionosphere of the Northern Hemisphere. There are obvious upflows occurring in the topside ionosphere around the SAPS region, exceeding 200 m s −1 at winter solstice, indicating an important relationship between SAPS and the local plasma upward motion. Both SAPS and ion upward velocities show similar seasonal variations, largest in winter and smallest in summer, irrespective of geomagnetic activity. A good correlation is found and a linear relationship is derived between SAPS and the ion upflow velocities. During December solstice the average upflow flux can reach about 2 × 10 8 cm −2 s −1 for more disturbed periods, which is comparable to the typical upflow flux in the dayside cusp region. The depression of the ion temperatures around the peak SAPS region can be understood in terms of the adiabatic cooling. The hot ion cools down when expanding into the low ion concentration region. The electron temperature elevates around the SAPS region because of the reduced Coulomb cooling in the low ion density region. Both the changes of ion and electron temperatures are larger in winter than in summer, however, for Kp 〈 4 the electron temperatures are almost seasonably independent. The present work highlights the important role of the SAPS-related frictional heating at mid-latitudes on the local formation of the strong upward flow, which might provide a direct ionospheric ion source for the ring current and plasmasphere in the duskside sector.

Description: Magnetic field generation in a jet-sheath plasma via the kinetic Kelvin-Helmholtz instability Annales Geophysicae, 31, 1535-1541, 2013 Author(s): K.-I. Nishikawa, P. Hardee, B. Zhang, I. Duţan, M. Medvedev, E. J. Choi, K. W. Min, J. Niemiec, Y. Mizuno, A. Nordlund, J. T. Frederiksen, H. Sol, M. Pohl, and D. H. Hartmann We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic jet and an unmagnetized sheath plasma. We have examined the strong magnetic fields generated by kinetic shear (Kelvin–Helmholtz) instabilities. Compared to the previous studies using counter-streaming performed by Alves et al. (2012), the structure of the kinetic Kelvin–Helmholtz instability (KKHI) of our jet-sheath configuration is slightly different, even for the global evolution of the strong transverse magnetic field. In our simulations the major components of growing modes are the electric field E z , perpendicular to the flow boundary, and the magnetic field B y , transverse to the flow direction. After the B y component is excited, an induced electric field E x , parallel to the flow direction, becomes significant. However, other field components remain small. We find that the structure and growth rate of KKHI with mass ratios m i / m e = 1836 and m i / m e = 20 are similar. In our simulations saturation in the nonlinear stage is not as clear as in counter-streaming cases. The growth rate for a mildly-relativistic jet case (γ j = 1.5) is larger than for a relativistic jet case (γ j = 15).

Description: Ionospheric response to total solar eclipse of 22 July 2009 in different Indian regions Annales Geophysicae, 31, 1549-1558, 2013 Author(s): S. Kumar, A. K. Singh, and R. P. Singh The variability of ionospheric response to the total solar eclipse of 22 July 2009 has been studied analyzing the GPS data recorded at the four Indian low-latitude stations Varanasi (100% obscuration), Kanpur (95% obscuration), Hyderabad (84% obscuration) and Bangalore (72% obscuration). The retrieved ionospheric vertical total electron content (VTEC) shows a significant reduction (reflected by all PRNs (satellites) at all stations) with a maximum of 48% at Varanasi (PRN 14), which decreases to 30% at Bangalore (PRN 14). Data from PRN 31 show a maximum of 54% at Kanpur and 26% at Hyderabad. The maximum decrement in VTEC occurs some time (2–15 min) after the maximum obscuration. The reduction in VTEC compared to the quiet mean VTEC depends on latitude as well as longitude, which also depends on the location of the satellite with respect to the solar eclipse path. The amount of reduction in VTEC decreases as the present obscuration decreases, which is directly related to the electron production by the photoionization process. The analysis of electron density height profile derived from the COSMIC (Constellation Observing System for Meteorology, Ionosphere & Climate) satellite over the Indian region shows significant reduction from 100 km altitude up to 800 km altitude with a maximum of 48% at 360 km altitude. The oscillatory nature in total electron content data at all stations is observed with different wave periods lying between 40 and 120 min, which are attributed to gravity wave effects generated in the lower atmosphere during the total solar eclipse.

Description: Application of nonlinear autoregressive moving average exogenous input models to geospace: advances in understanding and space weather forecasts Annales Geophysicae, 31, 1579-1589, 2013 Author(s): R. J. Boynton, M. A. Balikhin, S. A. Billings, and O. A. Amariutei The nonlinear autoregressive moving average with exogenous inputs (NARMAX) system identification technique is applied to various aspects of the magnetospheres dynamics. It is shown, from an example system, how the inputs to a system can be found from the error reduction ratio (ERR) analysis, a key concept of the NARMAX approach. The application of the NARMAX approach to the Dst (disturbance storm time) index and the electron fluxes at geostationary Earth orbit (GEO) are reviewed, revealing new insight into the physics of the system. The review of studies into the Dst index illustrate how the NARMAX approach is able to find a coupling function for the Dst index from data, which was then analytically justified from first principles. While the review of the electron flux demonstrates how NARMAX is able to reveal new insight into the physics of the acceleration and loss processes within the radiation belt.

Description: Support vector machines for TEC seismo-ionospheric anomalies detection Annales Geophysicae, 31, 173-186, 2013 Author(s): M. Akhoondzadeh Using time series prediction methods, it is possible to pursue the behaviors of earthquake precursors in the future and to announce early warnings when the differences between the predicted value and the observed value exceed the predefined threshold value. Support Vector Machines (SVMs) are widely used due to their many advantages for classification and regression tasks. This study is concerned with investigating the Total Electron Content (TEC) time series by using a SVM to detect seismo-ionospheric anomalous variations induced by the three powerful earthquakes of Tohoku (11 March 2011), Haiti (12 January 2010) and Samoa (29 September 2009). The duration of TEC time series dataset is 49, 46 and 71 days, for Tohoku, Haiti and Samoa earthquakes, respectively, with each at time resolution of 2 h. In the case of Tohoku earthquake, the results show that the difference between the predicted value obtained from the SVM method and the observed value reaches the maximum value (i.e., 129.31 TECU) at earthquake time in a period of high geomagnetic activities. The SVM method detected a considerable number of anomalous occurrences 1 and 2 days prior to the Haiti earthquake and also 1 and 5 days before the Samoa earthquake in a period of low geomagnetic activities. In order to show that the method is acting sensibly with regard to the results extracted during nonevent and event TEC data, i.e., to perform some null-hypothesis tests in which the methods would also be calibrated, the same period of data from the previous year of the Samoa earthquake date has been taken into the account. Further to this, in this study, the detected TEC anomalies using the SVM method were compared to the previous results (Akhoondzadeh and Saradjian, 2011; Akhoondzadeh, 2012) obtained from the mean, median, wavelet and Kalman filter methods. The SVM detected anomalies are similar to those detected using the previous methods. It can be concluded that SVM can be a suitable learning method to detect the novelty changes of a nonlinear time series such as variations of earthquake precursors.

Description: Magnetosheath dynamic pressure enhancements: occurrence and typical properties Annales Geophysicae, 31, 319-331, 2013 Author(s): M. O. Archer and T. S. Horbury The first comprehensive statistical study of large-amplitude (〉 100%) transient enhancements of the magnetosheath dynamic pressure reveals events of up to ~ 15 times the ambient dynamic pressure with durations up to 3 min and an average duration of around 30 s, predominantly downstream of the quasi-parallel shock. The dynamic pressure transients are most often dominated by velocity increases along with a small fractional increase in the density, though the velocity is generally only deflected by a few degrees. Superposed wavelet transforms of the magnetic field show that, whilst most enhancements exhibit changes in the magnetosheath magnetic field, the majority are not associated with changes in the Interplanetary Magnetic Field (IMF). However, there is a minority of enhancements that do appear to be associated with solar wind discontinuities which cannot be explained simply by random events. In general, it is found that during periods of magnetosheath dynamic pressure enhancements the IMF is steadier than usual. This suggests that a stable foreshock and hence foreshock structures or processes may be important in the generation of the majority of magnetosheath dynamic pressure enhancements.

Description: Response of polar mesosphere summer echoes to geomagnetic disturbances in the Southern and Northern Hemispheres: the importance of nitric oxide Annales Geophysicae, 31, 333-347, 2013 Author(s): S. Kirkwood, E. Belova, P. Dalin, M. Mihalikova, D. Mikhaylova, D. Murtagh, H. Nilsson, K. Satheesan, J. Urban, and I. Wolf The relationship between polar mesosphere summer echoes (PMSE) and geomagnetic disturbances (represented by magnetic K indices) is examined. Calibrated PMSE reflectivities for the period May 2006–February 2012 are used from two 52.0/54.5 MHz radars located in Arctic Sweden (68° N, geomagnetic latitude 65°) and at two different sites in Queen Maud Land, Antarctica (73°/72° S, geomagnetic latitudes 62°/63°). In both the Northern Hemisphere (NH) and the Southern Hemisphere (SH) there is a strong increase in mean PMSE reflectivity between quiet and disturbed geomagnetic conditions. Mean volume reflectivities are slightly lower at the SH locations compared to the NH, but the position of the peak in the lognormal distribution of PMSE reflectivities is close to the same at both NH and SH locations, and varies only slightly with magnetic disturbance level. Differences between the sites, and between geomagnetic disturbance levels, are primarily due to differences in the high-reflectivity tail of the distribution. PMSE occurrence rates are essentially the same at both NH and SH locations during most of the PMSE season when a sufficiently low detection threshold is used so that the peak in the lognormal distribution is included. When the local-time dependence of the PMSE response to geomagnetic disturbance level is considered, the response in the NH is found to be immediate at most local times, but delayed by several hours in the afternoon sector and absent in the early evening. At the SH sites, at lower magnetic latitude, there is a delayed response (by several hours) at almost all local times. At the NH (auroral zone) site, the dependence on magnetic disturbance is highest during evening-to-morning hours. At the SH (sub-auroral) sites the response to magnetic disturbance is weaker but persists throughout the day. While the immediate response to magnetic activity can be qualitatively explained by changes in electron density resulting from energetic particle precipitation, the delayed response can largely be explained by changes in nitric oxide concentrations. Observations of nitric oxide concentration at PMSE heights by the Odin satellite support this hypothesis. Sensitivity to geomagnetic disturbances, including nitric oxide produced during these disturbances, can explain previously reported differences between sites in the auroral zone and those at higher or lower magnetic latitudes. The several-day lifetime of nitric oxide can also explain earlier reported discrepancies between high correlations for average conditions (year-by-year PMSE reflectivities and K indices) and low correlations for minute-to-day timescales.

Description: Observations of poleward-propagating large-scale traveling ionospheric disturbances in southern China Annales Geophysicae, 31, 377-385, 2013 Author(s): F. Ding, W. Wan, B. Ning, B. Zhao, Q. Li, Y. Wang, L. Hu, R. Zhang, and B. Xiong We report here on two cases of poleward-propagating large-scale traveling ionospheric disturbances (LSTIDs) in China during a medium-scale storm between 27 May and 1 June 2011. The observations were conducted by making use of the Global Positioning System network and ionosondes in China and Southeast Asia. One northeastward-propagating LSTID occurred on the morning of 30 May, while the other was observed during the nighttime of 1 June. Both poleward-traveling LSTIDs occurred during the storm's recovery phase in southern China's low-latitude region (geomagnetic latitude ~ 7.3–24° N) and experienced severe dissipation during their propagation from south to north. Although the initial relative amplitude of the nighttime LSTID was ~ 60% larger than that of the morning event, the nighttime event dissipated more quickly than the morning event because of a strong nighttime enhancement in background total electronic content (TEC) during storm time, which led to strong ion-drag dissipation during the evening. The poleward-propagating LSTIDs exhibit a narrower latitudinal range, a smaller amplitude, and a slightly higher elevation compared with the equatorward-moving LSTIDs observed in the same region. Given these features, the poleward-propagating LSTIDs were likely excited by some local source near southern China. Excitation of secondary LSTIDs during the dissipation of some primary medium-scale disturbances from the lower atmosphere is a possible mechanism.

Description: A new method for solving the MHD equations in the magnetosheath Annales Geophysicae, 31, 419-437, 2013 Author(s): C. Nabert, K.-H. Glassmeier, and F. Plaschke We present a new analytical method to derive steady-state magnetohydrodynamic (MHD) solutions of the magnetosheath in different levels of approximation. With this method, we calculate the magnetosheath's density, velocity, and magnetic field distribution as well as its geometry. Thereby, the solution depends on the geomagnetic dipole moment and solar wind conditions only. To simplify the representation, we restrict our model to northward IMF with the solar wind flow along the stagnation streamline. The sheath's geometry, with its boundaries, bow shock and magnetopause, is determined self-consistently. Our model is stationary and time relaxation has not to be considered as in global MHD simulations. Our method uses series expansion to transfer the MHD equations into a new set of ordinary differential equations. The number of equations is related to the level of approximation considered including different physical processes. These equations can be solved numerically; however, an analytical approach for the lowest-order approximation is also presented. This yields explicit expressions, not only for the flow and field variations but also for the magnetosheath thickness, depending on the solar wind parameters. Results are compared to THEMIS data and offer a detailed explanation of, e.g., the pile-up process and the corresponding plasma depletion layer, the bow shock and magnetopause geometry, the magnetosheath thickness, and the flow deceleration.

Description: Investigation of a mesospheric bore event over northern China Annales Geophysicae, 31, 409-418, 2013 Author(s): Q. Li, J. Xu, J. Yue, X. Liu, W. Yuan, B. Ning, S. Guan, and J. P. Younger A mesospheric bore event was observed using an OH all-sky airglow imager (ASAI) at Xinglong (40.2° N, 117.4° E), in northern China, on the night of 8–9 January 2011. Simultaneous observations by a Doppler meteor radar, a broadband sodium lidar, and TIMED/SABER OH intensity and temperature measurements are used to investigate the characteristics and environment of the bore propagation and the possible relations with the Na density perturbations. The bore propagated from northeast to southwest and divided the sky into bright and dark halves. The calculations show that the bore has an average phase velocity of 68 m s −1 . The crests following the bore have a horizontal wavelength of ~ 22 km. These parameters are consistent with the hydraulic jump theory proposed by Dewan and Picard, as well as the previous bore reports. Simultaneous wind measurements from the Doppler meteor radar at Shisanling (40.3° N, 116.2° E) and temperature data from SABER on board the TIMED satellite are used to characterize the propagating environment of the bore. The result shows that a thermal-Doppler duct exists near the OH layer that supports the horizontal propagation of the bore. Simultaneous Na lidar observations at Yanqing (40.4° N, 116.0° E) suggest that there is a downward displacement of Na density during the passage of the mesospheric bore event.

Description: The ultra-fast Kelvin waves in the equatorial ionosphere: observations and modeling Annales Geophysicae, 31, 209-215, 2013 Author(s): A. N. Onohara, I. S. Batista, and H. Takahashi The main purpose of this study is to investigate the vertical coupling between the mesosphere and lower thermosphere (MLT) region and the ionosphere through ultra-fast Kelvin (UFK) waves in the equatorial atmosphere. The effect of UFK waves on the ionospheric parameters was estimated using an ionospheric model which calculates electrostatic potential in the E-region and solves coupled electrodynamics of the equatorial ionosphere in the E- and F-regions. The UFK wave was observed in the South American equatorial region during February–March 2005. The MLT wind data obtained by meteor radar at São João do Cariri (7.5° S, 37.5° W) and ionospheric F-layer bottom height ( h 'F) observed by ionosonde at Fortaleza (3.9° S; 38.4° W) were used in order to calculate the wave characteristics and amplitude of oscillation. The simulation results showed that the combined electrodynamical effect of tides and UFK waves in the MLT region could explain the oscillations observed in the ionospheric parameters.

Description: Payload charging events in the mesosphere and their impact on Langmuir type electric probes Annales Geophysicae, 31, 187-196, 2013 Author(s): T. A. Bekkeng, A. Barjatya, U.-P. Hoppe, A. Pedersen, J. I. Moen, M. Friedrich, and M. Rapp Three sounding rockets were launched from Andøya Rocket Range in the ECOMA campaign in December 2010. The aim was to study the evolution of meteoric smoke particles during a major meteor shower. Of the various instruments onboard the rocket payload, this paper presents the data from a multi-Needle Langmuir Probe (m-NLP) and a charged dust detector. The payload floating potential, as observed using the m-NLP instrument, shows charging events on two of the three flights. These charging events cannot be explained using a simple charging model, and have implications towards the use of fixed bias Langmuir probes on sounding rockets investigating mesospheric altitudes. We show that for a reliable use of a single fixed bias Langmuir probe as a high spatial resolution relative density measurement, each payload should also carry an additional instrument to measure payload floating potential, and an instrument that is immune to spacecraft charging and measures absolute plasma density.

Description: Electric potential differences across auroral generator interfaces Annales Geophysicae, 31, 251-261, 2013 Author(s): J. De Keyser and M. Echim Strong localized high-altitude auroral electric fields, such as those observed by Cluster, are often associated with magnetospheric interfaces. The type of high-altitude electric field profile (monopolar, bipolar, or more complicated) depends on the properties of the plasmas on either side of the interface, as well as on the total electric potential difference across the structure. The present paper explores the role of this cross-field electric potential difference in the situation where the interface is a tangential discontinuity. A self-consistent Vlasov description is used to determine the equilibrium configuration for different values of the transverse potential difference. A major observation is that there exist limits to the potential difference, beyond which no equilibrium configuration of the interface can be sustained. It is further demonstrated how the plasma densities and temperatures affect the type of electric field profile in the transition, with monopolar electric fields appearing primarily when the temperature contrast is large. These findings strongly support the observed association of monopolar fields with the plasma sheet boundary. The role of shear flow tangent to the interface is also examined.

Description: Variability of ionospheric TEC during solar and geomagnetic minima (2008 and 2009): external high speed stream drivers Annales Geophysicae, 31, 263-276, 2013 Author(s): O. P. Verkhoglyadova, B. T. Tsurutani, A. J. Mannucci, M. G. Mlynczak, L. A. Hunt, and T. Runge We study solar wind–ionosphere coupling through the late declining phase/solar minimum and geomagnetic minimum phases during the last solar cycle (SC23) – 2008 and 2009. This interval was characterized by sequences of high-speed solar wind streams (HSSs). The concomitant geomagnetic response was moderate geomagnetic storms and high-intensity, long-duration continuous auroral activity (HILDCAA) events. The JPL Global Ionospheric Map (GIM) software and the GPS total electron content (TEC) database were used to calculate the vertical TEC (VTEC) and estimate daily averaged values in separate latitude and local time ranges. Our results show distinct low- and mid-latitude VTEC responses to HSSs during this interval, with the low-latitude daytime daily averaged values increasing by up to 33 TECU (annual average of ~20 TECU) near local noon (12:00 to 14:00 LT) in 2008. In 2009 during the minimum geomagnetic activity (MGA) interval, the response to HSSs was a maximum of ~30 TECU increases with a slightly lower average value than in 2008. There was a weak nighttime ionospheric response to the HSSs. A well-studied solar cycle declining phase interval, 10–22 October 2003, was analyzed for comparative purposes, with daytime low-latitude VTEC peak values of up to ~58 TECU (event average of ~55 TECU). The ionospheric VTEC changes during 2008–2009 were similar but ~60% less intense on average. There is an evidence of correlations of filtered daily averaged VTEC data with Ap index and solar wind speed. We use the infrared NO and CO 2 emission data obtained with SABER on TIMED as a proxy for the radiation balance of the thermosphere. It is shown that infrared emissions increase during HSS events possibly due to increased energy input into the auroral region associated with HILDCAAs. The 2008–2009 HSS intervals were ~85% less intense than the 2003 early declining phase event, with annual averages of daily infrared NO emission power of ~ 3.3 × 10 10 W and 2.7 × 10 10 W in 2008 and 2009, respectively. The roles of disturbance dynamos caused by high-latitude winds (due to particle precipitation and Joule heating in the auroral zones) and of prompt penetrating electric fields (PPEFs) in the solar wind–ionosphere coupling during these intervals are discussed. A correlation between geoeffective interplanetary electric field components and HSS intervals is shown. Both PPEF and disturbance dynamo mechanisms could play important roles in solar wind–ionosphere coupling during prolonged (up to days) external driving within HILDCAA intervals.

Description: Case study of stratospheric gravity waves of convective origin over Arctic Scandinavia – VHF radar observations and numerical modelling Annales Geophysicae, 31, 239-250, 2013 Author(s): A. Réchou, J. Arnault, P. Dalin, and S. Kirkwood Orography is a well-known source of gravity and inertia-gravity waves in the atmosphere. Other sources, such as convection, are also known to be potentially important but the large amplitude of orographic waves over Scandinavia has generally precluded the possibility to study such other sources experimentally in this region. In order to better understand the origin of stratospheric gravity waves observed by the VHF radar ESRAD (Esrange MST radar) over Kiruna, in Arctic Sweden (67.88° N, 21.10° E), observations have been compared to simulations made using the Weather Research and Forecasting model (WRF) with and without the effects of orography and clouds. This case study concerns gravity waves observed from 00:00 UTC on 18 February to 12:00 UTC on 20 February 2007. We focus on the wave signatures in the static stability field and vertical wind deduced from the simulations and from the observations as these are the parameters which are provided by the observations with the best height coverage. As is common at this site, orographic gravity waves were produced over the Scandinavian mountains and observed by the radar. However, at the same time, southward propagation of fronts in the Barents Sea created short-period waves which propagated into the stratosphere and were transported, embedded in the cyclonic winds, over the radar site.

Description: Long-term trends in the ionospheric F2 region with different solar activity indices Annales Geophysicae, 31, 291-303, 2013 Author(s): J. Mielich and J. Bremer A new comprehensive data collection by Damboldt and Suessmann (2012a) with monthly fo F2 and M(3000)F2 median values is an excellent basis for the derivation of long-term trends in the ionospheric F2 region. Ionospheric trends have been derived only for stations with data series of at least 22 years (124 stations with fo F2 data and 113 stations with M(3000)F2 data) using a twofold regression analysis depending on solar and geomagnetic activity. Three main results have been derived: Firstly, it could be shown that the solar 10.7 cm radio flux F10.7 is a better index for the description of the solar activity than the relative solar sunspot number R as well as the solar EUV proxy E10.7. Secondly, the global mean fo F2 and hm F2 trends derived for the interval between 1948 and 2006 are in surprisingly good agreement with model calculations of an increasing atmospheric greenhouse effect (Rishbeth and Roble, 1992). Thirdly, during the years 2007 until 2009, the hm F2 values and to a smaller amount the fo F2 values strongly decrease. The reason for this effect is a reduction of the thermospheric density and ionization due to a markedly reduced solar EUV irradiation and extremely small geomagnetic activity during the solar cycle 23/24 minimum.

Description: Simultaneous measurement of OI 557.7 nm, O 2 (0, 1) Atmospheric Band and OH (6, 2) Meinel Band nightglow at Kolhapur (17° N), India Annales Geophysicae, 31, 197-208, 2013 Author(s): N. Parihar, A. Taori, S. Gurubaran, and G. K. Mukherjee Near-simultaneous measurements of OI 557.7 nm, O 2 (0, 1) Atmospheric Band and OH (6, 2) Meinel Band nightglow were carried out at Kolhapur (17° N), India during February–March 2007. Atmospheric temperatures around 87 and 94 km were derived from the knowledge of intensity measurements of spectral features OH (6, 2) Meinel Band and O 2 Atmospheric Band, respectively. An account of the behaviour of derived temperatures has been presented. The nocturnal behaviour of OH and O 2 temperatures is governed by the waves of tidal origin, whereas the signatures of planetary wave-like oscillations is noted in the night-to-night variation of two temperatures. This is probably the first report of planetary waves observed in nightglow temperatures from the Indian subcontinent.

Description: Refilling process in the plasmasphere: a 3-D statistical characterization based on Cluster density observations Annales Geophysicae, 31, 217-237, 2013 Author(s): G. Lointier, F. Darrouzet, P. M. E. Décréau, X. Vallières, S. Kougblénou, J. G. Trotignon, and J.-L. Rauch The Cluster mission offers an excellent opportunity to investigate the evolution of the plasma population in a large part of the inner magnetosphere, explored near its orbit's perigee, over a complete solar cycle. The WHISPER sounder, on board each satellite of the mission, is particularly suitable to study the electron density in this region, between 0.2 and 80 cm −3 . Compiling WHISPER observations during 1339 perigee passes distributed over more than three years of the Cluster mission, we present first results of a statistical analysis dedicated to the study of the electron density morphology and dynamics along and across magnetic field lines between L = 2 and L = 10. In this study, we examine a specific topic: the refilling of the plasmasphere and trough regions during extended periods of quiet magnetic conditions. To do so, we survey the evolution of the ap index during the days preceding each perigee crossing and sort out electron density profiles along the orbit according to three classes, namely after respectively less than 2 days, between 2 and 4 days, and more than 4 days of quiet magnetic conditions (ap ≤ 15 nT) following an active episode (ap 〉 15 nT). This leads to three independent data subsets. Comparisons between density distributions in the 3-D plasmasphere and trough regions at the three stages of quiet magnetosphere provide novel views about the distribution of matter inside the inner magnetosphere during several days of low activity. Clear signatures of a refilling process inside an expended plasmasphere in formation are noted. A plasmapause-like boundary, at L ~ 6 for all MLT sectors, is formed after 3 to 4 days and expends somewhat further after that. In the outer part of the plasmasphere ( L ~ 8), latitudinal profiles of median density values vary essentially according to the MLT sector considered rather than according to the refilling duration. The shape of these density profiles indicates that magnetic flux tubes are not fully replenished after 6 days of quiet conditions. In addition, the outer plasmasphere in the night and dawn sectors (22:00 to 10:00 MLT range) maintains an overall clear deficit of ionospheric population, when compared to the situation in the noon and dusk sectors (10:00 to 22:00 MLT range).

Description: Spatiotemporal variability analysis of diffuse radiation in China during 1981–2010 Annales Geophysicae, 31, 277-289, 2013 Author(s): X. L. Ren, H. L. He, L. Zhang, L. Zhou, G. R. Yu, and J. W. Fan Solar radiation is the primary driver of terrestrial plant photosynthesis and the diffuse component can enhance canopy light use efficiency (LUE), which in turn influences the carbon balance of terrestrial ecosystems. In this study we calculated the spatial data of diffuse radiation in China from 1981 to 2010, using a radiation decomposition model and spatial interpolation method based on observational data. Furthermore, we explored the spatiotemporal characteristics of diffuse radiation using GIS and trend analysis techniques. The results show the following: (1) The spatial patterns of perennial average of annual diffuse radiation during 1981–2010 are complex and inhomogeneous in China, generally lower in the north and higher in the south and west. The perennial average ranges from 1730.20 to 3064.41 MJ m −2 yr −1 across the whole country. (2) There is an increasing trend of annual diffuse radiation in China from 1981 to 2010 on the whole, with mean increasing amplitude of 7.03 MJ m −2 yr −1 per decade. Whereas a significant downtrend was observed in the first 10 years, distinct anomalies in 1982, 1983, 1991 and 1992 occurred due to the eruptions of El Chinchon and Pinatubo. (3) The spatial distribution of the temporal variability of diffuse radiation showed significant regional heterogeneity in addition to the seasonal differences. Northwestern China has the most evident downtrend, with highest decreasing rate of 6% per decade, while the Tibetan Plateau has the most evident uptrend, with highest increasing rate of up to 9% per decade. Such quantitative spatiotemporal characteristics of diffuse radiation are essential in regional scale modeling of terrestrial carbon dynamics.

Description: Comment on "Storming the Bastille: the effect of electric fields on the ionospheric F-layer" by Rishbeth et al. (2010) Annales Geophysicae, 31, 145-150, 2013 Author(s): B. T. Tsurutani, A. J. Mannuccci, O. P. Verkhoglyadova, and G. S. Lakhina No abstract available.

Description: Statistical study of foreshock cavitons Annales Geophysicae, 31, 2163-2178, 2013 Author(s): P. Kajdič, X. Blanco-Cano, N. Omidi, K. Meziane, C. T. Russell, J.-A. Sauvaud, I. Dandouras, and B. Lavraud In this work we perform a statistical analysis of 92 foreshock cavitons observed with the Cluster spacecraft 1 during the period 2001–2006. We analyze time intervals during which the spacecraft was located in the Earth's foreshock with durations longer than 10 min. Together these amount to ~ 50 days. The cavitons are transient structures in the Earth's foreshock. Their main signatures in the data include simultaneous depletions of the magnetic field intensity and plasma density, which are surrounded by a rim of enhanced values of these two quantities. Cavitons form due to nonlinear interaction of transverse and compressive ultra-low frequency (ULF) waves and are therefore always surrounded by intense compressive ULF fluctuations. They are carried by the solar wind towards the bow shock. This work represents the first systematic study of a large sample of foreshock cavitons. We find that cavitons appear for a wide range of solar wind and interplanetary magnetic field conditions and are therefore a common feature upstream of Earth's quasi-parallel bow shock with an average occurrence rate of ~ 2 events per day. We also discuss their observational properties in the context of other known upstream phenomena and show that the cavitons are a distinct structure in the foreshock.

Description: Theoretical constraints on the cross-tail width of bursty bulk flows Annales Geophysicae, 31, 2179-2192, 2013 Author(s): C. X. Chen The characteristic cross-tail width of bursty bulk flows (BBFs) in earth's plasma sheet was investigated at two stages of its life, one at its onset, the other when it is fully developed. Equilibrium domains with gradient of magnetic field are constructed. Interchange instability analysis of such domains yields the most unstable mode with the half wave length comparable with the observed cross-tail width of a flow burst and the inverse of growth rate comparable with its duration. The thickness of the plasma sheet for the most unstable mode is also comparable to the width of BBFs in the north–south direction. We found that viscosity, the dimension of the unstable domain, the thickness of the plasma sheet and gradient of the magnetic field together determine the most unstable mode. The ion Larmor radius plays an important role in viscosity as half effective mean free path. For a fully developed flow, however, velocity-caused pressure difference between the leading and trailing sides of a flow burst also plays a role. The equatorial cross section of flow is reshaped and its cross-tail width is changed as well. Representing the surrounding medium with empirical magnetic field and plasma models, the force balance of the fast flow is analyzed. The cross-section area of flow burst is estimated to be one to several square earth radii, and the cross-tail width of fast flow is estimated to be 1 to 3 earth radii, which is consistent with observations of BBFs.

Description: Ray tracing model of the auroral kilometric radiation generation in the 3-D plasma cavity Annales Geophysicae, 31, 1645-1652, 2013 Author(s): T. M. Burinskaya Propagation and amplification of the auroral kilometric radiation (AKR) in a three-dimensional plasma cavity is investigated using the approximation of the geometrical optics, and taking into account both the slightly relativistic electrons propagating inside a cavity and the background cold electrons. It is shown that the global magnetic field inhomogeneity plays a key role in a wave escape from a thin plasma cavity. The main contribution to the AKR spectrum is made by waves initially generated with the component of group velocity directed to the Earth and with the optimum relationship between the wave vector components, controlling the value of the linear grow rate and duration of the ray lifetime inside a source.

Description: Empirical regional models for the short-term forecast of M3000F2 during not quiet geomagnetic conditions over Europe Annales Geophysicae, 31, 1653-1671, 2013 Author(s): M. Pietrella Twelve empirical local models have been developed for the long-term prediction of the ionospheric characteristic M3000F2 , and then used as starting point for the development of a short-term forecasting empirical regional model of M3000F2 under not quiet geomagnetic conditions. Under the assumption that the monthly median measurements of M3000F2 are linearly correlated to the solar activity, a set of regression coefficients were calculated over 12 months and 24 h for each of 12 ionospheric observatories located in the European area, and then used for the long-term prediction of M3000F2 at each station under consideration. Based on the 12 long-term prediction empirical local models of M3000F2 , an empirical regional model for the prediction of the monthly median field of M3000F2 over Europe (indicated as RM_M3000F2 ) was developed. Thanks to the IFELM_foF2 models, which are able to provide short-term forecasts of the critical frequency of the F2 layer ( fo F2 STF ) up to three hours in advance, it was possible to considerer the Brudley–Dudeney algorithm as a function of fo F2 STF to correct RM_M3000F2 and thus obtain an empirical regional model for the short-term forecasting of M3000F2 (indicated as RM_M3000F2_BD ) up to three hours in advance under not quiet geomagnetic conditions. From the long-term predictions of M3000F2 provided by the IRI model, an empirical regional model for the forecast of the monthly median field of M3000F2 over Europe (indicated as IRI_RM_M3000F2 ) was derived. IRI_RM_M3000F2 predictions were modified with the Bradley–Dudeney correction factor, and another empirical regional model for the short-term forecasting of M3000F2 (indicated as IRI_RM_M3000F2_BD ) up to three hours ahead under not quiet geomagnetic conditions was obtained. The main results achieved comparing the performance of RM_M3000F2 , RM_M3000F2_BD , IRI_RM_M3000F2 , and IRI_RM_M3000F2_BD are (1) in the case of moderate geomagnetic activity, the Bradley–Dudeney correction factor does not improve significantly the predictions; (2) under disturbed geomagnetic conditions, the Bradley–Dudeney formula improves the predictions of RM_M3000F2 in the entire European area; (3) in the case of very disturbed geomagnetic conditions, the Bradley–Dudeney algorithm is very effective in improving the performance of IRI_RM_M3000F2 ; (4) under moderate geomagnetic conditions, the long-term prediction maps of M3000F2 generated by RM_M3000F2 can be considered as short-term forecasting maps providing very satisfactory results because quiet geomagnetic conditions are not so diverse from moderate geomagnetic conditions; (5) the forecasting maps originated by RM_M3000F2 , RM_M3000F2_BD , and IRI_RM_M3000F2_BD show some regions where the forecasts are not satisfactory, but also wide sectors where the M3000F2 forecasts quite faithfully match the M3000F2 observations, and therefore RM_M3000F2 , RM_M3000F2_BD , and IRI_RM_M3000F2_BD could be exploited to produce short-term forecasting maps of M3000F2 over Europe up to 3 h in advance.

Description: On the supply of heavy planetary material to the magnetotail of Mercury Annales Geophysicae, 31, 1673-1679, 2013 Author(s): D. C. Delcourt We examine the transport of low-energy heavy ions of planetary origin (O + , Na + , Ca + ) in the magnetosphere of Mercury. We show that, in contrast to Earth, these ions are abruptly energized after ejection into the magnetosphere due to enhanced curvature-related parallel acceleration. Regardless of their mass-to-charge ratio, the parallel speed of these ions is rapidly raised up to ~ 2 V E × B (denoting by V E × B the magnitude of the local E × B drift speed), in a like manner to Fermi-type acceleration by a moving magnetic mirror. This parallel energization is such that ions with very low initial energies (a few tenths of eVs) can overcome gravity and, regardless of species or convection rate, are transported over comparable distances into the nightside magnetosphere. The region of space where these ions reach the magnetotail is found to extend over altitudes similar to those where enhanced densities are noticeable in the MESSENGER data, viz., from ~ 1000 km up to ~ 6000 km in the pre-midnight sector. The observed density enhancements may thus follow from E × B related focusing of planetary material of dayside origin into the magnetotail. Due to the planetary magnetic field offset, an asymmetry is found between drift paths anchored in the Northern and Southern hemispheres, which puts forward a predominant role of heavy material originating in the Northern Hemisphere in populating the innermost region of Mercury's magnetotail.

Description: Evaluating the applicability of the finite element method for modelling of geoelectric fields Annales Geophysicae, 31, 1689-1698, 2013 Author(s): B. Dong, D. W. Danskin, R. J. Pirjola, D. H. Boteler, and Z. Z. Wang Geomagnetically induced currents in power systems are due to space weather events which create geomagnetic disturbances accompanied by electric fields at the surface of the Earth. The purpose of this paper is to evaluate the use of the finite element method (FEM) to calculate the magnetic and electric fields to which long transmission lines of power systems on the Earth are exposed. The well-known technique of FEM is used for the first time to simulate magnetic and electric fields applicable to power systems. Several test cases are modelled and compared with known solutions. It is shown that FEM is an effective modelling technique that can be applied to determine the electric fields which affect power systems. FEM enables an increased capability beyond the traditional methods for modelling electric and magnetic fields with layered earth conductivity structures, as spatially more complex structures can be considered using FEM. As an example results are presented for induction, due to a line current source, in adjacent regions with different layered conductivity structures. The results show the electric field away from the interface is the same as calculated for a single region; however near the interface the electric field is influenced by both regions.

Description: Stereoscopic determination of all-sky altitude map of aurora using two ground-based Nikon DSLR cameras Annales Geophysicae, 31, 1543-1548, 2013 Author(s): R. Kataoka, Y. Miyoshi, K. Shigematsu, D. Hampton, Y. Mori, T. Kubo, A. Yamashita, M. Tanaka, T. Takahei, T. Nakai, H. Miyahara, and K. Shiokawa A new stereoscopic measurement technique is developed to obtain an all-sky altitude map of aurora using two ground-based digital single-lens reflex (DSLR) cameras. Two identical full-color all-sky cameras were set with an 8 km separation across the Chatanika area in Alaska (Poker Flat Research Range and Aurora Borealis Lodge) to find localized emission height with the maximum correlation of the apparent patterns in the localized pixels applying a method of the geographical coordinate transform. It is found that a typical ray structure of discrete aurora shows the broad altitude distribution above 100 km, while a typical patchy structure of pulsating aurora shows the narrow altitude distribution of less than 100 km. Because of its portability and low cost of the DSLR camera systems, the new technique may open a unique opportunity not only for scientists but also for night-sky photographers to complementarily attend the aurora science to potentially form a dense observation network.

Description: Cluster as current sheet surveyor in the magnetotail Annales Geophysicae, 31, 1605-1610, 2013 Author(s): Y. Narita, R. Nakamura, and W. Baumjohann A novel analysis technique is presented to estimate the current sheet thickness unambiguously and directly, without associating time series data with spatial structure. The technique is a combination of eigenvalue analysis and minimum variance estimator adapted to Harris current sheet geometry, and needs one-time, four-point magnetic field data as provided by the Cluster spacecraft. Two current sheet parameters, thickness and distance to the spacecraft, can be determined at each time step of the magnetic field measurements. An example is shown from a Cluster magnetotail crossing under quiet magnetospheric conditions, yielding the result that the current sheet thickness is on the scale of the proton gyroradius. The analysis technique can also be used to track the dynamical evolution of the current sheet structure in three dimensions.

Description: Changes in the ultra-low frequency wave field during the precursor phase to the Sichuan earthquake: DEMETER observations Annales Geophysicae, 31, 1597-1603, 2013 Author(s): S. N. Walker, V. Kadirkamanathan, and O. A. Pokhotelov Electromagnetic phenomena observed in association with increases in seismic activity have been studied for several decades. These phenomena are generated during the precursory phases of an earthquake as well as during the main event. Their occurrence during the precursory phases may be used in short-term prediction of a large earthquake. In this paper, we examine ultra-low frequency (ULF) electric field data from the DEMETER satellite during the period leading up to the Sichuan earthquake. It is shown that there is an increase in ULF wave activity observed as DEMETER passes in the vicinity of the earthquake epicentre. This increase is most obvious at lower frequencies. Examination of the ULF spectra shows the possible occurrence of geomagnetic pearl pulsations, resulting from the passage of atmospheric gravity waves generated in the vicinity of the earthquake epicentre.

Description: Determination of wave vectors using the phase differencing method Annales Geophysicae, 31, 1611-1617, 2013 Author(s): S. N. Walker and I. Moiseenko Due to the collisionless nature of space plasmas, plasma waves play an important role in the redistribution of energy between the various particle populations in many regions of geospace. In order to fully comprehend such mechanisms it is necessary to characterise the nature of the waves present. This involves the determination of properties such as wave vector k . There are a number of methods used to calculate k based on the multipoint measurements that are now available. These methods rely on the fact that the same wave packet is simultaneously observed at two or more locations whose separation is small in comparison to the correlation length of the wave packet. This limitation restricts the analysis to low frequency (MHD) waves. In this paper we propose an extension to the phase differencing method to enable the correlation of measurements that were not made simultaneously but differ temporally by a number of wave periods. The method is illustrated using measurements of magnetosonic waves from the Cluster STAFF search coil magnetometer. It is shown that it is possible to identify wave packets whose coherence length is much less than the separation between the measurement locations. The resulting dispersion is found to agree with theoretical results.

Description: Effective CO 2 lifetime and future CO 2 levels based on fit function Annales Geophysicae, 31, 1591-1596, 2013 Author(s): G. R. Sonnemann and M. Grygalashvyly The estimated global CO 2 emission rates and the measured atmospheric CO 2 concentrations show that only a certain share of the emitted CO 2 accumulates in the atmosphere. For given atmospheric emissions of CO 2 , the effective lifetime determines its accumulation in the atmosphere and, consequently, its impact on the future global warming. We found that on average the inferred effective lifetime of CO 2 decreases as its atmospheric concentration increases, reducing the rate of its accumulation in the atmosphere. We derived a power function that fits the varying lifetimes. Based on this fitting function, we calculated the increase of CO 2 for different scenarios of future global emission rates.

Description: Enhanced EISCAT UHF backscatter during high-energy auroral electron precipitation Annales Geophysicae, 31, 1681-1687, 2013 Author(s): N. M. Schlatter, N. Ivchenko, T. Sergienko, B. Gustavsson, and B. U. E. Brändström Natural enhancements in the backscattered power of incoherent scatter radars up to 5 orders of magnitudes above the thermal backscatter are sometimes observed at high latitudes. Recently observations of enhancements in the backscattered power including a feature at zero Doppler shift have been reported. These enhancements are limited in altitude to tens of kilometers. The zero Doppler shift feature has been interpreted as a signature of electron density cavitation. Enhanced plasma lines during these observations have also been reported. We report on the first EISCAT UHF observations of enhanced backscattered radar power including a zero Doppler shift feature. The enhancements originated from two distinct and intermittent layers at about 200 km altitude. The altitude extent of the enhancements, observed during auroral high-energy electron precipitation, was 〈 2 km.

Description: Seasonal dependence of the longitudinal variations of nighttime ionospheric electron density and equivalent winds at southern midlatitudes Annales Geophysicae, 31, 1699-1708, 2013 Author(s): X. Luan and X. Dou It has been indicated that the observed Weddell Sea anomaly (WSA) appeared to be an extreme manifestation of the longitudinal variations in the Southern Hemisphere, since the WSA is characterized by greater evening electron density than the daytime density in the region near the Weddell Sea. In the present study, the longitudinal variations of the nighttime F2-layer peak electron density at southern midlatitudes are analyzed using the observations of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites between 2006 and 2008. It is found that significant longitudinal difference (〉 150%) relative to the minimum density at each local time prevails in all seasons, although the WSA phenomenon is only evident in summer under this solar minimum condition. Another interesting feature is that in summer, the maximum longitudinal differences occur around midnight (~ 23:00–00:00 LT) rather than in the evening (19:00–21:00 LT) in the evening, when the most prominent electron density enhancement occurs for the WSA phenomenon. Thus the seasonal–local time patterns of the electron density longitudinal variations during nighttime at southern midlatitudes cannot be simply explained in terms of the WSA. Meanwhile, the variations of the geomagnetic configuration and the equivalent magnetic meridional winds/upward plasma drifts are analyzed to explore their contributions to the longitudinal variations of the nighttime electron density. The maximum longitudinal differences are associated with the strongest wind-induced vertical plasma drifts after 21:00 LT in the Western Hemisphere. Besides the magnetic declination–zonal wind effects, the geographic meridional winds and the magnetic inclination also have significant effects on the upward plasma drifts and the resultant electron density.

Description: Extracting gravity wave parameters during the September 2002 Southern Hemisphere major sudden stratospheric warming using a SANAE imaging riometer Annales Geophysicae, 31, 1709-1719, 2013 Author(s): N. Mbatha, V. Sivakumar, H. Bencherif, and S. Malinga Using absorption data measured by imaging riometer for ionospheric studies (IRIS) located at the South Africa National Antarctic Expedition (SANAE), Antarctica (72° S, 3° W), we extracted the parameters of gravity waves (GW) of periods between 40 and 50 min during late winter/spring of the year 2002, a period of the unprecedented major sudden stratospheric warming (SSW) in the Southern Hemisphere middle atmosphere. During this period, an unprecedented substantial increase of temperature by about 25–30 K throughout the stratosphere was observed. During the period of the occurrence of the major stratospheric warming, there was a reduction of both the GW horizontal phase speeds and the horizontal wavelengths at 90 km. The GW phase speeds and horizontal wavelengths were observed to reach minimum values of about 7 m s −1 and 19 km, respectively, while during the quiet period the average value of the phase speed and horizontal wavelength was approximately 23 m s −1 and 62 km, respectively. The observed event is discussed in terms of momentum flux and also a potential interaction of gravity waves, planetary waves and mean circulation.

Description: GNSS troposphere tomography based on two-step reconstructions using GPS observations and COSMIC profiles Annales Geophysicae, 31, 1805-1815, 2013 Author(s): P. Xia, C. Cai, and Z. Liu Traditionally, balloon-based radiosonde soundings are used to study the spatial distribution of atmospheric water vapour. However, this approach cannot be frequently employed due to its high cost. In contrast, GPS tomography technique can obtain water vapour in a high temporal resolution. In the tomography technique, an iterative or non-iterative reconstruction algorithm is usually utilised to overcome rank deficiency of observation equations for water vapour inversion. However, the single iterative or non-iterative reconstruction algorithm has their limitations. For instance, the iterative reconstruction algorithm requires accurate initial values of water vapour while the non-iterative reconstruction algorithm needs proper constraint conditions. To overcome these drawbacks, we present a combined iterative and non-iterative reconstruction approach for the three-dimensional (3-D) water vapour inversion using GPS observations and COSMIC profiles. In this approach, the non-iterative reconstruction algorithm is first used to estimate water vapour density based on a priori water vapour information derived from COSMIC radio occultation data. The estimates are then employed as initial values in the iterative reconstruction algorithm. The largest advantage of this approach is that precise initial values of water vapour density that are essential in the iterative reconstruction algorithm can be obtained. This combined reconstruction algorithm (CRA) is evaluated using 10-day GPS observations in Hong Kong and COSMIC profiles. The test results indicate that the water vapor accuracy from CRA is 16 and 14% higher than that of iterative and non-iterative reconstruction approaches, respectively. In addition, the tomography results obtained from the CRA are further validated using radiosonde data. Results indicate that water vapour densities derived from the CRA agree with radiosonde results very well at altitudes above 2.5 km. The average RMS value of their differences above 2.5 km is 0.44 g m −3 .

Description: Corrigendum to "Estimates of aerosol absorption over India using multi-satellite retrieval" published in Ann. Geophys., 31, 1773–1778, 2013 Annales Geophysicae, 31, 1791-1791, 2013 Author(s): D. Narasimhan and S. K. Satheesh No abstract available.

Description: Frequency variations of gravity waves interacting with a time-varying tide Annales Geophysicae, 31, 1731-1743, 2013 Author(s): C. M. Huang, S. D. Zhang, F. Yi, K. M. Huang, Y. H. Zhang, Q. Gan, and Y. Gong Using a nonlinear, 2-D time-dependent numerical model, we simulate the propagation of gravity waves (GWs) in a time-varying tide. Our simulations show that when a GW packet propagates in a time-varying tidal-wind environment, not only its intrinsic frequency but also its ground-based frequency would change significantly. The tidal horizontal-wind acceleration dominates the GW frequency variation. Positive (negative) accelerations induce frequency increases (decreases) with time. More interestingly, tidal-wind acceleration near the critical layers always causes the GW frequency to increase, which may partially explain the observations that high-frequency GW components are more dominant in the middle and upper atmosphere than in the lower atmosphere. The combination of the increased ground-based frequency of propagating GWs in a time-varying tidal-wind field and the transient nature of the critical layer induced by a time-varying tidal zonal wind creates favorable conditions for GWs to penetrate their originally expected critical layers. Consequently, GWs have an impact on the background atmosphere at much higher altitudes than expected, which indicates that the dynamical effects of tidal–GW interactions are more complicated than usually taken into account by GW parameterizations in global models.

Description: Diurnal variation in gravity wave activity at low and middle latitudes Annales Geophysicae, 31, 2123-2135, 2013 Author(s): V. F. Andrioli, D. C. Fritts, P. P. Batista, B. R. Clemesha, and D. Janches We employ a modified composite day extension of the Hocking (2005) analysis method to study gravity wave (GW) activity in the mesosphere and lower thermosphere using 4 meteor radars spanning latitudes from 7° S to 53.6° S. Diurnal and semidiurnal modulations were observed in GW variances over all sites. Semidiurnal modulation with downward phase propagation was observed at lower latitudes mainly near the equinoxes. Diurnal modulations occur mainly near solstice and, except for the zonal component at Cariri (7° S), do not exhibit downward phase propagation. At a higher latitude (SAAMER, 53.6° S) these modulations are only observed in the meridional component where we can observe diurnal variation from March to May, and semidiurnal, during January, February, October (above 88 km) and November. Some of these modulations with downward phase progression correlate well with wind shear. When the wind shear is well correlated with the maximum of the variances the diurnal tide has its largest amplitudes, i.e., near equinox. Correlations exhibiting variations with tidal phases suggest significant GW-tidal interactions that have different characters depending on the tidal components and possible mean wind shears. Modulations that do not exhibit phase variations could be indicative of diurnal variations in GW sources.

Description: A study of solar and interplanetary parameters of CMEs causing major geomagnetic storms during SC 23 Annales Geophysicae, 31, 1285-1295, 2013 Author(s): C. Oprea, M. Mierla, D. Beşliu-Ionescu, O. Stere, and G. Mariş Muntean In this paper we analyse 25 Earth-directed and strongly geoeffective interplanetary coronal mass ejections (ICMEs) which occurred during solar cycle 23, using data provided by instruments on SOHO (Solar and Heliospheric Observatory), ACE (Advanced Composition Explorer) and geomagnetic stations. We also examine the in situ parameters, the energy transfer into magnetosphere, and the geomagnetic indexes. We compare observed travel times with those calculated by observed speeds projected into the plane of the sky and de-projected by a simple model. The best fit was found with the projected speeds. No correlation was found between the importance of a flare and the geomagnetic Dst (disturbance storm time) index. By comparing the in situ parameters with the Dst index we find a strong connection between some of these parameters (such as Bz, Bs · V and the energy transfer into the magnetosphere) with the strength of the geomagnetic storm. No correlation was found with proton density and plasma temperature. A superposed epoch analysis revealed a strong dependence of the Dst index on the southward component of interplanetary magnetic field, Bz, and to the Akasofu coupling function, which evaluates the energy transfer between the ICME and the magnetosphere. The analysis also showed that the geomagnetic field at higher latitudes is disturbed before the field around the Earth's equator.

Description: Quasi-16-day period oscillations observed in middle atmospheric ozone and temperature in Antarctica Annales Geophysicae, 31, 1279-1284, 2013 Author(s): T. D. Demissie, N. H. Kleinknecht, R. E. Hibbins, P. J. Espy, and C. Straub Nightly averaged mesospheric temperature derived from the hydroxyl nightglow at Rothera station (67°34' S, 68°08' W) and nightly midnight measurements of ozone mixing ratio obtained from Troll station (72°01' S, 2°32' E) in Antarctica have been used to investigate the presence and vertical profile of the quasi-16-day planetary wave in the stratosphere and mesosphere during the Antarctic winter of 2009. The variations caused by planetary waves on the ozone mixing ratio and temperature are discussed, and spectral and cross-correlation analyses are performed to extract the wave amplitudes and to examine the vertical structure of the wave from 34 to 80 km. The results show that while planetary-wave signatures with periods 3–12 days are strong below the stratopause, the oscillations associated with the 16-day wave are the strongest and present in both the mesosphere and stratosphere. The period of the wave is found to increase below 42 km due to the Doppler shifting by the strong eastward zonal wind. The 16-day oscillation in the temperature is found to be correlated and phase coherent with the corresponding oscillation observed in O 3 volume mixing ratio at all levels, and the wave is found to have vertical phase fronts consistent with a normal mode structure.

Description: Global characteristics of Pc1 magnetic pulsations during solar cycle 23 deduced from CHAMP data Annales Geophysicae, 31, 1507-1520, 2013 Author(s): J. Park, H. Lühr, and J. Rauberg We present a global climatology of Pc1 pulsations as observed by the CHAMP satellite from 2000 to 2010. The Pc1 center frequency and bandwidth are about 1 and 0.5 Hz, respectively. The ellipticity is mostly linear with the major axis almost aligned with the magnetic zonal direction. The diurnal variation of Pc1 occurrences shows a primary maximum early in the morning and a secondary maximum during pre-midnight hours. The annual variations of the occurrence rates exhibit a clear preference for local summer. The solar cycle dependence of the occurrence rate reveals a maximum at the declining phase (2004–2005). Neither magnetic activity nor solar wind velocity controls the Pc1 occurrence rate significantly. Pc1 occurrence rate peaks at subauroral latitudes, but the steep cutoff towards higher latitudes is due to auroral field-aligned currents masking the Pc1 pulsations. The center frequency of Pc1 pulsations does not show a clear dependence on latitude. The global distribution of Pc1 exhibits highest occurrence rates near the longitude sector of the South Atlantic Anomaly. Pc1 events at auroral latitudes, although they are rarely detected, show a clear occurrence peak around local noon. A majority of the auroral Pc1 events are observed during solar minimum years.

Description: Electron pitch-angle diffusion: resonant scattering by waves vs. nonadiabatic effects Annales Geophysicae, 31, 1485-1490, 2013 Author(s): A. V. Artemyev, K. G. Orlova, D. Mourenas, O. V. Agapitov, and V. V. Krasnoselskikh In this paper we investigate the electron pitch-angle diffusion coefficients in the night-side inner magnetosphere around the geostationary orbit ( L ~ 7) due to magnetic field deformation. We compare the effects of resonant wave–particle scattering by lower band chorus waves and the adiabaticity violation of electron motion due to the strong curvature of field lines in the vicinity of the equator. For a realistic magnetic field configuration, the nonadiabatic effects are more important than the wave–particle interactions for high energy (〉 1 MeV) electrons. For smaller energy, the scattering by waves is more effective than nonadiabatic one. Moreover, the role of nonadiabatic effects increases with particle energy. Therefore, to model electron scattering and transport in the night-side inner magnetosphere, it is important to take into account the peculiarities of high-energy electron dynamics.

Description: GPS tomography: validation of reconstructed 3-D humidity fields with radiosonde profiles Annales Geophysicae, 31, 1491-1505, 2013 Author(s): M. Shangguan, M. Bender, M. Ramatschi, G. Dick, J. Wickert, A. Raabe, and R. Galas Water vapor plays an important role in meteorological applications; GeoForschungsZentrum (GFZ) therefore developed a tomographic system to derive 3-D distributions of the tropospheric water vapor above Germany using GPS data from about 300 ground stations. Input data for the tomographic reconstructions are generated by the Earth Parameter and Orbit determination System (EPOS) software of the GFZ, which provides zenith total delay (ZTD), integrated water vapor (IWV) and slant total delay (STD) data operationally with a temporal resolution of 2.5 min (STD) and 15 min (ZTD, IWV). The water vapor distribution in the atmosphere is derived by tomographic reconstruction techniques. The quality of the solution is dependent on many factors such as the spatial coverage of the atmosphere with slant paths, the spatial distribution of their intersections and the accuracy of the input observations. Independent observations are required to validate the tomographic reconstructions and to get precise information on the accuracy of the derived 3-D water vapor fields. To determine the quality of the GPS tomography, more than 8000 vertical water vapor profiles at 13 German radiosonde stations were used for the comparison. The radiosondes were launched twice a day (at 00:00 UTC and 12:00 UTC) in 2007. In this paper, parameters of the entire profiles such as the wet refractivity, and the zenith wet delay have been compared. Before the validation the temporal and spatial distribution of the slant paths, serving as a basis for tomographic reconstruction, as well as their angular distribution were studied. The mean wet refractivity differences between tomography and radiosonde data for all points vary from −1.3 to 0.3, and the root mean square is within the range of 6.5–9. About 32% of 6803 profiles match well, 23% match badly and 45% are difficult to classify as they match only in parts.

Description: Ray tracing of whistler-mode chorus elements: implications for generation mechanisms of rising and falling tone emissions Annales Geophysicae, 31, 665-673, 2013 Author(s): K. Yamaguchi, T. Matsumuro, Y. Omura, and D. Nunn Using a well-established magnetospheric very-low-frequency (VLF) ray tracing method, in this work we trace the propagation of individual rising- and falling-frequency elements of VLF chorus from their generation point in the equatorial region of the magnetosphere through to at least one reflection at the lower-hybrid resonance point. Unlike recent work by Bortnik and co-workers, whose emphasis was on demonstrating that magnetospheric hiss has its origins in chorus, we here track the motion in the equatorial plane of the whole chorus element, paying particular regard to movement across field lines, rotation, and compression or expansion of the wave pulse. With a generation point for rising chorus at the equator, it was found the element wave pulse remained largely field aligned in the generation region. However, for a falling tone generation point at 4000 km upstream from the equator, by the time the pulse crosses the equator the wavefield had substantial obliquity, displacement, and compression, which has substantial implications for the theory of falling chorus generation.

Description: Non-adiabatic electron behaviour due to short-scale electric field structures at collisionless shock waves Annales Geophysicae, 31, 639-646, 2013 Author(s): V. See, R. F. Cameron, and S. J. Schwartz Under sufficiently high electric field gradients, electron behaviour within exactly perpendicular shocks is unstable to the so-called trajectory instability. We extend previous work paying special attention to short-scale, high-amplitude structures as observed within the electric field profile. Via test particle simulations, we show that such structures can cause the electron distribution to heat in a manner that violates conservation of the first adiabatic invariant. This is the case even if the overall shock width is larger than the upstream electron gyroradius. The spatial distance over which these structures occur therefore constitutes a new scale length relevant to the shock heating problem. Furthermore, we find that the spatial location of the short-scale structure is important in determining the total effect of non-adiabatic behaviour – a result that has not been previously noted.

Description: Eureka, 80° N, SKiYMET meteor radar temperatures compared with Aura MLS values Annales Geophysicae, 31, 1267-1277, 2013 Author(s): C. E. Meek, A. H. Manson, W. K. Hocking, and J. R. Drummond The meteor trail echo decay rates are analysed on-site to provide daily temperatures near 90 km. In order to get temperatures from trail decay times, either knowledge of the pressure or the background temperature height gradient near 90 km is required (Hocking, 1999). Hocking et al. (2004) have developed an empirical 90 km temperature gradient model depending only on latitude and time of year, which is used in the SKiYMET on-site meteor temperature analysis. Here we look at the sensitivity of the resulting temperature to the assumed gradient and compare it and the temperatures with daily AuraMLS averages near Eureka. Generally there is good agreement between radar and satellite for winter temperatures and their short-term variations. However there is a major difference in mid-summer both in the temperatures and the gradients. Increased turbulence in summer, which may overwhelm the ambipolar diffusion even at 90 km, is likely a major factor. These differences are investigated by generating ambipolar-controlled decay times from satellite pressure and temperature data at a range of heights and comparing with radar measurements. Our study suggests it may be possible to use these data to estimate eddy diffusion coefficients at heights below 90 km. Finally the simple temperature analysis (using satellite pressures), and a standard meteor wind analysis are used to compare mean diurnal variations of temperature ( T ) with those of zonal wind ( U ) and meridional wind ( V ) in composite multi-year monthly intervals.

Description: On the relationship between interplanetary coronal mass ejections and magnetic clouds Annales Geophysicae, 31, 1251-1265, 2013 Author(s): E. K. J. Kilpua, A. Isavnin, A. Vourlidas, H. E. J. Koskinen, and L. Rodriguez The relationship of magnetic clouds (MCs) to interplanetary coronal mass ejections (ICMEs) is still an open issue in space research. The view that all ICMEs would originate as magnetic flux ropes has received increasing attention, although near the orbit of the Earth only about one-third of ICMEs show clear MC signatures and often the MC occupies only a portion of the more extended region showing ICME signatures. In this work we analyze 79 events between 1996 and 2009 reported in existing ICME/MC catalogs (Wind magnetic cloud list and the Richardson and Cane ICME list) using near-Earth observations by ACE (Advanced Composition Explorer) and Wind. We perform a systematic comparison of cases where ICME and MC signatures coincided and where ICME signatures extended significantly beyond the MC boundaries. We find clear differences in the characteristics of these two event types. In particular, the events where ICME signatures continued more than 6 h past the MC rear boundary had 2.7 times larger speed difference between the ICME's leading edge and the preceding solar wind, 1.4 times higher magnetic fields, 2.1 times larger widths and they experienced three times more often strong expansion than the events for which the rear boundaries coincided. The events with significant mismatch in MC and ICME boundary times were also embedded in a faster solar wind and the majority of them were observed close to the solar maximum. Our analysis shows that the sheath, the MC and the regions of ICME-related plasma in front and behind the MC have different magnetic field, plasma and charge state characteristics, thus suggesting that these regions separate already close to the Sun. Our study shows that the geometrical effect (the encounter through the CME leg and/or far from the flux rope center) does not contribute much to the observed mismatch in the MC and ICME boundary times.

Description: Reconstruction of F2 layer peak electron density based on operational vertical total electron content maps Annales Geophysicae, 31, 1241-1249, 2013 Author(s): T. Gerzen, N. Jakowski, V. Wilken, and M. M. Hoque Electron density is the major determining parameter of the ionosphere. Especially the maximum electron density of the F2 layer in the ionosphere, Nm F2, is of particular interest with regard to the HF radio communication applications as well as for characterizing the ionosphere. In this paper we present a new method to generate global maps of Nm F2. The main principle behind this approach is to use the information about the current state of the ionosphere included in global total electron content (TEC) maps as well as the relationship between total electron content, equivalent slab thickness and F2 layer peak density. Modeling of slab thickness is an interim step in our reconstruction approach. Thus, results showing the diurnal and seasonal variations and effects of solar activity on the modeled slab thickness values are given. In addition a comparison of the reconstructed Nm F2 maps with measurements from several ionosonde stations as well as with the global Nm F2 model NPDM is presented. Since 2011 the described method has been used at DLR Neustrelitz to generate Nm F2 maps as operational service. These maps are freely available via the Space Weather Application Center Ionosphere SWACI ( http://swaciweb.dlr.de ).

Description: Detection of a plasmaspheric wind in the Earth's magnetosphere by the Cluster spacecraft Annales Geophysicae, 31, 1143-1153, 2013 Author(s): I. Dandouras Plumes, forming at the plasmapause and released outwards, constitute a well-established mode for plasmaspheric material release to the Earth's magnetosphere. They are associated to active periods and the related electric field change. In 1992, Lemaire and Shunk proposed the existence of an additional mode for plasmaspheric material release to the Earth's magnetosphere: a plasmaspheric wind, steadily transporting cold plasmaspheric plasma outwards across the geomagnetic field lines, even during prolonged periods of quiet geomagnetic conditions. This has been proposed on a theoretical basis. Direct detection of this wind has, however, eluded observation in the past. Analysis of ion measurements, acquired in the outer plasmasphere by the CIS experiment onboard the four Cluster spacecraft, provide now an experimental confirmation of the plasmaspheric wind. This wind has been systematically detected in the outer plasmasphere during quiet and moderately active conditions, and calculations show that it could provide a substantial contribution to the magnetospheric plasma populations outside the Earth's plasmasphere. Similar winds should also exist on other planets, or astrophysical objects, quickly rotating and having an atmosphere and a magnetic field.

Description: Poker Flat Incoherent Scatter Radar observations of anomalous electron heating in the E region Annales Geophysicae, 31, 1163-1176, 2013 Author(s): R. A. Makarevich, A. V. Koustov, and M. J. Nicolls A comprehensive 2-year dataset collected with the Poker Flat Incoherent Scatter Radar (PFISR) located near Fairbanks, Alaska (MLAT = 65.4° N) is employed to identify and analyse 22 events of anomalous electron heating (AEH) in the auroral E region. The overall AEH occurrence probability is conservatively estimated to be 0.3% from nearly-continuous observations of the E region by PFISR, although it increases to 0.7–0.9% in the dawn and dusk sectors where all AEH events were observed. The AEH occurrence variation with MLT is broadly consistent with those of events with high convection velocity (〉1000 m s −1 ) or electron temperature (〉 800 K), except for much smaller AEH probability and absence of AEH events near magnetic midnight. This suggests that high convection electric field by itself is necessary but not sufficient for measurable electron heating by two-stream plasma waves. The multi-point observations are utilised to investigate the fundamental dependence of the electron temperature on the convection electric field, focusing on the previously-proposed saturation effects at extreme electric fields. The AEH dataset was found to exhibit considerable scatter and, on average, similar rate of the electron temperature increase with the electric field up to 100 mV m −1 as compared with previous studies. At higher (highest) electric fields, the electron temperatures are below the linear trend on average (within uncertainty). By employing a simple fluid model of AEH, it is demonstrated that some of this deviation from the linear trend may be due to a stronger vibrational cooling at very large temperatures and electric fields.

Description: EISCAT and ESRAD radars observations of polar mesosphere winter echoes during solar proton events on 11–12 November 2004 Annales Geophysicae, 31, 1177-1190, 2013 Author(s): E. Belova, S. Kirkwood, and T. Sergienko Polar mesosphere winter echoes (PMWE) were detected by two radars, ESRAD at 52 MHz located near Kiruna, Sweden, and EISCAT at 224 MHz located near Tromsø, Norway, during the strong solar proton event on 11–12 November 2004. PMWE maximum volume reflectivity was estimated to be 3 × 10 −15 m −1 for ESRAD and 2 × 10 −18 m −1 for EISCAT. It was found that the shape of the echo power spectrum is close to Gaussian inside the PMWE layers, and outside of them it is close to Lorentzian, as for the standard ion line of incoherent scatter (IS). The EISCAT PMWE spectral width is about 5–7 m s −1 at 64–67 km and 7–10 m s −1 at 68–70 km. At the lower altitudes the PMWE spectral widths are close to those for the IS ion line derived from the EISCAT data outside the layers. At the higher altitudes the PMWE spectra are broader by 2–4 m s −1 than those for the ion line. The ESRAD PMWE spectral widths at 67–72 km altitude are 3–5 m s −1 , that is, 2–4 m s −1 larger than ion line spectral widths modelled for the ESRAD radar. The PMWE spectral widths for both EISCAT and ESRAD showed no dependence on the echo strength. It was found that all these facts cannot be explained by turbulent origin of the echoes. We suggested that evanescent perturbations in the electron gas generated by the incident infrasound waves may explain the observed PMWE spectral widths. However, a complete theory of radar scatter from this kind of disturbance needs to be developed before a full conclusion can be made.

Description: CloudSat observations of cloud-type distribution over the Indian summer monsoon region Annales Geophysicae, 31, 1155-1162, 2013 Author(s): K. V. Subrahmanyam and K. K. Kumar The three-dimensional distribution of various cloud types over the Indian summer monsoon (ISM) region using five years (2006–2010) of CloudSat observations during June-July-August-September months is discussed for the first time. As the radiative properties, latent heat released and microphysical properties of clouds differ largely depending on the cloud type, it becomes important to know what types of clouds occur over which region. In this regard, the present analysis establishes the three-dimensional distribution of frequency of occurrence of stratus (St), stratocumulus (Sc), nimbostratus (Ns), cumulus (Cu), altocumulus (Ac), altostratus (As), cirrus (Ci) and deep convective (DC) clouds over the ISM region. The results show that the various cloud types preferentially occur over some regions of the ISM, which are consistent during all the years of observations. It is found that the DC clouds frequently occur over northeast of Bay of Bengal (BoB), Ci clouds over a wide region of south BoB–Indian peninsula–equatorial Indian Ocean, and Sc clouds over the north Arabian Sea. Ac clouds preferentially occur over land, and a large amount of As clouds are found over BoB. The occurrence of both St and Ns clouds over the study region is much lower than all other cloud types.The interannual variability of all these clouds including their vertical distribution is discussed. It is envisaged that the present study opens up possibilities to quantify the feedback of individual cloud type in the maintenance of the ISM through radiative forcing and latent heat release.

Description: Relation of zonal plasma drift and wind in the equatorial F region as derived from CHAMP observations Annales Geophysicae, 31, 1035-1044, 2013 Author(s): J. Park and H. Lühr In this paper we estimate zonal plasma drift in the equatorial ionospheric F region without counting on ion drift meters. From June 2001 to June 2004 zonal plasma drift velocity is estimated from electron, neutral, and magnetic field observations of Challenging Mini-satellite Payload (CHAMP) in the 09:00–20:00 LT sector. The estimated velocities are validated against ion drift measurements by the Republic of China Satellite-1/Ionospheric Plasma and Electrodynamics Instrument (ROCSAT-1/IPEI) during the same period. The correlation between the CHAMP (altitude ~ 400 km) estimates and ROCSAT-1 (altitude ~ 600 km) observations is reasonably high ( R ≈ 0.8). The slope of the linear regression is close to unity. However, the maximum westward drift and the westward-to-eastward reversal occur earlier for CHAMP estimates than for ROCSAT-1 measurements. In the equatorial F region both zonal wind and plasma drift have the same direction. Both generate vertical currents but with opposite signs. The wind effect (F region wind dynamo) is generally larger in magnitude than the plasma drift effect (Pedersen current generated by vertical E field), thus determining the direction of the F region vertical current.

Description: Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum Annales Geophysicae, 31, 513-527, 2013 Author(s): J. H. Zoennchen, U. Nass, and H. J. Fahr The Lyman-α Detectors (LAD) on board the two TWINS 1/2-satellites allow for the simultaneous stereo imaging of the resonant emission glow of the H-geocorona from very different orbital positions. Terrestrial exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation. During the past solar minimum, relevant solar parameters that influence these emissions were quite stable. Here, we use simultaneous LAD1/2-observations from TWINS1 and TWINS2 between June 2008 and June 2010 to study seasonal variations in the H-geocorona. Data are combined to produce two datasets containing (summer) solstice and (combined spring and fall) equinox emissions. In the range from 3 to 10 Earth radii ( R E ), a three-dimensional (3-D) mathematical model is used that allows for density asymmetries in longitude and latitude. At lower geocentric distances (〈 3 R E ), a best fitting r -dependent (Chamberlain, 1963)-like model is adapted to enable extrapolation of our information to lower heights. We find that dawn and dusk H-geocoronal densities differ by up to a factor of 1.3 with higher densities on the dawn side. Also, noon densities are greater by up to a factor of 2 compared to the dawn and dusk densities. The density profiles are aligned well with the Earth–Sun line and there are clear density depletions over both poles that show additional seasonal effects. These solstice and equinox empirical fits can be used to determine H-geocoronal densities for any day of the year for solar minimum conditions.

Description: Significance of Wave-Particle Interaction Analyzer for direct measurements of nonlinear wave-particle interactions Annales Geophysicae, 31, 503-512, 2013 Author(s): Y. Katoh, M. Kitahara, H. Kojima, Y. Omura, S. Kasahara, M. Hirahara, Y. Miyoshi, K. Seki, K. Asamura, T. Takashima, and T. Ono In the upcoming JAXA/ERG satellite mission, Wave Particle Interaction Analyzer (WPIA) will be installed as an onboard software function. We study the statistical significance of the WPIA for measurement of the energy transfer process between energetic electrons and whistler-mode chorus emissions in the Earth's inner magnetosphere. The WPIA measures a relative phase angle between the wave vector E and velocity vector v of each electron and computes their inner product W , where W is the time variation of the kinetic energy of energetic electrons interacting with plasma waves. We evaluate the feasibility by applying the WPIA analysis to the simulation results of whistler-mode chorus generation. We compute W using both a wave electric field vector observed at a fixed point in the simulation system and a velocity vector of each energetic electron passing through this point. By summing up W i of an individual particle i to give W int , we obtain significant values of W int as expected from the evolution of chorus emissions in the simulation result. We can discuss the efficiency of the energy exchange through wave-particle interactions by selecting the range of the kinetic energy and pitch angle of the electrons used in the computation of W int . The statistical significance of the obtained W int is evaluated by calculating the standard deviation σ W of W int . In the results of the analysis, positive or negative W int is obtained at the different regions of velocity phase space, while at the specific regions the obtained W int values are significantly greater than σ W , indicating efficient wave-particle interactions. The present study demonstrates the feasibility of using the WPIA, which will be on board the upcoming ERG satellite, for direct measurement of wave-particle interactions.

Description: New plasmapause model derived from CHAMP field-aligned current signatures Annales Geophysicae, 31, 529-539, 2013 Author(s): B. Heilig and H. Lühr We introduce a new model for the plasmapause location in the equatorial plane. The determination of the L -shell bounding the plasmasphere is based on magnetic field observations made by the CHAMP satellite in the topside ionosphere. Related signals are medium-scale field-aligned currents (MSFAC) (some 10 km scale size). The mid-latitude boundary of these MSFACs is used for determining the plasmapause. We are presenting a procedure for detecting the MSFAC boundary. Reliable L -values are obtained on the night side, whenever the solar zenith angle is below 90°. This means, the boundary is not determined well in the 08:00 to 16:00 magnetic local time (MLT) sector. The radial distance of the boundary is closely controlled by the magnetic activity index Kp. Over the Kp range 0 to 9, the L -value varies from 6 to 2 R E . Conversely, the dependence on solar flux is insignificant. For a fixed Kp level, the obtained L -values of the boundary form a ring on an MLT dial plot with a centre somewhat offset from the geomagnetic pole. This Kp and local time dependent feature is used for predicting the location of the MSFAC boundary at all MLTs based on a single L -value determination by CHAMP. We compared the location of the MSFAC boundary during the years 2001–2002 with the L -value of the plasmapause, determined from in situ observations by the IMAGE spacecraft. The mean difference in radial distance is within a 1 R E range for all local times and Kp values. The plasmapause is generally found earthward of the FAC boundary, except for the duskside. By considering this systematic displacement and by taking into account the diurnal variation and Kp-dependence of the residuals, we are able to construct an empirical model of the plasmapause location that is based on MSFAC measurements from CHAMP. Our new model PPCH-2012 agrees with IMAGE in situ observations within a standard deviation of 0.79 R E .

Description: The relationship of thermospheric density anomaly with electron temperature, small-scale FAC, and ion up-flow in the cusp region, as observed by CHAMP and DMSP satellites Annales Geophysicae, 31, 541-554, 2013 Author(s): G. N. Kervalishvili and H. Lühr We present in a statistical study a comparison of thermospheric mass density enhancements (ρ rel ) with electron temperature ( T e ), small-scale field-aligned currents (SSFACs), and vertical ion velocity ( V z ) at high latitudes around noon magnetic local time (MLT). Satellite data from CHAMP (CHAllenging Minisatellite Payload) and DMSP (Defense Meteorological Satellite Program) sampling the Northern Hemisphere during the years 2002–2005 are used. In a first step we investigate the distribution of the measured quantities in a magnetic latitude (MLat) versus MLT frame. All considered variables exhibit prominent peak amplitudes in the cusp region. A superposed epoch analysis was performed to examine causal relationship between the quantities. The occurrence of a thermospheric relative mass density anomaly, ρ rel 〉1.2, in the cusp region is defining an event. The location of the density peak is taken as a reference latitude (Δ MLat = 0°). Interestingly, all the considered quantities, SSFACs, T e , and V z are co-located with the density anomaly. The amplitudes of the peaks exhibit different characters of seasonal variation. The average relative density enhancement of the more prominent density peaks considered in this study amounts to 1.33 during all seasons. As expected, SSFACs are largest in summer with average amplitudes equal to 2.56 μA m −2 , decaying to 2.00 μA m −2 in winter. The event related enhancements of T e and V z are both largest in winter (Δ T e =730 K, V z =136 m s −1 ) and smallest in summer (Δ T e = 377 K, V z = 57 m s −1 . Based on the similarity of the seasonal behaviour we suggest a close relationship between these two quantities. A correlation analysis supports a linear relation with a high coefficient greater than or equal to 0.93, irrespective of season. Our preferred explanation is that dayside reconnection fuels Joule heating of the thermosphere causing air upwelling and at the same time heating of the electron gas that pulls up ions along affected flux tubes.

Description: Joint radio and optical observations of the most radio-powerful intracloud lightning discharges Annales Geophysicae, 31, 563-580, 2013 Author(s): A. R. Jacobson, T. E. L. Light, T. Hamlin, and R. Nemzek The most radio-powerful intracloud lightning emissions are associated with a phenomenon variously called "narrow bipolar events" or "compact intracloud discharges". This article examines in detail the coincidence and timing relationship between, on the one hand, the most radio-powerful intracloud lightning events and, on the other hand, optical outputs (or lack thereof) of the same discharge process. This is done, first, using coordinated very high frequency (VHF) and optical observations from the FORTE satellite and, second, using coordinated sferic and all-sky optical observations from the Los Alamos Sferic Array. In both cases, it is found that the sought coincidences are exceedingly rare. Moreover, in the handful of coincidences between optical and intense radio emissions that have been identified, the radio emissions differ from their usual behavior, by being accompanied by approximately simultaneous "conventional" lightning radio emissions. It is implied that the most radio-powerful intracloud emission process essentially differs from ordinary incandescent lightning.

Description: Analysis of the substructure within a complex magnetic cloud on 3–4 September 2008 Annales Geophysicae, 31, 555-562, 2013 Author(s): K. Andreeova, E. K. J. Kilpua, H. Hietala, H. E. J. Koskinen, A. Isavnin, and R. Vainio In this paper we have analyzed a substructure found within a leading part of a north–south-oriented magnetic cloud (MC) observed on 3–4 September 2008 in the near-Earth solar wind by multiple spacecraft (ACE, Wind, THEMIS B and C). The MC was preceded by a stream interface (SI) and followed by a high-speed stream (HSS). The identified substructure featured a strong depletion of suprathermal halo electrons and showed distinct magnetic field and plasma signatures. It occurred where suprathermal electron flow within a cloud changed from bidirectional to unidirectional, indicating change in the field line connectivity to the Sun. We found that the substructure maintained roughly its integrity from the first Lagrangian point to the vicinity of the Earth's bow shock in the front edge of the MC, but revealed small changes in the structure which could be explained either by temporal evolution or spatial configuration of the spacecraft.

Description: A 20-day period standing oscillation in the northern winter stratosphere Annales Geophysicae, 31, 755-764, 2013 Author(s): K. Hocke, S. Studer, O. Martius, D. Scheiben, and N. Kämpfer Observations of the ozone profile by a ground-based microwave radiometer in Switzerland indicate a dominant 20-day oscillation in stratospheric ozone, possibly related to oscillations of the polar vortex edge during winter. For further understanding of the nature of the 20-day oscillation, the ozone data set of ERA Interim meteorological reanalysis is analyzed at the latitude belt of 47.5° N and in the time from 1979 to 2010. Spectral analysis of ozone time series at 7 hPa indicates that the 20-day oscillation is maximal at two locations: 7.5° E, 47.5° N and 60° E, 47.5° N. Composites of the stream function are derived for different phases of the 20-day oscillation of stratospheric ozone at 7 hPa in the Northern Hemisphere. The streamline at Ψ = −2 × 10 7 m 2 s −1 is in the vicinity of the polar vortex edge. The other streamline at Ψ = 4 × 10 7 m 2 s 1 surrounds the Aleutian anticyclone and goes to the subtropics. The composites show 20-day period standing oscillations at the polar vortex edge and in the subtropics above Northern Africa, India, and China. The 20-day period standing oscillation above Aral Sea and India is correlated to the strength of the Aleutian anticyclone.

Description: Tropopause fold occurrence rates over the Antarctic station Troll (72° S, 2.5° E) Annales Geophysicae, 31, 591-598, 2013 Author(s): M. Mihalikova and S. Kirkwood One of the important mechanisms of stratosphere–troposphere exchange, which brings ozone-rich stratospheric air to low altitudes in extratropical regions, is transport related to tropopause folds. The climatology of folds has been studied at high latitudes of the Northern Hemisphere with the help of radars and global models. Global models supply information about fold occurrence rates at high latitudes of the Southern Hemisphere as well, but so far comparisons with direct measurements are rare. The Moveable Atmospheric Radar for Antarctica (MARA), a 54.5 MHz wind-profiler radar, has been operated at the Norwegian year-round station Troll, Antarctica (72° S, 2.5° E) since December 2011. Frequent tropopause fold signatures have been observed. In this study, based on MARA observations, an occurrence rate statistics of tropopause folds from December 2011 until November 2012 has been made, and radar data have been compared with the analysis from the European Center for Medium-Range Weather Forecasting (ECMWF). The fold occurrence rates exhibit an annual cycle with winter maximum and summer minimum and suggest significantly higher occurrence rates for the given location than those obtained previously by global model studies.

Description: Strong Kelvin wave activity observed during the westerly phase of QBO – a case study Annales Geophysicae, 31, 581-590, 2013 Author(s): U. Das and C. J. Pan Temperature data from Global Positioning System based Radio Occultation (GPS RO) soundings of the Formosa Satellite mission 3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC or F-3/C) micro satellites have been investigated in detail to study the Kelvin wave (KW) properties during September 2008 to February 2009 using the two-dimensional Fourier transform. It is observed that there was strong KW activity during November and December 2008; large wave amplitudes are observed from above the tropopause to 40 km – the data limit of F-3/C. KW of wavenumbers E1 and E2 with time periods 7.5 and 13 days, dominated during this period and the vertical wavelengths of these waves varied from 12 to 18 km. This event is very interesting as the QBO during this period was westerly in the lower stratosphere (up to ~ 26 km) and easterly above, whereas, climatological studies show that KW get attenuated during westerlies and their amplitudes maximise during easterlies and westerly shears. In the present study, however, the eastward propagating KW crossed the westerly lower stratosphere as the vertical extent of the westerly wind regime was less than the vertical wavelengths of the KW. The waves might have deposited eastward momentum in the upper stratosphere at 26–40 km, thereby reducing the magnitude of the easterly wind by as much as 10 m s −1 . The outgoing long wave radiation (OLR) is also investigated and it is found that these KW are produced due to deep convections in the lower atmosphere.

Description: Convective cells of internal gravity waves in the earth's atmosphere with finite temperature gradient Annales Geophysicae, 31, 459-462, 2013 Author(s): O. Onishchenko, O. Pokhotelov, and V. Fedun In this paper, we have investigated vortex structures (e.g. convective cells) of internal gravity waves (IGWs) in the earth's atmosphere with a finite vertical temperature gradient. A closed system of nonlinear equations for these waves and the condition for existence of solitary convective cells are obtained. In the atmosphere layers where the temperature decreases with height, the presence of IGW convective cells is shown. The typical parameters of such structures in the earth's atmosphere are discussed.

Description: Ion temperature anisotropy effects on threshold conditions of a shear-modified current driven electrostatic ion-acoustic instability in the topside auroral ionosphere Annales Geophysicae, 31, 451-457, 2013 Author(s): P. J. G. Perron, J.-M. A. Noël, K. Kabin, and J.-P. St-Maurice Temperature anisotropies may be encountered in space plasmas when there is a preferred direction, for instance, a strong magnetic or electric field. In this paper, we study how ion temperature anisotropy can affect the threshold conditions of a shear-modified current driven electrostatic ion-acoustic (CDEIA) instability. In particular, this communication focuses on instabilities in the context of topside auroral F-region situations and in the limit where finite Larmor radius corrections are small. We derived a new fluid-like expression for the critical drift which depends explicitly on ion anisotropy. More importantly, for ion to electron temperature ratios typical of F-region, solutions of the kinetic dispersion relation show that ion temperature anisotropy may significantly lower the drift threshold required for instability. In some cases, a perpendicular to parallel ion temperature ratio of 2 and may reduce the relative drift required for the onset of instability by a factor of approximately 30, assuming the ion-acoustic speed of the medium remains constant. Therefore, the ion temperature anisotropy should be considered in future studies of ion-acoustic waves and instabilities in the high-latitude ionospheric F-region.

Description: Thermospheric neutral temperatures derived from charge-exchange produced N 2 + Meinel (1,0) rotational distributions Annales Geophysicae, 31, 463-471, 2013 Author(s): C. K. Mutiso, M. D. Zettergren, J. M. Hughes, and G. G. Sivjee Thermalized rotational distributions of neutral and ionized N 2 and O 2 have long been used to determine neutral temperatures ( T n ) during auroral conditions. In both bright E-region (≲150 km) auroras, and in higher-altitude auroras, spectral distributions of molecular emissions employed to determine T n in the E-region cannot likewise be used to obtain T n in the F-region. Nevertheless, charge-exchange reactions between high-altitude (≳130 km) species provide an exception to this situation. In particular, the charge-exchange reaction O + ( 2 D) + N 2 (X) → N + 2 (A 2 Π u , ν' = 1 + O( 3 P) yields thermalized N 2 + Meinel (1,0) emissions, which, albeit weak, can be used to derive neutral temperatures at altitudes of ~130 km and higher. In this work, we present N 2 + Meinel (1,0) rotational temperatures and brightnesses obtained at Svalbard, Norway, during various auroral conditions. We calculate T n at thermospheric altitudes of 130–180 km from thermalized rotational populations of N 2 + Meinel (1,0); these emissions are excited by soft electron (≲1 keV) impact and charge-exchange reactions. We model the contributions of the respective excitation mechanisms, and compare derived brightnesses to observations. The agreement between the two is good. Emission heights obtained from optical data, modeling, and ISR data are consistent. Obtaining thermospheric T n from charge-exchange excited N 2 + Meinel (1,0) emissions provides an additional means of remotely sensing the neutral atmosphere, although certain limiting conditions are necessary. These include precipitation of low-energy electrons, and a non-sunlit emitting layer.

Description: Effect of the solar wind conditions on the ionospheric equivalent current systems Annales Geophysicae, 31, 489-501, 2013 Author(s): J. J. Zhang, C. Wang, B. B. Tang, and H. Li We employ a global magnetohydrodynamics (MHD) model, namely the PPMLR-MHD model, to investigate the effect of the solar wind conditions, such as the interplanetary magnetic field (IMF) clock angle, southward IMF magnitude and solar wind speed, on the average pattern of the ionospheric equivalent current systems (ECS). A new method to derive ECS from the MHD model is proposed and applied, which takes account of the oblique magnetic field line effects. The model results indicate that when the IMF is due northward, the ECS are very weak while the current over polar region is stronger than the lower latitude; when the IMF rotates southward, the two-cell current system dominates, the eastward electrojet on the afternoon sector and the westward electrojet on the dawn sector increase rapidly while the westward electrojet is stronger than the eastward electrojet. Under southward IMF, the intensity of the westward electrojet and eastward electrojet both increase with the increase of the southward IMF magnitude and solar wind speed, and the increase is very sharp for the westward electrojet. Furthermore, we compare the geomagnetic perturbations on the ground represented by the simulated average ECS with the observation-based statistical results under similar solar wind conditions. It is found that the model results generally match with the observations, but the underestimation of the eastward equivalent current on the dusk sector is the main limitation of the present model.

Description: Modeling solar flare induced lower ionosphere changes using VLF/LF transmitter amplitude and phase observations at a midlatitude site Annales Geophysicae, 31, 765-773, 2013 Author(s): E. D. Schmitter Remote sensing of the ionosphere bottom using long wave radio signal propagation is a still going strong and inexpensive method for continuous monitoring purposes. We present a propagation model describing the time development of solar flare effects. Based on monitored amplitude and phase data from VLF/LF transmitters gained at a mid-latitude site during the currently increasing solar cycle no. 24 a parameterized electron density profile is calculated as a function of time and fed into propagation calculations using the LWPC (Long Wave Propagation Capability). The model allows to include lower ionosphere recombination and attachment coefficients, as well as to identify the relevant forcing X-ray wavelength band, and is intended to be a small step forward to a better understanding of the solar–lower ionosphere interaction mechanisms within a consistent framework.

Description: Simultaneous observations of a Mesospheric Inversion Layer and turbulence during the ECOMA-2010 rocket campaign Annales Geophysicae, 31, 775-785, 2013 Author(s): A. Szewczyk, B. Strelnikov, M. Rapp, I. Strelnikova, G. Baumgarten, N. Kaifler, T. Dunker, and U.-P. Hoppe From 19 November to 19 December 2010 the fourth and final ECOMA rocket campaign was conducted at Andøya Rocket Range (69° N, 16° E) in northern Norway. We present and discuss measurement results obtained during the last rocket launch labelled ECOMA09 when simultaneous and true common volume in situ measurements of temperature and turbulence supported by ground-based lidar observations reveal two Mesospheric Inversion Layers (MIL) at heights between 71 and 73 km and between 86 and 89 km. Strong turbulence was measured in the region of the upper inversion layer, with the turbulent energy dissipation rates maximising at 2 W kg −1 . This upper MIL was observed by the ALOMAR Weber Na lidar over the period of several hours. The spatial extension of this MIL as observed by the MLS instrument onboard AURA satellite was found to be more than two thousand kilometres. Our analysis suggests that both observed MILs could possibly have been produced by neutral air turbulence.

Description: Comparison of the characteristics of ionospheric parameters obtained from FORMOSAT-3 and digisonde over Ascension Island Annales Geophysicae, 31, 787-794, 2013 Author(s): Y. J. Chuo, C. C. Lee, W. S. Chen, and B. W. Reinisch Electron density profile data obtained from the FORMOSAT-3 radio occultation (RO) measurements over Ascension Island are used to study the bottomside thickness parameter B0 in the International Reference Ionosphere (IRI) model, scale height around the F region peak height, and other F2 region parameters. The RO data were collected when the radio occultation occurred at Ascension Island (345.6° E, 8.0° S) during the solar minimum activity period from May 2006 to April 2008. Results show that the B0 values are in moderate agreement with the ground-based observations in the equinox period (correlation coefficient r = 0.682) and winter ( r = 0.570), with a strong correlation in summer ( r = 0.750). The seasonal and diurnal variations in B0 over Ascension Island show peak values during the daytime and in winter. In addition, the B0 values were underestimated and overestimated in the RO measurements during the daytime and nighttime, respectively. Moreover, the comparison of scale heights shows that scale heights obtained from the retrieved data and digisonde observations are weakly correlation in all three seasons. Furthermore, although the effective scale height ( H T ) values were reverse of those obtained from the RO measurements and are higher during the nighttime than in the daytime, they are in good agreement with those from ground-based observations. This paper also provides a comprehensive discussion of the effect of the asymmetric ionospheric electron density profiles on RO measurements.

Description: Variability and trend of diurnal temperature range in China and their relationship to total cloud cover and sunshine duration Annales Geophysicae, 31, 795-804, 2013 Author(s): X. Xia This study aims to investigate the effect of total cloud cover (TCC) and sunshine duration (SSD) in the variation of diurnal temperature range (DTR) in China during 1954–2009. As expected, the inter-annual variation of DTR was mainly determined by TCC. Analysis of trends of 30-year moving windows of DTR and TCC time series showed that TCC changes could account for that of DTR in some cases. However, TCC decreased during 1954–2009, which did not support DTR reduction across China. DTRs under sky conditions such as clear, cloudy and overcast showed nearly the same decreasing rate that completely accounted for the overall DTR reduction. Nevertheless, correlation between SSD and DTR was weak and not significant under clear sky conditions in which aerosol direct radiative effect should be dominant. Furthermore, 30–60% of DTR reduction was associated with DTR decrease under overcast conditions in south China. This implies that aerosol direct radiative effect appears not to be one of the main factors determining long-term changes in DTR in China.

Description: Studies of gravity wave propagation in the mesosphere observed by MU radar Annales Geophysicae, 31, 845-858, 2013 Author(s): H. Y. Lue, F. S. Kuo, S. Fukao, and T. Nakamura Mesospheric data were analyzed by a composite method combining phase and group velocity tracing technique and the spectra method of Stokes parameter analysis to obtain the propagation parameters of atmospheric gravity waves (AGW) in the height ranges between 63.6 and 99.3 km, observed using the MU radar at Shigaraki in Japan in the months of November and July in the years 1986, 1988 and 1989. The data of waves with downward phase velocity and the data of waves with upward phase velocity were independently treated. First, the vertical phase velocity and vertical group velocity as well as the characteristic wave period for each wave packet were obtained by phase and group velocity tracing technique. Then its horizontal wavelength, intrinsic wave period and horizontal group velocity were obtained by the dispersion relation. The intrinsic frequency and azimuth of wave vector of each wave packet were checked by Stokes parameters analysis. The results showed that the waves with intrinsic periods in the range 30 min–4.5 h had horizontal wavelength ranging from 25 to 240 km, vertical wavelength from 2.5 to 12 km, and horizontal group velocities from 15 to 60 m s −1 . Both upward moving wave packets and downward moving wave packets had horizontal group velocities mostly directed in the sector between directions NNE (north-north-east) and SEE in the month of November, and mostly in the sector between directions NW and SWS in the month of July. Comparing with mean wind directions, the gravity waves appeared to be more likely to propagate along with mean wind than against it. This apparent prevalence for downstream wave packets was found to be caused by a systematic filtering effect existing in the process of phase and group velocity tracing analysis: A significant portion of upstream wave packets might have been Doppler shifted out of the vertical range in phase and group velocity tracing analysis.

Description: Analysis and parameterisation of ionic reactions affecting middle atmospheric HO x and NO y during solar proton events Annales Geophysicae, 31, 909-956, 2013 Author(s): P. T. Verronen and R. Lehmann In the polar regions, precipitation of solar high-energy protons and electrons affects the neutral composition of the middle atmosphere. Here we use the Sodankylä Ion and Neutral Chemistry model to calculate ionic production and loss rates of neutral HO x and NO y species, imposed by particle precipitation, for a range of atmospheric conditions and levels of ionization. We also analyse in detail the ionic reaction sequences leading to the HO x and NO y changes. Our results show that particle impact ionization and positive ion chemistry cause net production of N, NO, HNO 2 , H, andOH from N 2 and H 2 O. On the other hand, negative ion chemistry redistributes the NO y species, without net production or loss, so that NO, NO 2 , and N 2 O 5 are converted to HNO 3 and NO 3 . Based on the model results, we provide tables of so-called P/Q numbers (i.e. production and loss rates of neutral species divided by ionization rates) at altitudes between 20 and 90 km. These numbers can be easily used to parameterise the ion chemistry effects when modelling atmospheric response to particle precipitation. Compared to earlier studies, our work is the first to consider in detail the NO y effect of negative ion chemistry, and the diurnal and seasonal variability of the P/Q numbers.

Description: Improved analysis of all-sky meteor radar measurements of gravity wave variances and momentum fluxes Annales Geophysicae, 31, 889-908, 2013 Author(s): V. F. Andrioli, D. C. Fritts, P. P. Batista, and B. R. Clemesha The advantages of using a composite day analysis for all-sky interferometric meteor radars when measuring mean winds and tides are widely known. On the other hand, problems arise if this technique is applied to Hocking's (2005) gravity wave analysis for all-sky meteor radars. In this paper we describe how a simple change in the procedure makes it possible to use a composite day in Hocking's analysis. Also, we explain how a modified composite day can be constructed to test its ability to measure gravity wave momentum fluxes. Test results for specified mean, tidal, and gravity wave fields, including tidal amplitudes and gravity wave momentum fluxes varying strongly with altitude and/or time, suggest that the modified composite day allows characterization of monthly mean profiles of the gravity wave momentum fluxes, with good accuracy at least at the altitudes where the meteor counts are large (from 89 to 92.5 km). In the present work we also show that the variances measured with Hocking's method are often contaminated by the tidal fields and suggest a method of empirical correction derived from a simple simulation model. The results presented here greatly increase our confidence because they show that our technique is able to remove the tide-induced false variances from Hocking's analysis.

Description: Discontinuities and Alfvénic fluctuations in the solar wind Annales Geophysicae, 31, 871-887, 2013 Author(s): G. Paschmann, S. Haaland, B. Sonnerup, and T. Knetter We examine the Alfvénicity of a set of 188 solar wind directional discontinuities (DDs) identified in the Cluster data from 2003 by Knetter (2005), with the objective of separating rotational discontinuities (RDs) from tangential ones (TDs). The DDs occurred over the full range of solar wind velocities and magnetic shear angles. By performing the Walén test in the de Hoffmann–Teller (HT) frame, we show that 77 of the 127 crossings for which a good HT frame was found had plasma flow speeds exceeding 80% of the Alfvén speed at an average angular deviation of 7.7°; 33 cases had speeds exceeding 90% of the Alfvén speed at an average angle of 6.4°. We show that the angular deviation between flow velocity (in the HT frame) and the Alfvén velocity can be obtained from a reduced form of the Walén correlation coefficient. The corresponding results from the Walén test expressed in terms of jumps in flow speed and corresponding jumps in Alfvén speed are similar: 66 of the same 127 cases had velocity jumps exceeding 80% with average angular deviation of 5.8°, and 22 exceeding 90% of the jump in Alfvén speed, with average angular deviation 6.2°. We conclude that a substantial fraction of the 127 events can be identified as RDs. We present further evidence for coupling across the DDs by showing that, for most of the 127 crossings, the HT frame velocities, evaluated separately on the two sides of the DD, are nearly the same – a result required for RDs but not for TDs. We also show that the degree of Alfvénicity is nearly the same for the DDs and fluctuations in which the DDs are embedded. Whatever process causes deviations from ideal Alfvénicity appears to operate equally for the DDs as for the surrounding fluctuations. Finally, our study has established a unique relation between the strahl electron pitch angle and the sign of the Walén slope, implying antisunward propagation in the plasma frame for all 127 cases.

Description: The polar cliff in the morning sector of the ionosphere Annales Geophysicae, 31, 957-966, 2013 Author(s): G. W. Prölss By "polar cliff" we mean the steep increase in the ionization density observed in the morning sector of the polar ionosphere. Here the properties of this remarkable feature are investigated. The data set consists of electron density and temperature measurements obtained by the Dynamics Explorer 2 satellite. Only data recorded in the Northern Hemisphere winter are considered (solar zenith angle ≥ 90°). We find that for moderately disturbed conditions, the foot of the polar cliff is located below 60° invariant latitude. Here, within about 4°, the density increases by a factor of 4, on average. The actual location of the polar cliff depends primarily on the level of geomagnetic activity, its associated density increase on geographic longitude and altitude. As to the longitudinal variations, they are attributed to asymmetries in the background ionization density at middle latitudes. Using a superposed epoch type of averaging procedure, mean latitudinal profiles of the polar cliff and the associated electron temperature changes are derived. Since these differ significantly from those derived for the afternoon/evening sector, we conclude that the subauroral ionospheric trough does not extend into the morning sector. As to the origin of the polar cliff in the morning sector, local auroral particle precipitation should play only a secondary role.

Description: Role of convection in hydration of tropical UTLS: implication of AURA MLS long-term observations Annales Geophysicae, 31, 967-981, 2013 Author(s): S. Jain, A. R. Jain, and T. K. Mandal This paper addresses various characteristic features associated with the hydration of the tropical upper troposphere and lower stratosphere (UTLS) in association with the boreal monsoonal convective activity occurring over three different tropical regions viz. Asian region, American region and African region. Analysis of water vapor mixing ratio (WVMR) data obtained from AURA MLS (v3.3), ISCCP D1 cloud top pressure data and outgoing long-wave radiation data (OLR) from NOAA reanalysis has brought out two significant results. Firstly, high altitude clouds and high WVMR regions are mostly associated with the low OLR region. Convection over Asian region is very deep and spread over a large geographical area as compared to African or American region. Magnitude of WVMR in the pressure range of 261.0–146.8 hPa is observed to be higher over the Asian region, whereas, close to the tropopause level (~ 100 hPa) it is comparable or more over the American region as compared to Asian and African regions. Secondly, the vertical ascent rate of water vapour obtained from AURA MLS data suggest that convection associated transport might have dominated up to 146.8–121.2 hPa in addition to slow large scale diabatic transport, which appears to be prominent above the pressure range of 100–82.5 hPa. Within the pressure levels of 121.2 to 82.5 hPa, the influence of convective transport appears to be gradually decreasing. However, there are other possible processes like ice sublimation or cirrus jumping which could contribute to the hydration of the tropical lower stratosphere. Present analysis has, thus, brought out the significance of convection in water vapour transport and distribution in the tropical UTLS. Another interesting feature which is observed is the anomalous increase in temperature during boreal winters (November–April) over all three convective regions. Such phenomenon is only observed within the pressure range of 100.0–261.0 hPa; however it is relatively weaker for levels 〉 177.8 hPa.

Description: New low-frequency electromagnetic modes associated with neutral dynamics in partially ionised plasma Annales Geophysicae, 31, 983-993, 2013 Author(s): A. A. Shaikh and A. C. Das We have investigated the low frequency electromagnetic (EM) modes in inhomogeneous, magnetised partially ionised plasma by incorporating neutral dynamics. We have derived a general EM dispersion relation by using a two-fluids magnetohydrodynamics (MHD) model. Our analysis shows that the neutral dynamics is playing an extremely important role in the physics of magnetised partially ionised plasma by giving rise to new kind of EM modes. We found (1) the new instability is linked with compressibility of neutral particles, the collision between neutral and charged species and the relative streaming in hot/cold, inhomogeneous, magnetised partially ionised plasma, (2) and that neutral dynamics is responsible for the modified (complex) inertial effect on magnetic field lines. Its consequences on the propagation characteristics of Alfvén wave and cyclotron frequency are discussed. Furthermore, a new mode similar to the Langmuir mode is reported. Finally, we discuss our results, for limiting cases, that may be appropriate for applications to space plasma environments including probable mechanism of escaping H + and O − from the Martian atmosphere.

Description: High-precision measurement of satellite range and velocity using the EISCAT radar Annales Geophysicae, 31, 859-870, 2013 Author(s): J. Markkanen, T. Nygrén, M. Markkanen, M. Voiculescu, and A. Aikio This paper is a continuation of an earlier work by Nygrén et al. (2012), where the velocity of a hard target was determined from a set of echo pulses reflected by the target flying through the radar beam. Here the method is extended to include the determination of range at a high accuracy. The method is as follows. First, the flight time of the pulse from the transmitter to the target is determined at an accuracy essentially better than the accuracy given by the sampling interval. This method makes use of the fact that the receiver filtering creates slopes at the phase flips of the phase modulated echo pulse. A precise flight time is found by investigating the echo amplitude within this slope. A value of velocity is calculated from each echo pulse as explained in the earlier paper. Next, the ranges together with velocities from a single beam pass are combined to a measurement vector for a linear inversion problem. The solution of the inversion problem gives the time-dependent range and velocity from the time interval of satellite flight through the radar beam. The method is demonstrated using the EISCAT (European Incoherent Scatter) UHF radar and radio pulses reflected by a satellite. The achieved standard deviations of range are about 5–50 cm and those of velocity are about 3–25 mm s −1 .

Description: Magnetic susceptibility from electron holes Annales Geophysicae, 31, 1191-1193, 2013 Author(s): R. A. Treumann and W. Baumjohann A recent theory of magnetic field amplification in electron holes is extended to derive the magnetic susceptibility of an electron-hole gas propagating in a magnetic flux tube along the ambient magnetic field. It is shown that the hole gas behaves diamagnetic adding some small amount to the well-known Landau susceptibility in the hole-carrying volume.

Description: On the electron temperature downstream of the solar wind termination shock Annales Geophysicae, 31, 1205-1212, 2013 Author(s): I. V. Chashei and H. J. Fahr In this paper we study the temperatures of electrons convected with the solar wind to large solar distances and finally transported over the solar wind termination shock. Nearly nothing, unless at high energies in the cosmic ray regime, is known about the thermodynamical behaviour of these distant electrons from in~situ plasma observations. Hence it is tacitly assumed these electrons, due to their adiabatic behaviour and vanishing heat conduction or energization processes, have rapidly cooled off to very low temperatures once they eventually arrive at the solar wind termination shock (at about 100 AU). In this paper we show that such electrons, however, at their passage over the termination shock due to the shock–electric field action undergo an over-adiabatic heating and therefore appear on the downstream side as a substantially heated plasma species. Looking quantitatively into this heating process we find that solar wind electrons achieve temperatures of the order of 2–4 × 10 6 K downstream of the termination shock, depending on the upstream solar wind bulk velocity and the shock compression ratio. Hence these electrons therewith play an important dynamical role in structuring this shock and determining the downstream plasma flow properties. Furthermore, they present an additional ionization source for incoming neutral interstellar hydrogen and excite X-ray emission. They also behave similar to cosmic ray electrons and extend to some limited region upstream of the shock of the order of 0.1 AU by spatial diffusion and thereby also modify the upstream solar wind properties.

Description: Fluid simulations of non-resonant anisotropic ion heating Annales Geophysicae, 31, 1195-1204, 2013 Author(s): D. Laveder, T. Passot, and P. L. Sulem The finite Larmor radius (FLR)-Landau fluid model, which extends the usual anisotropic magnetohydrodynamics to magnetized collisionless plasmas by retaining linear Landau damping and finite Larmor radius corrections down to the sub-ionic scales in the quasi-transverse directions, is used to study the non-resonant heating of the plasma by randomly driven Alfvén waves. One-dimensional numerical simulations, free from any artificial dissipation, are used to analyze the influence on the thermal dynamics, of the beta parameter and of the separation between the driving and the ion scales. While the gyrotropic heat fluxes play a dominant role when the plasma is driven at large scales, leading to a parallel heating of the ions by Landau damping, a different regime develops when the driving acts at scales comparable to the ion Larmor radius. Perpendicular heating and parallel cooling of the ions are then observed, an effect that is mostly due to the work of the non-gyrotropic pressure force and that can be viewed as the fluid signature of the so-called stochastic heating. A partial characterization of the plasma by global quantities (such as the magnetic compressibility and the density-magnetic field correlations that provide information on the dominant type of waves) is also presented. The enhancement of the parallel electron heating by a higher level of fast magnetosonic waves is in particular pointed out.

Description: Thunderstorm incidence in southeastern Brazil estimated from different data sources Annales Geophysicae, 31, 1213-1219, 2013 Author(s): O. Pinto Jr., K. P. Naccarato, and I. R. C. A. Pinto This paper describes a comparative analysis of the thunderstorm incidence in southeastern Brazil obtained from thunderstorm days observed at two different epochs (from 1910 to 1951 and from 1971 to 1984) and from lightning data provided by the Brazilian lightning location system RINDAT (from 1999 to 2006) and the Lightning Imaging Sensor (LIS) on board the Tropical Rainfall Measuring Mission (TRMM) satellite (from 1998 to 2010). The results are interpreted in terms of the main synoptic patterns associated with thunderstorm activity in this region, indicating that the prevailing synoptic pattern associated with thunderstorm activity is the occurrence of frontal systems and their modulation by the South Atlantic Convergence Zone (SACZ) and topography. Evidence of urban effects is also found. The results are also discussed in the context of practical applications involving their use in the Brazilian lightning protection standards, suggesting that the present version of the Brazilian standards should be revised incorporating RINDAT and LIS data. Finally, the results are important to improve our knowledge about the limitations of the different techniques used to record the thunderstorm activity and support future climatic studies.

Description: Comments on quiet daily variation derivation in "Identification of the IMF sector structure in near-real time by ground magnetic data" by Janzhura and Troshichev (2011) Annales Geophysicae, 31, 1221-1225, 2013 Author(s): P. Stauning The description presented in the paper of the relations of the solar wind sector structure to the derivation of the quiet daily variation (QDC) in polar magnetic recordings used for calculation of polar cap (PC) indices is found to be unclear and not properly justified. The presented example on inclusion of a solar sector term in an actual QDC series is found to be questionable even on the authors' premises.

Description: A nonlinear interaction event between a 16-day wave and a diurnal tide from meteor radar observations Annales Geophysicae, 31, 2039-2048, 2013 Author(s): K. M. Huang, A. Z. Liu, S. D. Zhang, F. Yi, C. M. Huang, Q. Gan, Y. Gong, and Y. H. Zhang Relative to extensive studies of interactions between the quasi 2-day wave and tides, nonlinear interaction of the 16-day wave with tides was reported less, in particular interaction with the diurnal tide. We present an observational study of a possible nonlinear interaction event between the 16-day wave and the diurnal tide based on meteor radar measurement at Maui. An obvious 16-day wave can be observed from raw wind data. Its maximum meridional wind amplitude can attain 18.0 m s −1 at a height of 92 km during the time of our attention, which is larger than that in previous reports. Sum and difference interactions between the 16-day wave and the diurnal tide are observed to have rather different intensities. Because sum nonlinear interaction is very intense, the secondary sum wave with a period of 22.59 h is stronger than the diurnal tide. However, weak spectrum of the secondary difference wave is hardly identified. The beat of the diurnal tide with the secondary sum wave leads to substantial modulation of the diurnal tide at a period of 16 days. Moreover, this strong secondary sum wave further interacts with the 16-day wave to generate a new secondary wave with a period of 21.33 h. Such an interaction may be also regarded as a third-order nonlinear interaction between the 16-day wave and the diurnal tide with two-step interaction. Hence, the third-order nonlinear interaction between planetary waves and tides may occur significantly in the MLT region.

Description: Zonal velocity of the equatorial plasma bubbles over Kolhapur, India Annales Geophysicae, 31, 2077-2084, 2013 Author(s): D. P. Nade, A. K. Sharma, S. S. Nikte, P. T. Patil, R. N. Ghodpage, M. V. Rokade, S. Gurubaran, A. Taori, and Y. Sahai This paper presents the observations of zonal drift velocities of equatorial ionospheric plasma bubbles and their comparison with model values. These velocities are determined by nightglow OI 630.0 nm images. The nightglow observations have been carried out from the low latitude station Kolhapur (16.8° N, 74.2° E; 10.6° N dip lat.) during clear moonless nights. Herein we have presented the drift velocities of equatorial plasma bubbles for the period of February–April 2011. Out of 80 nights, 39 showed the occurrence of equatorial plasma bubbles (49%). These 39 nights correspond to magnetically quiet days (ΣKp 〈 26). The average eastward zonal velocities (112 ± 10 m s −1 ) of equatorial plasma bubbles increased from evening sector to 21:00 IST (Indian Standard Time = Universal Time + 05:30:00 h), reach maximum about 165 ± 30 m s −1 and then decreases with time. The calculated velocities are in good agreement with that of recently reported values obtained with models with occasional differences; possible mechanisms of which are discussed.

Description: Meteor smoke influences on the D-region charge balance – review of recent in situ measurements and one-dimensional model results Annales Geophysicae, 31, 2049-2062, 2013 Author(s): C. Baumann, M. Rapp, A. Kero, and C.-F. Enell This work investigates the influence of meteoric smoke particles (MSP) on the charge balance in the D-region ionosphere. Both experimental in situ measurements and a one-dimensional ionospheric model reveal a clear impact of MSP on the ionospheric composition of the D-region. The study reviews rocket-borne in situ measurements of electron and positive ion density, which show a distinct deficit of electrons in comparison to positive ions between 80 and 95 km. This deficit can be explained by the ambient negatively charged MSP measured simultaneously with a Faraday cup. The influence of MSP on the D-region charge balance is addressed with a simplified ionospheric model with only six components, i.e. electrons, positive and negative ions and neutral and charged MSP (both signs). The scheme includes reactions of plasma captured by MSP and MSP photo reactions as well as the standard ionospheric processes, e.g. ion-ion recombination. The model shows that the capture of plasma constituents by MSP is an important process leading to scavenging of electrons. Since Faraday cup measurements are biased towards heavy MSP because of aerodynamical filtering, we have applied an estimate of this filter on the modelled MSP densities. By doing that, we find good qualitative agreement between the experimental data and our model results. In addition, the model study reveals an increase of positive ions in the presence of MSP. That is primarily caused by the reduced dissociative recombination with electrons which have been removed from the gas phase by the MSP.

Description: Characteristics of the Taylor microscale in the solar wind/foreshock: magnetic field and electron velocity measurements Annales Geophysicae, 31, 2063-2075, 2013 Author(s): C. Gurgiolo, M. L. Goldstein, W. H. Matthaeus, A. Viñas, and A. N. Fazakerley The Taylor microscale is one of the fundamental turbulence scales. Not easily estimated in the interplanetary medium employing single spacecraft data, it has generally been studied through two point correlations. In this paper we present an alternative, albeit mathematically equivalent, method for estimating the Taylor microscale (λ T ). We make two independent determinations employing multi-spacecraft data sets from the Cluster mission, one using magnetic field data and a second using electron velocity data. Our results using the magnetic field data set yields a scale length of 1538 ± 550 km, slightly less than, but within the same range as, values found in previous magnetic-field-based studies. During time periods where both magnetic field and electron velocity data can be used, the two values can be compared. Relative comparisons show λ T computed from the velocity is often significantly smaller than that from the magnetic field data. Due to a lack of events where both measurements are available, the absolute λ T based on the electron fluid velocity is not able to be determined.