ALBERT

Description: Beyond multifractional Brownian motion: new stochastic models for geophysical modelling Nonlinear Processes in Geophysics, 20, 643-655, 2013 Author(s): J. Lévy Véhel Multifractional Brownian motion (mBm) has proved to be a useful tool in various areas of geophysical modelling. Although a versatile model, mBm is of course not always an adequate one. We present in this work several other stochastic processes which could potentially be useful in geophysics. The first alternative type is that of self-regulating processes : these are models where the local regularity is a function of the amplitude, in contrast to mBm where it is tuned exogenously. We demonstrate the relevance of such models for digital elevation maps and for temperature records. We also briefly describe two other types of alternative processes, which are the counterparts of mBm and of self-regulating processes when the intensity of local jumps is considered in lieu of local regularity: multistable processes allow one to prescribe the local intensity of jumps in space/time, while this intensity is governed by the amplitude for self-stabilizing processes .

Description: Simplified PRBMs of spatial compliant multi-beam modules for planar motion Mechanical Sciences, 4, 311-318, 2013 Author(s): G. Hao PRBMs (pseudo-rigid-body models) have been becoming important engineering technologies/methods in the field of compliant mechanisms to simplify the design and analysis through the use of the knowledge body of rigid-body mechanisms coupling with springs. This article addresses the PRBMs of spatial multi-beam modules for planar motion, which are composed of three or more symmetrical wire/slender beams parallel to each other where the planar twisting DOF (degree of freedom) is assumed to be very small for specific applications/loading conditions. Simplified PRBMs are firstly proposed through replacing each beam in spatial multi-beam module with a rigid-body link plus two identical spherical joints at its two ends. The characteristics factor, bending stiffness and twisting stiffness for the spherical joint are determined. Load-displacement equations are then derived for a class of spatial multi-beam modules and general spatial multi-beam modules using the virtual work principle and kinematic relationships. Finally, nonlinear FEA (finite element analysis) is employed with comparisons with the PRBMs. The present PRBMs have shown the ability to predict the primary nonlinear constraint characteristics such as load-stiffening effect, cross-axis coupling in the two primary translational directions and buckling load.

Description: Non-extensivity and long-range correlations in the earthquake activity at the West Corinth rift (Greece) Nonlinear Processes in Geophysics, 20, 713-724, 2013 Author(s): G. Michas, F. Vallianatos, and P. Sammonds In the present work the statistical properties of the earthquake activity in a highly seismic region, the West Corinth rift (Central Greece), are being studied by means of generalized statistical physics. By using a dataset that covers the period 2001–2008, we investigate the earthquake energy distribution and the distribution of the time intervals (interevent times) between the successive events. As has been reported previously, these distributions exhibit complex statistical properties and fractality. By using detrended fluctuation analysis (DFA), a well-established method for detection of long-range correlations in non-stationary signals, it is shown that long-range correlations are also present in the earthquake activity. The existence of these properties motivates us to use non-extensive statistical physics (NESP) to investigate the statistical properties of the frequency-magnitude and the interevent time distributions, along with other well-known relations in seismology, such as the gamma distribution for interevent times. The results of the analysis indicate that the statistical properties of the earthquake activity can be successfully reproduced by means of NESP and that the earthquake activity at the West Corinth rift is correlated at all-time scales.

Description: Designing hybrid flexure systems and elements using Freedom and Constraint Topologies Mechanical Sciences, 4, 319-331, 2013 Author(s): J. B. Hopkins In this paper we introduce the principles necessary to synthesize hybrid flexure systems and elements. Flexure systems consist of rigid bodies that are joined together by flexure elements that elastically deform to guide the system's rigid bodies with desired degrees of freedom (DOFs). The principles introduced here for synthesizing hybrid flexure systems and elements are extensions of the Freedom and Constraint Topologies (FACT) synthesis approach. FACT utilizes a comprehensive library of geometric shapes from which designers can rapidly consider and compare a multiplicity of flexure concepts that achieve any desired set of DOFs. Prior to this paper, designers primarily used these shapes to synthesize parallel and serial flexure systems and elements. With this paper, designers may now use these same shapes to synthesize more general flexures that consist of various combinations of parallel and serial systems and elements (i.e., hybrid configurations). As such, designers can access a larger body of flexure solutions that satisfy demanding design requirements. Instructions for helping designers utilize or avoid the advantages and challenges of over-, under-, and exact-constraint are also provided. Hybrid systems and elements are analysed and designed as case studies.

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: The impact of initial spread calibration on the RELO ensemble and its application to Lagrangian dynamics Nonlinear Processes in Geophysics, 20, 621-641, 2013 Author(s): M. Wei, G. Jacobs, C. Rowley, C. N. Barron, P. Hogan, P. Spence, O. M. Smedstad, P. Martin, P. Muscarella, and E. Coelho A number of real-time ocean model forecasts were carried out successfully at Naval Research Laboratory (NRL) to provide modeling support and numerical guidance to the CARTHE GLAD at-sea experiment during summer 2012. Two RELO ensembles and three single models using NCOM and HYCOM with different resolutions were carried out. A calibrated ensemble system with enhanced spread and reliability was developed to better support this experiment. The calibrated ensemble is found to outperform the un-calibrated ensemble in forecasting accuracy, skill, and reliability for all the variables and observation spaces evaluated. The metrics used in this paper include RMS error, anomaly correlation, PECA, Brier score, spread reliability, and Talagrand rank histogram. It is also found that even the un-calibrated ensemble outperforms the single forecast from the model with the same resolution. The advantages of the ensembles are further extended to the Lagrangian framework. In contrast to a single model forecast, the RELO ensemble provides not only the most likely Lagrangian trajectory for a particle in the ocean, but also an uncertainty estimate that directly reflects the complicated ocean dynamics, which is valuable for decision makers. The examples show that the calibrated ensemble with more reliability can capture trajectories in different, even opposite, directions, which would be missed by the un-calibrated ensemble. The ensembles are applied to compute the repelling and attracting Lagrangian coherent structures (LCSs), and the uncertainties of the LCSs, which are hard to obtain from a single model forecast, are estimated. It is found that the spatial scales of the LCSs depend on the model resolution. The model with the highest resolution produces the finest, small-scale, LCS structures, while the model with lowest resolution generates only large-scale LCSs. The repelling and attracting LCSs are found to intersect at many locations and create complex mesoscale eddies. The fluid particles and drifters in the middle of these tangles are subject to attraction and repulsion simultaneously from these two kinds of LCSs. As a result, the movements of particles near the Deepwater Horizon (DWH) location are severely limited. This is also confirmed by the Lagrangian trajectories predicted by the ensembles.

Description: Multifractal properties of embedded convective structures in orographic precipitation: toward subgrid-scale predictability Nonlinear Processes in Geophysics, 20, 605-620, 2013 Author(s): M. Nogueira, A. P. Barros, and P. M. A. Miranda Rain and cloud fields produced by fully nonlinear idealized cloud resolving numerical simulations of orographic convective precipitation display statistical multiscaling behavior, implying that multifractal diagnostics should provide a physically robust basis for the downscaling and sub-grid scale parameterizations of moist processes. Our results show that the horizontal scaling exponent function (and respective multiscaling parameters) of the simulated rainfall and cloud fields varies with atmospheric and terrain properties, particularly small-scale terrain spectra, atmospheric stability, and advective timescale. This implies that multifractal diagnostics of moist processes for these simulations are fundamentally transient, exhibiting complex nonlinear behavior depending on atmospheric conditions and terrain forcing at each location. A particularly robust behavior found here is the transition of the multifractal parameters between stable and unstable cases, which has a clear physical correspondence to the transition from stratiform to organized (banded and cellular) convective regime. This result is reinforced by a similar behavior in the horizontal spectral exponent. Finally, our results indicate that although nonlinearly coupled fields (such as rain and clouds) have different scaling exponent functions, there are robust relationships with physical underpinnings between the scaling parameters that can be explored for hybrid dynamical-statistical downscaling.

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: A top-down model to generate ensembles of runoff from a large number of hillslopes Nonlinear Processes in Geophysics, 20, 683-704, 2013 Author(s): P. R. Furey, V. K. Gupta, and B. M. Troutman We hypothesize that total hillslope water loss for a rainfall–runoff event is inversely related to a function of a lognormal random variable, based on basin- and point-scale observations taken from the 21 km 2 Goodwin Creek Experimental Watershed (GCEW) in Mississippi, USA. A top-down approach is used to develop a new runoff generation model both to test our physical-statistical hypothesis and to provide a method of generating ensembles of runoff from a large number of hillslopes in a basin. The model is based on the assumption that the probability distributions of a runoff/loss ratio have a space–time rescaling property. We test this assumption using streamflow and rainfall data from GCEW. For over 100 rainfall–runoff events, we find that the spatial probability distributions of a runoff/loss ratio can be rescaled to a new distribution that is common to all events. We interpret random within-event differences in runoff/loss ratios in the model to arise from soil moisture spatial variability. Observations of water loss during events in GCEW support this interpretation. Our model preserves water balance in a mean statistical sense and supports our hypothesis. As an example, we use the model to generate ensembles of runoff at a large number of hillslopes for a rainfall–runoff event in GCEW.

Description: A mechanism for catastrophic filter divergence in data assimilation for sparse observation networks Nonlinear Processes in Geophysics, 20, 705-712, 2013 Author(s): G. A. Gottwald and A. J. Majda We study catastrophic filter divergence in data assimilation procedures whereby the forecast model develops severe numerical instabilities leading to a blow-up of the solution. Catastrophic filter divergence can occur in sparse observational grids with small observational noise for intermediate observation intervals and finite ensemble sizes. Using a minimal five-dimensional model, we establish that catastrophic filter divergence is a numerical instability of the underlying forecast model caused by the filtering procedure producing analyses which are not consistent with the true dynamics, and stiffness caused by the fast attraction of the inconsistent analyses towards the attractor during the forecast step.

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: Sub-inertial modulation of nonlinear Kelvin waves in the coastal zone Nonlinear Processes in Geophysics, 20, 357-364, 2013 Author(s): D. V. Stepanov and V. V. Novotryasov Observational evidence is presented for interaction between nonlinear internal Kelvin waves at the ω t,i (where the ω t is the semidiurnal frequency and the ω i is the inertial frequency) and random oscillations of the background coastal current at the sub-inertial Ω frequency in the Japan/East Sea. Enhanced coastal currents at the sum ω + and difference ω-frequencies ω ± =ω t,i ± Ω have properties of propagating Kelvin waves, which suggests permanent energy exchange from the sub-inertial band to the mesoscale ω ± band. This interaction may be responsible for a greater-than-predicted intensification, steepening and breaking of boundary-trapped Kelvin waves. The problem of interaction between the nonlinear Kelvin wave at the frequency ω and the low-frequency narrowband noise with representative frequency Ω≪ω is investigated using the theory of nonlinear weak dispersion waves.

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: A graph-theoretic approach to sparse matrix inversion for implicit differential algebraic equations Mechanical Sciences, 4, 243-250, 2013 Author(s): H. Yoshimura In this paper, we propose an efficient numerical scheme to compute sparse matrix inversions for Implicit Differential Algebraic Equations of large-scale nonlinear mechanical systems. We first formulate mechanical systems with constraints by Dirac structures and associated Lagrangian systems. Second, we show how to allocate input-output relations to the variables in kinematical and dynamical relations appearing in DAEs by introducing an oriented bipartite graph. Then, we also show that the matrix inversion of Jacobian matrix associated to the kinematical and dynamical relations can be carried out by using the input-output relations and we explain solvability of the sparse Jacobian matrix inversion by using the bipartite graph. Finally, we propose an efficient symbolic generation algorithm to compute the sparse matrix inversion of the Jacobian matrix, and we demonstrate the validity in numerical efficiency by an example of the stanford manipulator.

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: Application of wavelet transform for evaluation of hydrocarbon reservoirs: example from Iranian oil fields in the north of the Persian Gulf Nonlinear Processes in Geophysics, 20, 231-238, 2013 Author(s): M. R. Saadatinejad and H. Hassani The Persian Gulf and its surrounding area are some of the biggest basins and have a very important role in producing huge amounts of hydrocarbon, and this potential was evaluated in order to explore the target for geoscientists and petroleum engineers. Wavelet transform is a useful and applicable technique to reveal frequency contents of various signals in different branches of science and especially in petroleum studies. We applied two major capacities of continuous mode of wavelet transform in seismic investigations. These investigations were operated to detect reservoir geological structures and some anomalies related to hydrocarbon to develop and explore new petroleum reservoirs in at least 4 oilfields in the southwest of Iran. It had been observed that continuous wavelet transform results show some discontinuities in the location of faults and are able to display them more clearly than other seismic methods. Moreover, continuous wavelet transform, utilizing Morlet wavelet, displays low-frequency shadows on 4 different iso-frequency vertical sections to identify reservoirs containing gas. By comparing these different figures, the presence of low-frequency shadows under the reservoir could be seen and we can relate these variations from anomalies at different frequencies as an indicator of the presence of hydrocarbons in the target reservoir.

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: Distributed allocation of mobile sensing swarms in gyre flows Nonlinear Processes in Geophysics, 20, 657-668, 2013 Author(s): K. Mallory, M. A. Hsieh, E. Forgoston, and I. B. Schwartz We address the synthesis of distributed control policies to enable a swarm of homogeneous mobile sensors to maintain a desired spatial distribution in a geophysical flow environment, or workspace. In this article, we assume the mobile sensors (or robots) have a "map" of the environment denoting the locations of the Lagrangian coherent structures or LCS boundaries. Using this information, we design agent-level hybrid control policies that leverage the surrounding fluid dynamics and inherent environmental noise to enable the team to maintain a desired distribution in the workspace. We discuss the stability properties of the ensemble dynamics of the distributed control policies. Since realistic quasi-geostrophic ocean models predict double-gyre flow solutions, we use a wind-driven multi-gyre flow model to verify the feasibility of the proposed distributed control strategy and compare the proposed control strategy with a baseline deterministic allocation strategy. Lastly, we validate the control strategy using actual flow data obtained by our coherent structure experimental testbed.

Description: Four-dimensional ensemble-variational data assimilation for global deterministic weather prediction Nonlinear Processes in Geophysics, 20, 669-682, 2013 Author(s): M. Buehner, J. Morneau, and C. Charette The goal of this study is to evaluate a version of the ensemble-variational data assimilation approach (EnVar) for possible replacement of 4D-Var at Environment Canada for global deterministic weather prediction. This implementation of EnVar relies on 4-D ensemble covariances, obtained from an ensemble Kalman filter, that are combined in a vertically dependent weighted average with simple static covariances. Verification results are presented from a set of data assimilation experiments over two separate 6-week periods that used assimilated observations and model configuration very similar to the currently operational system. To help interpret the comparison of EnVar versus 4D-Var, additional experiments using 3D-Var and a version of EnVar with only 3-D ensemble covariances are also evaluated. To improve the rate of convergence for all approaches evaluated (including EnVar), an estimate of the cost function Hessian generated by the quasi-Newton minimization algorithm is cycled from one analysis to the next. Analyses from EnVar (with 4-D ensemble covariances) nearly always produce improved, and never degraded, forecasts when compared with 3D-Var. Comparisons with 4D-Var show that forecasts from EnVar analyses have either similar or better scores in the troposphere of the tropics and the winter extra-tropical region. However, in the summer extra-tropical region the medium-range forecasts from EnVar have either similar or worse scores than 4D-Var in the troposphere. In contrast, the 6 h forecasts from EnVar are significantly better than 4D-Var relative to radiosonde observations for both periods and in all regions. The use of 4-D versus 3-D ensemble covariances only results in small improvements in forecast quality. By contrast, the improvements from using 4D-Var versus 3D-Var are much larger. Measurement of the fit of the background and analyzed states to the observations suggests that EnVar and 4D-Var can both make better use of observations distributed over time than 3D-Var. In summary, the results from this study suggest that the EnVar approach is a viable alternative to 4D-Var, especially when the simplicity and computational efficiency of EnVar are considered. Additional research is required to understand the seasonal dependence of the difference in forecast quality between EnVar and 4D-Var in the extra-tropics.

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: Temperature distribution and Hadley circulation in an axisymmetric model Nonlinear Processes in Geophysics Discussions, 1, 1621-1655, 2014 Author(s): N. Tartaglione The impact of the temperature distribution on the Hadley circulation simulated by an axisymmetric model is studied. The temperature distributions that drive the model are modulated here by two parameters, n and k , the former controlling the horizontal broadness and the latter defining change in the vertical lapse rate. In the present study, the changes of the temperature distribution mimic changes of the energy input of the atmospheric system leaving as an invariant the equator-poles difference. Both equinoctial and time-dependent Hadley circulations are simulated and results compared. The results give evidence that concentrated temperature distributions enhance the meridional circulation and jet wind speed intensities even with a lower energy input. The meridional circulation and the subtropical jet stream widths are controlled by the broadness of horizontal temperature rather than the vertical lapse rate k , which is important only when the temperature distribution is concentrated at the equator. The jet stream position does not show any dependence with n and k , except when the temperature distribution is very wide and in such a case the jet is located at the mid-latitude. Using n = 2 and k = 1 we have the formulation of the potential temperature adopted in classical literature. A comparison with other works is performed and our results show that the model running in different configurations (equinoctial, solstitial and time-dependent) yields results similar to one another.

Description: Energy exchange and wave action conservation for magnetohydrodynamic (MHD) waves in a general, slowly varying medium Annales Geophysicae, 32, 1495-1510, 2014 Author(s): A. D. M. Walker Magnetohydrodynamic (MHD) waves in the solar wind and magnetosphere are propagated in a medium whose velocity is comparable to or greater than the wave velocity and which varies in both space and time. In the approximation where the scales of the time and space variation are long compared with the period and wavelength, the ray-tracing equations can be generalized and then include an additional first-order differential equation that determines the variation of frequency. In such circumstances the wave can exchange energy with the background: wave energy is not conserved. In such processes the wave action theorem shows that the wave action, defined as the ratio of the wave energy to the frequency in the local rest frame, is conserved. In this paper we discuss ray-tracing techniques and the energy exchange relation for MHD waves. We then provide a unified account of how to deal with energy transport by MHD waves in non-uniform media. The wave action theorem is derived directly from the basic MHD equations for sound waves, transverse Alfvén waves, and the fast and slow magnetosonic waves. The techniques described are applied to a number of illustrative cases. These include a sound wave in a medium undergoing a uniform compression, an isotropic Alfvén wave in a steady-state shear layer, and a transverse Alfvén wave in a simple model of the magnetotail undergoing compression. In each case the nature and magnitude of the energy exchange between wave and background is found.

Description: The double layers in the plasma sheet boundary layer during magnetic reconnection Nonlinear Processes in Geophysics Discussions, 1, 1657-1671, 2014 Author(s): J. Guo and B. Yu We studied the evolutions of double layers which appear after the magnetic reconnection through two-dimensional electromagnetic particle-in-cell simulation. The simulation results show that the double layers are formed in the plasma sheet boundary layer after magnetic reconnection. At first, the double layers which have unipolar structures are formed. And then the double layers turn into bipolar structures, which will couple with another new weak bipolar structure. Thus a new double layer or tripolar structure comes into being. The double layers found in our work are about several ten Debye lengths, which accords with the observation results. It is suggested that the electron beam formed during the magnetic reconnection is responsible for the production of the double layers.

Description: On the influence of CMEs on the global 3-D coronal electron density Annales Geophysicae, 29, 1019-1028, 2011 Author(s): M. Kramar, J. Davila, H. Xie, and S. Antiochos In order to analize the influence of a Coronal Mass Ejection (CME) on the coronal streamer belt, we made 3-D reconstructions of the electron density in the corona at heliospheric distances from 1.5 to 4 R ⊙ for periods before and after a CME occured. The reconstructions were performed using a tomography technique. We studied two CME cases: (i) a slow CME on 1 June 2008; (ii) two fast CMEs on 31 December 2007 and 2 January 2008. For the first case of slow CME, it was found: (i) the potential magnetic field configuration in the CME initiation region before the CME does not agree with the coronal density structure while after the CME the agreement between the field and density is much better. This could be manifistation of that that the field was non-potential before the CME and after the CME the field relaxes towards a more potential state. (ii) It was shown that the dimming caused by the slow CME is not due to rotation of the corona and a line-of-sight (LOS) effect but a streamer blow out effect took place.

Description: Corrigendum to "Breeding and predictability in the baroclinic rotating annulus using a perfect model" published in Nonlin. Processes Geophys., 15, 469–487, 2008 Nonlinear Processes in Geophysics, 18, 359-359, 2011 Author(s): R. M. B. Young and P. L. Read No abstract available.

Description: Longitudinal study of the ionospheric response to the geomagnetic storm of 15 May 2005 and manifestation of TADs Annales Geophysicae, 29, 1063-1070, 2011 Author(s): S. Sharma, P. Galav, N. Dashora, and R. Pandey Response of low latitude ionosphere to the geomagnetic storm of 15 May 2005 has been studied using total electron content (TEC) data, obtained from three GPS stations namely, Yibal, Udaipur and Kunming situated near the northern crest of equatorial ionization anomaly at different longitudes. Solar wind parameters, north-south component of the interplanetary magnetic field (IMF B z ) and AE index data have been used to infer the strength of the geomagnetic storm. A large value of eastward interplanetary electric field at 06:15 UT, during the time of maximum southward IMF B z has been used to infer the transmission of an eastward prompt penetration electric field (PPEF) which resulted in a peak in TEC at 07:45 UT due to the local uplift of plasma in the low latitudes near the anomaly crest over a wide range of longitudes. Wave-like modulations superposed over the second enhancement in TEC between 09:15 UT to 10:30 UT have been observed at all the three stations. The second enhancement in TEC along with the modulations of up to 5 TECU have been attributed to the combined effect of super plasma fountain and traveling atmospheric disturbances (TAD). Observed large enhancements in TEC are a cause of concern for satellite based navigation and ground positioning. Increased [O/N 2 ] ratio between 09:15 UT to 10:15 UT when modulations in TEC have been also observed, confirms the presence of TADs over a wide range of longitudes.

Description: Volume cross section of auroral radar backscatter and RMS plasma fluctuations inferred from coherent and incoherent scatter data: a response on backscatter volume parameters Annales Geophysicae, 29, 1081-1092, 2011 Author(s): M. V. Uspensky, P. Janhunen, A. V. Koustov, and K. Kauristie Norway and Finland STARE radar measurements in the eastward auroral electrojet are combined with EISCAT CP-1 measurements of the electron density and electric field vector in the common scattering volume to investigate the variation of the auroral radar volume cross section (VCS) with the flow angle of observations (radar look direction with respect to the E × B electron drift). The data set available consists of ~6000 points for flow angles of 40–85° and electron drifts between 500 and 2000 m s −1 . The EISCAT electron density N(h) -profile data are used to estimate the effective electron density, aspect angle and thickness of the backscattering layer. It is shown that the flow angle variation of the VCS is rather weak, only ~5 dB within the range of the considered flow angles. The VCS values themselves respond almost linearly to the square of both the electron drift velocity magnitude and the effective electron density. By adopting the inferred shape of the VCS variation with the flow angle and the VCS dependence upon wavelength, the relative amplitude of electrostatic electron density fluctuations over all scales is estimated. Inferred values of 2–4 percent react nearly linearly to the electron drift velocity in the range of 500–1000 m s −1 but the rate of increase slows down at electron drifts 〉1000 m s −1 and density fluctuations of ~5.5 percent due to, perhaps, progressively growing nonlinear wave losses.

Description: Flux quanta, magnetic field lines, merging – some sub-microscale relations of interest in space plasma physics Annales Geophysicae, 29, 1121-1127, 2011 Author(s): R. A. Treumann, R. Nakamura, and W. Baumjohann We clarify the notion of magnetic field lines in plasma by referring to sub-microscale (quantum mechanical) particle dynamics. It is demonstrated that magnetic field lines in a field of strength B carry single magnetic flux quanta Φ 0 = h/e . The radius of a field line in the given magnetic field B is calculated. It is shown that such field lines can merge and annihilate only over the length ℓ ∥ of their strictly anti-parallel sections, for which case we estimate the power generated. The length ℓ ∥ becomes a function of the inclination angle θ of the two merging magnetic flux tubes (field lines). Merging is possible only in the interval 1 2 πθ≤π. This provides a sub-microscopic basis for "component reconnection" in classical macro-scale reconnection. We also find that the magnetic diffusion coefficient in plasma appears in quanta D 0 m = e Φ 0 / m e = h/m e . This lets us conclude that the bulk perpendicular plasma resistivity is limited and cannot be less than η 0⊥ =μ 0 e Φ 0 / m e =μ 0 h/m e ~10 −9 Ohm m. This resistance is an invariant.

Description: Could periodic patterns in human mortality be sensitive to solar activity? Annales Geophysicae, 29, 1113-1120, 2011 Author(s): R. Díaz-Sandoval, R. Erdélyi, and R. Maheswaran Seasonal behaviour of human diseases have been observed and reported in the literature for years. Although the Sun plays an essential role in the origin and evolution of life on Earth, it is barely taken into account in biological processes for the development of a specific disease. Higher mortality rates occur during the winter season in the Northern Hemisphere for several diseases, particularly diseases of the cardiovascular and respiratory systems. This increment has been associated with seasonal and social causes. However, is there more behind these correlations, in particular in terms of solar variability? In this paper we attempt to make a first step towards answering this question. A detailed wavelet analysis of periodicities for diseases from England and Wales seem to reveal that mortality periodicities (3 days to half a year) could be due to the Earth's position around the Sun. Moreover, crosswavelet and wavelet coherence analysis show common features between medical diseases and solar proxies around solar maximum activity suggesting that this relation, if any, has to be searched in times of high solar activity.

Description: Excitation of zero-frequency magnetic field-aligned currents by ionospheric heating Annales Geophysicae, 29, 1147-1152, 2011 Author(s): A. V. Streltsov and T. R. Pedersen Time-dependent, three-dimensional numerical simulations of the reduced MHD model describing shear Alfvén waves in the magnetosphere provide an interesting prediction superficially similar to results of several ionospheric heating experiments conducted at high altitudes. In these experiments, heating of the ionospheric F-region with a constant/zero-frequency beam of HF waves causes luminous structures in the ionosphere in the form of a ring or a solid spot with a characteristic size comparable to the size of the heated spot. Simulations suggest that spots/rings or similar optical appearance might be associated with a magnetic field-aligned current system produced by the ionospheric heating. Two of the most interesting features of this current system are (1) strong localization across the ambient magnetic field and (2) distinctive non-symmetrical luminous signatures (ring/spot) in magnetically conjugate locations in the ionosphere.

Description: Strategies for coupling global and limited-area ensemble Kalman filter assimilation Nonlinear Processes in Geophysics, 18, 415-430, 2011 Author(s): D. Merkova, I. Szunyogh, and E. Ott This paper compares the forecast performance of four strategies for coupling global and limited area data assimilation: three strategies propagate information from the global to the limited area process, while the fourth strategy feeds back information from the limited area to the global process. All four strategies are formulated in the Local Ensemble Transform Kalman Filter (LETKF) framework. Numerical experiments are carried out with the model component of the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) and the NCEP Regional Spectral Model (RSM). The limited area domain is an extended North-America region that includes part of the north-east Pacific. The GFS is integrated at horizontal resolution T62 (about 150 km in the mid-latitudes), while the RSM is integrated at horizontal resolution 48 km. Experiments are carried out both under the perfect model hypothesis and in a realistic setting. The coupling strategies are evaluated by comparing their deterministic forecast performance at 12-h and 48-h lead times. The results suggest that the limited area data assimilation system has the potential to enhance the forecasts at 12-h lead time in the limited area domain at the synoptic and sub-synoptic scales (in the global wave number range of about 10 to 40). There is a clear indication that between the forecast performance of the different coupling strategies those that cycle the limited area assimilation process produce the most accurate forecasts. In the realistic setting, at 12-h forecast time the limited area systems produce more modest improvements compared to the global system than under the perfect model hypothesis, and at 48-h forecast time the global forecasts are more accurate than the limited area forecasts.

Description: An air shower array for LOFAR: LORA Astrophysics and Space Sciences Transactions, 7, 195-199, 2011 Author(s): S. Thoudam, G. v. Aar, M. v. d. Akker, L. Bähren, A. Corstanje, H. Falcke, J. R. Hörandel, A. Horneffer, C. James, M. Mevius, O. Scholten, K. Singh, and S. ter Veen LOFAR is a new form of radio telescope which can detect radio emission from air showers induced by very high-energy cosmic rays. It can also look for radio emission from particle cascades on the Moon induced by ultra high-energy cosmic rays or neutrinos. To complement the radio detection, we are setting up a small particle detector array LORA (LOfar Radboud Air shower array) within an area of ~300 m diameter in the LOFAR core. It will help in triggering and confirming the radio detection of air showers with the LOFAR antennas. In this paper, we present a short overview about LORA and discuss its current status.

Description: Spectral methods for internal waves: indistinguishable density profiles and double-humped solitary waves Nonlinear Processes in Geophysics, 18, 351-358, 2011 Author(s): M. Dunphy, C. Subich, and M. Stastna Internal solitary waves are widely observed in both the oceans and large lakes. They can be described by a variety of mathematical theories, covering the full spectrum from first order asymptotic theory (i.e. Korteweg-de Vries, or KdV, theory), through higher order extensions of weakly nonlinear-weakly nonhydrostatic theory, to fully nonlinear-weakly nonhydrostatic theories and finally exact theory based on the Dubreil-Jacotin-Long (DJL) equation that is formally equivalent to the full set of Euler equations. We discuss how spectral and pseudospectral methods allow for the computation of novel phenomena in both approximate and exact theories. In particular we construct markedly different density profiles for which the coefficients in the KdV theory are very nearly identical. These two density profiles yield qualitatively different behaviour for both exact, or fully nonlinear, waves computed using the DJL equation and in dynamic simulations of the time dependent Euler equations. For exact, DJL, theory we compute exact solitary waves with two-scales, or so-called double-humped waves.

Description: A nonlinear method of removing harmonic noise in geophysical data Nonlinear Processes in Geophysics, 18, 367-379, 2011 Author(s): Y. Jeng and C.-S. Chen A nonlinear, adaptive method to remove the harmonic noise that commonly resides in geophysical data is proposed in this study. This filtering method is based on the ensemble empirical mode decomposition algorithm in conjunction with the logarithmic transform. We present a synthetic model study to investigate the capability of signal reconstruction from the decomposed data, and compare the results with those derived from other 2-D adaptive filters. Applications to the real seismic data acquired by using an ocean bottom seismograph and to a shot gather of the ground penetrating radar demonstrate the robustness of this method. Our work proposes a concept that instead of Fourier-based approaches, the harmonic noise removal in geophysical data can be achieved effectively by using an alternative nonlinear adaptive data analysis method, which has been applied extensively in other scientific studies.

Description: Comparison of correlation analysis techniques for irregularly sampled time series Nonlinear Processes in Geophysics, 18, 389-404, 2011 Author(s): K. Rehfeld, N. Marwan, J. Heitzig, and J. Kurths Geoscientific measurements often provide time series with irregular time sampling, requiring either data reconstruction (interpolation) or sophisticated methods to handle irregular sampling. We compare the linear interpolation technique and different approaches for analyzing the correlation functions and persistence of irregularly sampled time series, as Lomb-Scargle Fourier transformation and kernel-based methods. In a thorough benchmark test we investigate the performance of these techniques. All methods have comparable root mean square errors (RMSEs) for low skewness of the inter-observation time distribution. For high skewness, very irregular data, interpolation bias and RMSE increase strongly. We find a 40 % lower RMSE for the lag-1 autocorrelation function (ACF) for the Gaussian kernel method vs. the linear interpolation scheme,in the analysis of highly irregular time series. For the cross correlation function (CCF) the RMSE is then lower by 60 %. The application of the Lomb-Scargle technique gave results comparable to the kernel methods for the univariate, but poorer results in the bivariate case. Especially the high-frequency components of the signal, where classical methods show a strong bias in ACF and CCF magnitude, are preserved when using the kernel methods. We illustrate the performances of interpolation vs. Gaussian kernel method by applying both to paleo-data from four locations, reflecting late Holocene Asian monsoon variability as derived from speleothem δ 18 O measurements. Cross correlation results are similar for both methods, which we attribute to the long time scales of the common variability. The persistence time (memory) is strongly overestimated when using the standard, interpolation-based, approach. Hence, the Gaussian kernel is a reliable and more robust estimator with significant advantages compared to other techniques and suitable for large scale application to paleo-data.

Description: Comment on "The PC index: review of methods", by McCreadie and Menvielle (2010) Annales Geophysicae, 29, 1137-1146, 2011 Author(s): P. Stauning The Polar Cap (PC) index is a controversial topic within the IAGA scientific community. Since 1997 discussions of the validity of the index to be endorsed as an official IAGA index have ensued. The article: "The PC index: review of methods", written by two members of the IAGA PC index committee, H. McCreadie and M. Menvielle, holds a critical review of some aspects of the methods used to derive PC index values. However, a number of incorrect statements and factual errors have been found and shall be called attention to and discussed in this commentary. Further critical comments concern the Corrigendum issued by the same authors and published in Ann. Geophys., 29, 813–814, 2011.

Description: The role of the bow shock in solar wind-magnetosphere coupling Annales Geophysicae, 29, 1129-1135, 2011 Author(s): R. E. Lopez, V. G. Merkin, and J. G. Lyon In this paper we examine the role of the bow shock in coupling solar wind energy to the magnetosphere using global magnetohydrodynamic simulations of the solar wind-magnetosphere interaction with southward IMF. During typical solar wind conditions, there are two significant dynamo currents in the magnetospheric system, one in the high-latitude mantle region tailward of the cusp and the other in the bow shock. As the magnitude of the (southward) IMF increases and the solar wind becomes a low Mach number flow, there is a significant change in solar wind-magnetosphere coupling. The high-latitude magnetopause dynamo becomes insignificant compared to the bow shock and a large load appears right outside the magnetopause. This leaves the bow shock current as the only substantial dynamo current in the system, and the only place where a significant amount of mechanical energy is extracted from the solar wind. That energy appears primarily as electromagnetic energy, and the Poynting flux generated at the bow shock feeds energy back into the plasma, reaccelerating it to solar wind speeds. Some small fraction of that Poynting flux is directed into the magnetosphere, supplying the energy needed for magnetospheric dynamics. Thus during periods when the solar wind flow has a low Mach number, the main dynamo in the solar wind-magnetosphere system is the bow shock.

Description: Measurements of aerosol intensive properties over Visakhapatnam, India for 2007 Annales Geophysicae, 29, 973-985, 2011 Author(s): K. Niranjan, B. Spandana, T. Anjana Devi, V. Sreekanth, and B. L. Madhavan Information on the aerosol intensive properties like Single Scattering Albedo (SSA) and asymmetry parameter are very limited, particularly over the peninsular India, though extensive reports are available on the aerosol bulk properties. In view of the importance of these parameters in evaluating the aerosol radiative forcing, we present for the first time the temporal variation in SSA with measurements on aerosol absorption and scattering coefficients over Visakhapatnam (17.72° N, 83.32° E; located on the east coast of India) for the year 2007. The inferred SSA was in the range of 0.65 and 0.9 with an annual mean of 0.76 ± 0.013 and with a probable value of 0.80, indicating a marginal atmospheric warming over the region. The mixed layer contribution to column Aerosol Optical depth is found to be 35 % in summer while it is well above 35 % in winter, indicating the confinement of aerosol within the boundary layer during winter. The asymmetry parameter which represents the angular scattering in radiative forcing estimation is found to be around 0.65 ± 0.1 for the location. The day to day variability in SSA is found to be well correlated with the variations in surface BC mass concentrations and/or the relative dominance of the fine/coarse mode aerosol. The results are discussed in light of the aerosol physical and optical properties and the asymmetry parameter.

Description: Overview of radio detection of cosmic ray air showers in the MHz range, and prospects for a large scale experiment Astrophysics and Space Sciences Transactions, 7, 207-210, 2011 Author(s): Maximilien Melissas 1 for the Pierre AUGER Collaboration 2,* Since its revival in the last decade, radio detection of cosmic ray air showers has made tremendous progress. Today, several experiments are routinely detecting radio signals associated with air showers. Large cosmic ray observatories such as the Pierre Auger Observatory are also pursuing radio detection activities. As an introduction, in this article we will summarize the main results from the first generation of radio detection experiments: LOPES and CODALEMA. Then, we will show which questions concerning the radio emission mechanisms can be answered from larger-scale experiments like the Auger Engineering Radio Array (AERA), which is a 20 km 2 antenna array under construction close to other enhancement devices at the Pierre Auger Observatory.

Description: Brief communication "On one mechanism of low frequency variability of the Antarctic Circumpolar Current" Nonlinear Processes in Geophysics, 18, 361-365, 2011 Author(s): O. G. Derzho and B. de Young In this paper we present a simple analytical model for low frequency and large scale variability of the Antarctic Circumpolar Current (ACC). The physical mechanism of the variability is related to temporal and spatial variations of the cyclonic mean flow (ACC) due to circularly propagating nonlinear barotropic Rossby wave trains. It is shown that the Rossby wave train is a fundamental mode, trapped between the major fronts in the ACC. The Rossby waves are predicted to rotate with a particular angular velocity that depends on the magnitude and width of the mean current. The spatial structure of the rotating pattern, including its zonal wave number, is defined by the specific form of the stream function-vorticity relation. The similarity between the simulated patterns and the Antarctic Circumpolar Wave (ACW) is highlighted. The model can predict the observed sequence of warm and cold patches in the ACW as well as its zonal number.

Description: Corrigendum to "Downward auroral currents from the reconnection Hall-region", published in Ann. Geophys., 29, 679–685, 2011 Annales Geophysicae, 29, 1061-1061, 2011 Author(s): R. A. Treumann, R. Nakamura, and W. Baumjohann No abstract available.

Description: Hybrid models of solar wind plasma heating Annales Geophysicae, 29, 1071-1079, 2011 Author(s): L. Ofman, A.-F. Viñas, and P. S. Moya Remote sensing and in-situ observations show that solar wind ions are often hotter than electrons, and the heavy ions flow faster than the protons by up to an Alfvén speed. Turbulent spectrum of Alfvénic fluctuations and shocks were detected in solar wind plasma. Cross-field inhomogeneities in the corona were observed to extend to several tens of solar radii from the Sun. The acceleration and heating of solar wind plasma is studied via 1-D and 2-D hybrid simulations. The models describe the kinetics of protons and heavy ions, and electrons are treated as neutralizing fluid.The expansion of the solar wind is considered in 1-D hybrid model. A spectrum of Alfvénic fluctuations is injected at the computational boundary, produced by differential streaming instability, or initial ion temperature anisotropy, and the parametric dependence of the perpendicular heating of H + -He ++ solar wind plasma is studied. It is found that He ++ ions are heated efficiently by the Alfvénic wave spectrum below the proton gyroperiod.

Description: A semi-phenomenological approach to explain the event-size distribution of the Drossel-Schwabl forest-fire model Nonlinear Processes in Geophysics, 18, 381-388, 2011 Author(s): S. Hergarten and R. Krenn We present a novel approach to explain the complex scaling behavior of the Drossel-Schwabl forest-fire model in two dimensions. Clusters of trees are characterized by their size and perimeter only, whereas spatial correlations are neglected. Coalescence of clusters is restricted to clusters of similar sizes. Our approach derives the value of the scaling exponent τ of the event size distribution directly from the scaling of the accessible perimeter of percolation clusters. We obtain τ = 1.19 in the limit of infinite growth rate, in perfect agreement with numerical results. Furthermore, our approach predicts the unusual transition from a power law to an exponential decay even quantitatively, while the exponential decay at large event sizes itself is reproduced only qualitatively.

Description: Double rank-ordering technique of ROMA (Rank-Ordered Multifractal Analysis) for multifractal fluctuations featuring multiple regimes of scales Nonlinear Processes in Geophysics, 18, 405-414, 2011 Author(s): S. W. Y. Tam and T. Chang Rank-Ordered Multifractal Analysis (ROMA), a technique capable of deciphering the multifractal characteristics of intermittent fluctuations, was originally applied to the results of a magnetohydrodynamic (MHD) simulation. Application of ROMA to measured fluctuations in the auroral zone, due to the dominant physical effects changing from kinetic to MHD as the scale increases, requires an additional level of rank-ordering in order to divide the domain of scales into regimes. An algorithm for the additional step in this double rank-ordering technique is discussed, and is demonstrated in the application to the electric field fluctuations in the auroral zone as an example. As a result of the double rank-ordering, ROMA is able to take into account the nonlinear crossover behavior characterized by the multiple regimes of time scales by providing a scaling variable and a scaling function that are global to all the time scales.

Description: Measurements of cosmic-ray antiprotons with PAMELA Astrophysics and Space Sciences Transactions, 7, 225-228, 2011 Author(s): J. Wu on behalf of the PAMELA collaboration The PAMELA experiment is a satellite-borne apparatus designed to study charged particles, and especially antiparticles, in the cosmic radiation. The apparatus is mounted on the Resurs DK1 satellite which was launched on 15 June 2006. PAMELA has been traveling around the earth along an elliptical and semi-polar orbit for almost five years. It mainly consists of a permanent magnetic spectrometer, a time of flight system and an electromagnetic imaging calorimeter, which allows antiprotons to be identified from a dominating cosmic-ray background. New measurements of the cosmic-ray antiproton flux and the antiproton-to-proton flux ratio between 60 MeV and 180 GeV are presented, employing data collected between June 2006 and December 2008. Compared to previous experiments, PAMELA extends the energy range of antiproton measurements and provides significantly higher statistics. The derived antiproton flux and antiproton-to-proton flux ratio indicates that the main source of cosmic-ray antiprotons is considered to be secondary production and no primary contribution has to be invoked.

Description: The cosmic ray energy spectrum in the range 10 16 –10 18 eV measured by KASCADE-Grande Astrophysics and Space Sciences Transactions, 7, 229-234, 2011 Author(s): M. Bertaina, W.D. Apel, J.C. Arteaga-Velázquez, K. Bekk, J. Blümer, H. Bozdog, I.M. Brancus, P. Buchholz, E. Cantoni, A. Chiavassa, F. Cossavella, K. Daumiller, V. de Souza, F. Di Pierro, P. Doll, R. Engel, J. Engler, M. Finger, D. Fuhrmann, P.L. Ghia, H.J. Gils, R. Glasstetter, C. Grupen, A. Haungs, D. Heck, J.R. Hörandel, T. Huege, P.G. Isar, K.-H. Kampert, D. Kang, D. Kickelbick, H.O. Klages, K. Link, P. Łuczak, M. Ludwig, H.J. Mathes, H.J. Mayer, M. Melissas, J. Milke, B. Mitrica, C. Morello, G. Navarra, S. Nehls, J. Oehlschläger, S. Ostapchenko, S. Over, N. Palmieri, M. Petcu, T. Pierog, H. Rebel, M. Roth, H. Schieler, F. Schröder, O. Sima, G. Toma, G.C. Trinchero, H. Ulrich, A. Weindl, J. Wochele, M. Wommer, and J. Zabierowski The KASCADE-Grande experiment, located at Campus North of the Karlsruhe Institute of Technology (Germany) is a multi-component extensive air-shower experiment devoted to the study of cosmic rays and their interactions at primary energies 10 14 –10 18 eV. One of the main goals of the experiment is the measurement of the all particle energy spectrum in the 10 16 –10 18 eV range, i.e. extending the range accessible by KASCADE alone. The Grande detector samples the charged component ( N ch ) of the air shower while the original KASCADE array provides in addition a measurement of the muon component ( N μ ). The combined information of N ch and N μ is used to estimate the energy on an event-by-event basis and to derive the all-particle energy spectrum. Since the calibration of the observables in terms of the primary energy depends on Monte Carlo simulations, three different methods with partially different sources of uncertainties, have been considered and compared to each other to derive the systematics on the energy spectrum. The different methods employed to derive the spectrum and their uncertainties, as well as the implications of the obtained result, are discussed in detail.

Description: On the effect of IMF turning on ion dynamics at Mercury Annales Geophysicae, 29, 987-996, 2011 Author(s): D. C. Delcourt, T. E. Moore, and M.-C. H. Fok We investigate the effect of a rotation of the Interplanetary Magnetic Field (IMF) on the transport of magnetospheric ion populations at Mercury. We focus on ions of planetary origin and investigate their large-scale circulation using three-dimensional single-particle simulations. We show that a nonzero B X component of the IMF leads to a pronounced asymmetry in the overall circulation pattern. In particular, we demonstrate that the centrifugal acceleration due to curvature of the E × B drift paths is more pronounced in one hemisphere than the other, leading to filling of the magnetospheric lobes and plasma sheet with more or less energetic material depending upon the hemisphere of origin. Using a time-varying electric and magnetic field model, we investigate the response of ions to rapid (a few tens of seconds) re-orientation of the IMF. We show that, for ions with gyroperiods comparable to the field variation time scale, the inductive electric field should lead to significant nonadiabatic energization, up to several hundreds of eVs or a few keVs. It thus appears that IMF turning at Mercury should lead to localized loading of the magnetosphere with energetic material of planetary origin (e.g., Na + ).

Description: Instability of combined gravity-inertial-Rossby waves in atmospheres and oceans Annales Geophysicae, 29, 997-1003, 2011 Author(s): J. F. McKenzie The properties of the instability of combined gravity-inertial-Rossby waves on a β-plane are investigated. The wave-energy exchange equation shows that there is an exchange of energy with the background stratified medium. The energy source driving the instability lies in the background enthalpy released by the gravitational buoyancy force. It is shown that if the phase speed of the westward propagating low frequency-long wavelength Rossby wave exceeds the Poincaré-Kelvin (or "equivalent" shallow water) wave speed, instability arises from the merging of Rossby and Poincaré modes. There are two key parameters in this instability condition; namely, the equatorial/rotational Mach (or Froude) number M and the latitude θ 0 of the β-plane. In general waves equatorward of a critical latitude for given M can be driven unstable, with corresponding growth rates of the order of a day or so. Although these conclusions may only be safely drawn for short wavelengths corresponding to a JWKB wave packet propagating internally and located far from boundaries, nevertheless such a local instability may play a significant role in atmosphere-ocean dynamics.

Description: The correlation between solar and geomagnetic activity – Part 3: An integral response model Annales Geophysicae, 29, 1005-1018, 2011 Author(s): Z. L. Du An integral response model is proposed to describe the relationship between geomagnetic activity ( aa index) and solar activity (represented by sunspot number R z ): The aa at a given time t is the integral of R z at past times ( t '≤ t ) multiplied by an exponential decay factor of the time differences ( e −(t−t')/τ ), where τ is the decay time scale (~40 months). The correlation coefficient of aa with the reconstructed series based on this model ( r f =0.85) is much higher than that of aa with R z ( r 0 =0.61). If this model is applied to each solar cycle, the correlation coefficient will be higher ( r f =0.95). This model can naturally explain some phenomena related to aa and R z , such as (i) the significant increase in the aa index (and its baseline) over the twentieth century; (ii) the longer lag times of aa to R z at solar cycle maxima than at minima; and (iii) the variations in the correlations related to solar and Hale cycles. These results demonstrate that aa depends not only on the present R z but also on past values. The profile of aa can be better predicted from R z by this model than by point-point correspondence.

Description: Directional correlations between UHECRs and neutrinos observed with IceCube Astrophysics and Space Sciences Transactions, 7, 201-205, 2011 Author(s): R. Lauer 1,2 for the IceCube Collaboration * Ultra-high energy cosmic rays (UHECR) above an energy threshold of tens of EeV might undergo only small deflections due to interstellar magnetic fields. Their arrival directions would then point to regions of possible hadronic acceleration processes, which are likely to be also sources of high energy neutrinos. To search for such cosmic accelerators, we present here the first multi-messenger analysis of directional correlations between neutrino candidates from the IceCube Observatory and UHECR events. Data taken with IceCube in a configuration of 22 strings provided the basis for using published events from both the Pierre Auger Observatory and the HiRes experiment as reference directions in a search for coincidences with neutrinos. The analysis was optimized according to strict blindness criteria and showed an excess of neutrino candidates close to UHECR directions with a probability of 1% to occur as a random fluctuation, consistent with a background-only hypothesis. An extension of this analysis to include newer IceCube data, taken with 40 strings and using a likelihood approach, is discussed in the outlook.

Description: Cluster observations of trapped ions interacting with magnetosheath mirror modes Annales Geophysicae, 29, 1049-1060, 2011 Author(s): J. Soucek and C. P. Escoubet Mirror modes are among the most intense low frequency plasma wave phenomena observed in the magnetosheaths of magnetized planets. They appear as large amplitude non-propagating fluctuations in the magnetic field magnitude and plasma density. These structures are widely accepted to represent a non-linear stage of the mirror instability, dominant in plasmas with large ion beta and a significant ion temperature anisotropy T ⊥ / T ∥ 〉1. It has long been recognized that the mirror instability both in the linear and non-linear stage is a kinetic process and that the behavior of resonant particles at small parallel velocities is crucial for its development and saturation. While the dynamics of the instability and the effect of trapped particles have been studied extensively in theoretical models and numerical simulations, only spurious observations of the trapped ions were published to date. In this work we used data from the Cluster spacecraft to perform the first detailed experimental study of ion velocity distribution associated with mirror mode oscillations. We show a conclusive evidence for the predicted cooling of resonant ions at small parallel velocities and heating of trapped ions at intermediate pitch angles.

Description: MHD waves generated by high-frequency photospheric vortex motions Annales Geophysicae, 29, 1029-1035, 2011 Author(s): V. Fedun, S. Shelyag, G. Verth, M. Mathioudakis, and R. Erdélyi In this paper, we discuss simulations of MHD wave generation and propagation through a three-dimensional open magnetic flux tube in the lower solar atmosphere. By using self-similar analytical solutions for modelling the magnetic field in Cartesian coordinate system, we have constructed a 3-D magnetohydrostatic configuration which is used as the initial condition for non-linear MHD wave simulations. For a driver we have implemented a high-frequency vortex-type motion at the footpoint region of the open magnetic flux tube. It is found that the implemented swirly source is able to excite different types of wave modes, i.e. sausage, kink and torsional Alfvén modes. Analysing these waves by magneto-seismology tools could provide insight into the magnetic structure of the lower solar atmosphere.

Description: Average properties and small-scale variations of the mesospheric Na and Fe layers as observed simultaneously by two closely colocated lidars at 30° N Annales Geophysicae, 29, 1037-1048, 2011 Author(s): L. Chen and F. Yi We report the average properties and small-scale variation features of the mesospheric Na and Fe layers at 30° N from extensive simultaneous and common-volume Na and Fe lidar measurements at Wuhan, China. The annual mean Na and Fe density profiles are derived in terms of an averaging method taken from an early literature. The mean Na and Fe profiles preserve the sharp gradients present in most individual density profiles near the layer bottom. Near the bottommost of the layers the mean Na and Fe scale heights are respectively −0.42 and −0.30 km. The mean layer parameters coincide well with the previous report. The Na and Fe densities in the lowest several kilometers of the layers consistently exhibit nearly the same time variations. A clear-cut distinction between the Na and Fe time variations always appears in an altitude range near 90 km. A relatively weak positive correlation between them persistently occurs also in an altitude range near 100 km. The mean increase and decrease rates for both Na and Fe are altitude dependent and have a single-peak structure. The time constant of the layer variation is ~0.07–2.0 h for Na and ~0.02–1.7 h for Fe, suggesting that the variability is dominated by small-scale processes. However, there is also a slow net increase in each of the annual mean column abundances (Na and Fe) during night.

Description: Observation of the Fermi pulsar catalog at TeV energies with the Tibet air shower experiment Astrophysics and Space Sciences Transactions, 7, 211-215, 2011 Author(s): M. Amenomori, X. J. Bi, D. Chen, S. W. Cui, Danzengluobu, L. K. Ding, X. H. Ding, C. Fan, C. F. Feng, Zhaoyang Feng, Z. Y. Feng, X. Y. Gao, Q. X. Geng, Q. B. Gou, H. W. Guo, H. H. He, M. He, K. Hibino, N. Hotta, Haibing Hu, H. B. Hu, J. Huang, Q. Huang, H. Y. Jia, L. Jiang, F. Kajino, K. Kasahara, Y. Katayose, C. Kato, K. Kawata, Labaciren, G. M. Le, A. F. Li, H. C. Li, J. Y. Li, C. Liu, Y.-Q. Lou, H. Lu, X. R. Meng, K. Mizutani, J. Mu, K. Munakata, H. Nanjo, M. Nishizawa, M. Ohnishi, I. Ohta, S. Ozawa, T. Saito, T. Y. Saito, M. Sakata, T. K. Sako, M. Shibata, A. Shiomi, T. Shirai, H. Sugimoto, M. Takita, Y. H. Tan, N. Tateyama, S. Torii, H. Tsuchiya, S. Udo, B. Wang, H. Wang, Y. Wang, Y. G. Wang, H. R. Wu, L. Xue, Y. Yamamoto, C. T. Yan, X. C. Yang, S. Yasue, Z. H. Ye, G. C. Yu, A. F. Yuan, T. Yuda, H. M. Zhang, J. L. Zhang, N. J. Zhang, X. Y. Zhang, Y. Zhang, Yi Zhang, Ying Zhang, Zhaxisangzhu, and X. X. Zhou Using the Tibet-III air shower array, we search for steady TeV γ-rays from 18 pulsars in the Fermi Large Area Telescope pulsar catalog. Among them, we observe 8 sources including the Crab instead of the expected 0.41 sources at a significance of 2 σ or more excess. Under the assumption of Poisson distribution, the chance probability is estimated to be 1.4×10 −8 . When the Crab is excluded, it becomes 1.8×10 −7 . These low chance probabilities clearly show that the Fermi pulsars have a statistically significant correlations with TeV γ-ray excesses.

Description: Polarisation in the auroral red line during coordinated EISCAT Svalbard Radar/optical experiments Annales Geophysicae, 29, 1101-1112, 2011 Author(s): M. Barthélémy, J. Lilensten, F. Pitout, C. Simon Wedlund, R. Thissen, D. Lorentzen, F. Sigernes, J. Moen, G. Gronoff, I. McCrea, H. Rothkael, H. Ménager, and A. Aruliah The polarisation of the atomic oxygen red line in the Earth's thermosphere is observed in different configurations with respect to the magnetic field line at high latitude during several coordinated Incoherent Scatter radar/optical experiment campaigns. When pointing northward with a line-of-sight nearly perpendicular to the magnetic field, we show that, as expected, the polarisation is due to precipitated electrons with characteristic energies of a few hundreds of electron Volts. When pointing toward the zenith or southward with a line-of-sight more parallel to the magnetic field, we show that the polarisation practically disappears. This confirms experimentally the predictions deduced from the recent discovery of the red line polarisation. We show that the polarisation direction is parallel to the magnetic field line during geomagnetic activity intensification and that these results are in agreement with theoretical work.

Description: New insights into the stratospheric and mesosphere-lower thermospheric ozone response to the abrupt changes in solar forcing Annales Geophysicae, 29, 1093-1099, 2011 Author(s): Karanam Kishore Kumar, K. V. Subrahmanyam, and Sherine Rachel John Using a unique set of satellite based observations of the vertical distribution of ozone during the recent annular solar eclipse of 15 January 2010, we demonstrate for the first time, a complete picture of the response of stratospheric ozone to abrupt changes in solar forcing. The stratospheric ozone decreased after the maximum obscuration of the Sun and then gradually increased with time. A dramatic increase in stratospheric ozone of up to 4 ppmv is observed 3 h after the maximum obscuration of the Sun. The present study also reports for the first time the mesosphere-lower thermospheric ozone response to solar eclipse. Thus it is envisaged that the present results will have important implications in understanding the ozone response to abrupt changes in solar forcing and time-scales involved in such response.

Description: The Proton and Helium cosmic ray spectra from 50 GeV to 15 TeV Astrophysics and Space Sciences Transactions, 7, 235-238, 2011 Author(s): A. V. Karelin, S. V. Borisov, A. M. Galper, and S. A. Voronov for PAMELA collaboration To date, very little has been done in the field of research of the nuclei cosmic ray spectra in the energy range between 1 and 10 TeV per nucleon and especially there were even fewer direct measurements in space. The PAMELA experiment (Casolini et al., 2008) has a possibility to make progress in this range. The method of the measurement of the helium and proton cosmic ray spectra with the energy higher than 50 GeV/nucleon was developed mainly with the use of the PAMELA calorimeter. This analysis method and the results obtained based on 3 years of measurements are presented.

Description: Recent results from VHE gamma astrophysics related to fundamental physics and cosmic rays Astrophysics and Space Sciences Transactions, 7, 217-224, 2011 Author(s): A. De Angelis High-energy photons are a powerful probe for astrophysics and for fundamental physics under extreme conditions. During the recent years, our knowledge of the most violent phenomena in the universe has impressively progressed thanks to the advent of new detectors for very-high-energy (VHE) gamma rays (above 100 GeV). Ground-based detectors like the Cherenkov telescopes (H.E.S.S., MAGIC and VERITAS) recently discovered more than 80 new sources. This talk reviews the present status of VHE gamma astrophysics, with emphasis on the recent results and on the experimental developments, keeping an eye on the future. The impact on fundamental physics and on cosmic-ray physics is emphasized.

Description: Corrigendum to "The PC index: review of methods" published in Ann. Geophys., 28, 1887–1903, 2010 Annales Geophysicae, 29, 813-814, 2011 Author(s): H. McCreadie and M. Menvielle No abstract available.

Description: Pre-monsoon aerosol characteristics over the Indo-Gangetic Basin: implications to climatic impact Annales Geophysicae, 29, 789-804, 2011 Author(s): A. K. Srivastava, S. Tiwari, P. C. S. Devara, D. S. Bisht, Manoj K. Srivastava, S. N. Tripathi, P. Goloub, and B. N. Holben Sun/sky radiometer observations over the Indo-Gangetic Basin (IGB) region during pre-monsoon (from April–June 2009) have been processed to analyze various aerosol characteristics in the central and eastern IGB region, represented by Kanpur and Gandhi College, respectively, and their impacts on climate in terms of radiative forcing. Monthly mean aerosol optical depth (AOD at 500 nm) and corresponding Angstrom Exponent (AE at 440–870 nm, given within the brackets) was observed to be about 0.50 (0.49) and 0.51 (0.65) in April, 0.65 (0.74) and 0.67 (0.91) in May and 0.69 (0.45) and 0.77 (0.71) in June at Kanpur and Gandhi College, respectively. Results show a positive gradient in AOD and AE from central to eastern IGB region with the advancement of the pre-monsoon, which may be caused due to diverse geographical location of the stations having different meteorological conditions and emission sources. Relatively lower SSA was observed at the eastern IGB (0.89) than the central IGB (0.92) region during the period, which suggests relative dominance of absorbing aerosols at the eastern IGB as compared to central IGB region. The absorbing aerosol optical properties over the station suggest that the atmospheric absorption over central IGB region is mainly due to dominance of coarse-mode dust particles; however, absorption over eastern IGB region is mainly due to dominance of fine-particle pollution. The derived properties from sun/sky radiometer during pre-monsoon period are used in a radiative-transfer model to estimate aerosol radiative forcing at the top-of-the atmosphere (TOA) and at the surface over the IGB region. Relatively large TOA and surface cooling was observed at the eastern IGB as compared to the central IGB region. This translates into large heating of the atmosphere ranging from 0.45 to 0.55 K day −1 at Kanpur and from 0.45 to 0.59 K day −1 at Gandhi College.

Description: Annual and semiannual variations of vertical total electron content during high solar activity based on GPS observations Annales Geophysicae, 29, 865-873, 2011 Author(s): M. P. Natali and A. Meza Annual, semiannual and seasonal variations of the Vertical Total Electron Content (VTEC) have been investigated during high solar activity in 2000. In this work we use Global IGS VTEC maps and Principal Component Analysis to study spatial and temporal ionospheric variability. The behavior of VTEC variations at two-hour periods, at noon and at night is analyzed. Particular characteristics associated with each period and the geomagnetic regions are highlighted. The variations at night are smaller than those obtained at noon. At noon it is possible to see patterns of the seasonal variation at high latitude, and patterns of the semiannual anomaly at low latitudes with a slow decrease towards mid latitudes. At night there is no evidence of seasonal or annual anomaly for any region, but it was possible to see the semiannual anomaly at low latitudes with a sudden decrease towards mid latitudes. In general, the semiannual behavior shows March–April equinox at least 40 % higher than September one. Similarities and differences are analyzed also with regard to the same analysis done for a period of low solar activity.

Description: Modelling of spacecraft spin period during eclipse Annales Geophysicae, 29, 875-882, 2011 Author(s): E. Georgescu, F. Plaschke, U. Auster, K.-H. Fornaçon, and H. U. Frey The majority of scientific satellites investigating the Earth magnetosphere are spin stabilized. The attitude information comes usually from a sun sensor and is missing in the umbra; hence, the accurate experimental determination of vector quantities is not possible during eclipses. The spin period of the spacecraft is generally not constant during these times because the moment of inertia changes due to heat dissipation. The temperature dependence of the moment of inertia for each spacecraft has a specific signature determined by its design and distribution of mass. We developed an "eclipse-spin" model for the spacecraft spin period behaviour using magnetic field vector measurements close to the Earth, where the magnetic field is dominated by the dipole field, and in the magnetospheric lobes, where the magnetic field direction is mostly constant. The modelled spin periods give us extraordinarily good results with accumulated phase deviations over one hour of less than 10 degrees. Using the eclipse spin model satellite experiments depending on correct spin phase information can deliver science data even during eclipses. Two applications for THEMIS B, one in the lobe and the other in the lunar wake, are presented.

Description: Young supernova remnants and the knee in the cosmic ray spectrum Astrophysics and Space Sciences Transactions, 7, 179-182, 2011 Author(s): A. Erlykin, T. Wibig, and A. W. Wolfendale It has recently been suggested that neutron stars inside the shells of young supernova remnants (SNR) are the sources of PeV cosmic rays and that the interaction of the particles with the radiation field in the SNR causes electron pair production, which has relevance to recent observations of "high" positron fluxes. Furthermore, the character of the interaction is such that the well-known knee in the cosmic ray energy spectrum can be explained. Our examination of the mechanism leads us to believe that the required parameters of SN and pulsars are so uncommon that the knee and positron fraction can only be explained if a single, local and recent SN – and associated pulsar – are concerned. In this case the mechanism can be valid.

Description: Measurement of the all-particle cosmic ray energy spectrum with IceTop Astrophysics and Space Sciences Transactions, 7, 175-178, 2011 Author(s): F. Kislat 1 for the IceCube collaboration 2 IceTop is an air shower array at the geographic South Pole forming the surface component of the IceCube Neutrino Observatory. In this paper, a method to reconstruct shower size and primary energy spectrum from data measured by IceTop is described. Data were divided into three zenith angle bins. Using Monte Carlo based on the SIBYLL and Fluka hadronic interaction models, agreement with an isotropic flux can only be achieved under the assumption of a mixed composition. A preliminary all-particle energy spectrum is presented.

Description: Apparent temperature anisotropies due to wave activity in the solar wind Annales Geophysicae, 29, 909-917, 2011 Author(s): D. Verscharen and E. Marsch The fast solar wind is a collisionless plasma permeated by plasma waves on many different scales. A plasma wave represents the natural interplay between the periodic changes of the electromagnetic field and the associated coherent motions of the plasma particles. In this paper, a model velocity distribution function is derived for a plasma in a single, coherent, large-amplitude wave. This model allows one to study the kinetic effects of wave motions on particle distributions. They are by in-situ spacecraft measured by counting, over a certain sampling time, the particles coming from various directions and having different energies. We compare our results with the measurements by the Helios spacecraft, and thus find that by assuming high wave activity we are able to explain key observed features of the measured distributions within the framework of our model. We also address the recent discussions on nonresonant wave–particle interactions and apparent heating. The applied time-averaging procedure leads to an apparent ion temperature anisotropy which is connected but not identical to the intrinsic temperature of the underlying distribution function.

Description: Primary energy reconstruction from the charged particle densities recorded at 500 m distance from shower core with the KASCADE-Grande detector Astrophysics and Space Sciences Transactions, 7, 191-194, 2011 Author(s): G. Toma, W. D. Apel, J. C. Arteaga, K. Bekk, M. Bertaina, J. Blümer, H. Bozdog, I. M. Brâncuş, P. Buchholz, E. Cantoni, A. Chiavassa, F. Cossavella, K. Daumiller, V. de Souza, F. Di Pierro, P. Doll, R. Engel, J. Engler, M. Finger, D. Fuhrmann, P. L. Ghia, H. J. Gils, R. Glasstetter, C. Grupen, A. Haungs, D. Heck, J. R. Hörandel, T. Huege, P. G. Isar, K.-H. Kampert, D. Kang, D. Kickelbick, H. O. Klages, K. Link, P. \L uczak, M. Ludwig, H. J. Mathes, H. J. Mayer, M. Melissas, J. Milke, B. Mitrică, C. Morello, G. Navarra, S. Nehls, J. Oehlschläger, S. Ostapchenko, S. Over, N. Palmieri, M. Petcu, T. Pierog, H. Rebel, M. Roth, H. Schieler, F. G. Schröder, O. Sima, G. C. Trinchero, H. Ulrich, A. Weindl, J. Wochele, M. Wommer, and J. Zabierowski Previous EAS investigations have shown that for a fixed primary energy the charged particle density becomes independent of the primary mass at certain (fixed) distances from the shower axis. This feature can be used as an estimator for the primary energy. We present results on the reconstruction of the primary energy spectrum of cosmic rays from the experimentally recorded S(500) observable (the density of charged particles at a distance of 500 m to the shower core as measured in a plane normal to the shower axis) using the KASCADE-Grande detector array. The KASCADE-Grande experiment is hosted by the Karlsruhe Institute for Technology – Campus North, Karlsruhe, Germany, 110 m a.s.l. and operated by an international collaboration. The obtained primary energy spectrum is presented along with the result of another reconstruction technique presently employed at KASCADE-Grande.

Description: Elastic envelope inversion using multicomponent seismic data without low frequency Nonlinear Processes in Geophysics Discussions, 1, 1757-1802, 2014 Author(s): C. Huang, L. Dong, Y. Liu, and B. Chi Low frequency is a key issue to reduce the nonlinearity of elastic full waveform inversion. Hence, the lack of low frequency in recorded seismic data is one of the most challenging problems in elastic full waveform inversion. Theoretical derivations and numerical analysis are presented in this paper to show that envelope operator can retrieve strong low frequency modulation signal demodulated in multicomponent data, no matter what the frequency bands of the data is. With the benefit of such low frequency information, we use elastic envelope of multicomponent data to construct the objective function and present an elastic envelope inversion method to recover the long-wavelength components of the subsurface model, especially for the S-wave velocity model. Numerical tests using synthetic data for the Marmousi-II model prove the effectiveness of the proposed elastic envelope inversion method, especially when low frequency is missing in multicomponent data and when initial model is far from the true model. The elastic envelope can reduce the nonlinearity of inversion and can provide an excellent starting model.

Description: Large eddy simulation of sediment transport over rippled beds Nonlinear Processes in Geophysics, 21, 1169-1184, 2014 Author(s): J. C. Harris and S. T. Grilli Wave-induced boundary layer (BL) flows over sandy rippled bottoms are studied using a numerical model that applies a one-way coupling of a "far-field" inviscid flow model to a "near-field" large eddy simulation (LES) Navier–Stokes (NS) model. The incident inviscid velocity and pressure fields force the LES, in which near-field, wave-induced, turbulent bottom BL flows are simulated. A sediment suspension and transport model is embedded within the coupled flow model. The numerical implementation of the various models has been reported elsewhere, where we showed that the LES was able to accurately simulate both mean flow and turbulent statistics for oscillatory BL flows over a flat, rough bed. Here we show that the model accurately predicts the mean velocity fields and suspended sediment concentration for oscillatory flows over full-scale vortex ripples. Tests show that surface roughness has a significant effect on the results. Beyond increasing our insight into wave-induced oscillatory bottom BL physics, sophisticated coupled models of sediment transport such as that presented have the potential to make quantitative predictions of sediment transport and erosion/accretion around partly buried objects in the bottom, which is important for a vast array of bottom deployed instrumentation and other practical ocean engineering problems.

Description: The transient variation of the complexes of the low latitude ionosphere within the equatorial ionization anomaly region of Nigeria Nonlinear Processes in Geophysics Discussions, 1, 1855-1903, 2014 Author(s): A. B. Rabiu, B. O. Ogunsua, I. A. Fuwape, and J. A. Laoye The quest to find an index for proper characterization and description of the dynamical response of the ionosphere to external influences and its various internal irregularities has led to the study of the day to day variations of the chaoticity and dynamical complexity of the ionosphere. This study was conducted using Global Positioning System (GPS) Total Electron Content (TEC) time series, measured in the year 2011, from 5 GPS receiver stations in Nigeria which lies within the Equatorial Ionization Anomaly region. The nonlinear aspect of the TEC time series were obtained by detrending the data. The detrended TEC time series were subjected to various analyses for phase space reconstruction and to obtain the values of chaotic quantifiers which are Lyapunov exponents LE, correlation dimension, and Tsallis entropy for the study of dynamical complexity. The results show positive Lyapunov exponents for all days which indicate chaoticity of the ionosphere with no definite pattern for both quiet and disturbed days. However values of LE were lower for the storm period compared to its nearest relative quiet periods for all the stations. Considering all the days of the year the daily/transient variations show no definite pattern for each month but day to day values of Lyapunov exponent for the entire year show a wavelike semiannual variation pattern with lower values around March, April, September and October, a change in pattern which demonstrates the self-organized critical phenomenon of the system. This can be seen from the correlation dimension with values between 2.7 and 3.2 with lower values occurring mostly during storm periods demonstrating a phase transition from higher dimension during the quiet periods to lower dimension during storms for most of the stations. The values of Tsallis entropy show similar variation pattern with that of Lyapunov exponent with a lot of agreement in their comparison, with all computed values of Lyapunov exponent correlating with values of Tsallis entropy within the range of 0.79 to 0.82. These results show that Lyapunov quantifiers can be used together as indices in the study of the variations of the dynamical complexity of the ionosphere. The presence of chaos and high variations in the dynamical complexity, even at quiet periods in the ionosphere may be due to the internal dynamics and inherent irregularities of the ionosphere which exhibit non-linear properties. However, this inherent dynamics may be complicated by external factors like geomagnetic storms. This may be the main reason for the drop in the values of Lyapunov exponent and Tsallis entropy during storms. The results also show a strong interplay between determinism and stochasticity, as the ionosphere shows its response to changes in solar activities and in its internal dynamics. The dynamical behavior of the ionosphere throughout the year as described by these quantifiers, were discussed in this work.

Description: Dependence of sandpile avalanche frequency–size distribution on coverage extent and compactness of embedded toppling threshold heterogeneity: implications for the variation of Gutenberg–Richter b value Nonlinear Processes in Geophysics, 21, 1185-1193, 2014 Author(s): L.-Y. Chiao and Q. Liu The effects of the spatiotemporal evolution of failure threshold heterogeneity on the dynamics of fault criticality, and thus on regional seismogenesis, have attracted strong interest in the field of regional seismotectonics. The heterogeneity might be a manifestation of the macroscopic distribution and multiscale strength variation of asperities, the distinct regional stress level, and (microscopically) heterogeneous fault surface roughness or friction regimes. In this study, rather than attempting to mimic the complex microscale slipping physics on a fault surface, sandpile cellular automata were implemented with a straightforward toppling rule. The objective is to examine the influence of distinct configurations of the embedded heterogeneous toppling threshold field on the global system avalanche event statistics. The examination results revealed that increasing the coverage extent and decreasing the compactness of the heterogeneous failure threshold, rather than the magnitude, range of contrast, diversity, or the geometric configuration of the threshold heterogeneity, leads to a systematic increase in the scaling exponent of the avalanche event power law statistics, implying the importance of mutual interaction among toppling sites with distinct thresholds. For tectonic provinces with differing stress regimes evolving spatio temporally, it is postulated that the distinct extent and compactness of the heterogeneous failure threshold are critical factors that manifest in the reported dynamic variations of seismicity scaling.

Description: Evaluation of empirical mode decomposition for quantifying multi-decadal variations in sea level records Nonlinear Processes in Geophysics Discussions, 1, 1833-1854, 2014 Author(s): D. P. Chambers The ability of empirical mode decomposition (EMD) to extract multidecadal variability from sea level records is tested using three simulations: one based on a series of purely sinusoidal modes, one based on scaled climate indices of El Niño and the Pacific Decadal Oscillation (PDO), and the final one including a single month with an extreme sea level event. All simulations include random noise of similar variance to high-frequency variability in the San Francisco tide gauge record. The intrinsic mode functions (IMFs) computed using EMD were compared to the prescribed oscillations. In all cases, the longest-period modes are significantly distorted, with incorrect amplitudes and phases. This affects the estimated acceleration computed from the longest periodic IMF. In these simulations, the acceleration was underestimated in the case with purely sinusoidal modes, and overestimated by nearly 100% in the case with prescribed climate modes. Additionally, in all cases, extra low-frequency modes uncorrelated with the prescribed variability are found. These experiments suggest that using EMD to identify multidecadal variability and accelerations in sea level records should be used with caution.

Description: Benjamin–Feir instability of waves in the presence of current Nonlinear Processes in Geophysics Discussions, 1, 1803-1832, 2014 Author(s): I. V. Shugan, H. H. Hwung, and R. Y. Yang The development of Benjamin–Feir instability of Stokes waves in the presence of variable current is presented. We employ a model of a resonance system having three coexisting nonlinear waves and nonuniform current. The model is free from the narrow-band approximation for surface waves and relatively weak adverse current. The modulation instability of Stokes waves in nonuniform moving media has special properties. Interaction with countercurrent accelerates the growth of sideband modes on a short spatial scale. An increase in initial wave steepness intensifies the wave energy exchange accompanied by wave breaking dissipation, results in asymmetry of sideband modes and a frequency downshift with an energy transfer jump to the lower sideband mode, and depresses the higher sideband and carrier wave. Nonlinear waves may even overpass the blocking barrier produced by strong adverse current. The frequency downshift of the energy peak is permanent and the system does not revert to its initial state. We find reasonable correspondence between the results of model simulations and available experimental results for wave interaction with blocking opposing current. Large transient or freak waves with amplitude and steepness several times those of normal waves may form during temporal nonlinear focusing of the resonant waves accompanied by energy income from sufficiently strong opposing current. We employ the resonance model for the estimation of the maximum amplification of wave amplitudes as a function of gradually increasing opposing current and compare the result obtained with recently published experimental results and modeling results obtained with the nonlinear Schrödinger equation.

Description: The numerical simulation on ionospheric perturbations in electric field before large earthquakes Annales Geophysicae, 32, 1487-1493, 2014 Author(s): S. F. Zhao, X. M. Zhang, Z. Y. Zhao, and X. H. Shen Many observational results have shown electromagnetic abnormality in the ionosphere before large earthquakes. The theoretical simulation can help us to understand the internal mechanism of these anomalous electromagnetic signals resulted from seismic regions. In this paper, the horizontal and vertical components of electric and magnetic field at the topside ionosphere are simulated by using the full wave method that is based on an improved transfer matrix method in the lossy anisotropic horizontally stratified ionosphere. Taken account into two earthquakes with electric field perturbations recorded by the DEMETER satellite, the numerical results reveal that the propagation and penetration of ULF (ultra-low-frequency) electromagnetic waves into the ionosphere is related to the spatial distribution of electron and ion densities at different time and locations, in which the ion density has less effect than electron density on the field intensity. Compared with different frequency signals, the minimum values of electric and magnetic field excited by earthquakes can be detected by satellite in current detection capability have also been calculated, and the lower frequency wave can be detected easier.

Description: A comparison between VEGA 1, 2 and Giotto flybys of comet 1P/Halley: implications for Rosetta Annales Geophysicae, 32, 1441-1453, 2014 Author(s): M. Volwerk, K.-H. Glassmeier, M. Delva, D. Schmid, C. Koenders, I. Richter, and K. Szegö Three flybys of comet 1P/Halley, by VEGA 1, 2 and Giotto, are investigated with respect to the occurrence of mirror mode waves in the cometosheath and field line draping in the magnetic pile-up region around the nucleus. The time interval covered by these flybys is approximately 8 days, which is also the approximate length of an orbit or flyby of Rosetta around comet 67P/Churyumov–Gerasimenko. Thus any significant changes observed around Halley are changes that might occur for Rosetta during one pass of 67P/CG. It is found that the occurrence of mirror mode waves in the cometosheath is strongly influenced by the dynamical pressure of the solar wind and the outgassing rate of the comet. Field line draping happens in the magnetic pile-up region. Changes in nested draping regions (i.e. regions with different B x directions) can occur within a few days, possibly influenced by changes in the outgassing rate of the comet and thereby the conductivity of the cometary ionosphere.

Description: Instability and change detection in exponential families and generalized linear models, with a study of Atlantic tropical storms Nonlinear Processes in Geophysics, 21, 1133-1143, 2014 Author(s): Y. Lu and S. Chatterjee Exponential family statistical distributions, including the well-known normal, binomial, Poisson, and exponential distributions, are overwhelmingly used in data analysis. In the presence of covariates, an exponential family distributional assumption for the response random variables results in a generalized linear model. However, it is rarely ensured that the parameters of the assumed distributions are stable through the entire duration of the data collection process. A failure of stability leads to nonsmoothness and nonlinearity in the physical processes that result in the data. In this paper, we propose testing for stability of parameters of exponential family distributions and generalized linear models. A rejection of the hypothesis of stable parameters leads to change detection. We derive the related likelihood ratio test statistic. We compare the performance of this test statistic to the popular normal distributional assumption dependent cumulative sum (Gaussian CUSUM) statistic in change detection problems. We study Atlantic tropical storms using the techniques developed here, so to understand whether the nature of these tropical storms has remained stable over the last few decades.

Description: Magnetic curvature driven Rayleigh-Taylor instability revisited Annales Geophysicae, 29, 411-413, 2011 Author(s): O. A. Pokhotelov and O. G. Onishchenko The problem of incomplete finite ion Larmor radius (FLR) stabilization of the magnetic curvature driven Rayleigh-Taylor instability (RTI) in low beta plasma with homogeneous ion temperature is investigated. For this purpose a model hydrodynamic description of nonlinear flute waves with arbitrary spatial scales compared to the ion Larmor radius is developed. It is shown that the RTI is not stabilized by FLR effects in a plasma with cold electrons when the ratio of characteristic spatial scale of the plasma inhomogeneity to local effective radius of curvature of the magnetic field lines is larger than 1/4. The crucial role in the absence of the complete FLR stabilization plays the contribution of the compressibility of the polarization part of the ion velocity.

Description: Forecasting relativistic electron flux using dynamic multiple regression models Annales Geophysicae, 29, 415-420, 2011 Author(s): H.-L. Wei, S. A. Billings, A. Surjalal Sharma, S. Wing, R. J. Boynton, and S. N. Walker The forecast of high energy electron fluxes in the radiation belts is important because the exposure of modern spacecraft to high energy particles can result in significant damage to onboard systems. A comprehensive physical model of processes related to electron energisation that can be used for such a forecast has not yet been developed. In the present paper a systems identification approach is exploited to deduce a dynamic multiple regression model that can be used to predict the daily maximum of high energy electron fluxes at geosynchronous orbit from data. It is shown that the model developed provides reliable predictions.

Description: Airglow observations over the equatorial ionization anomaly zone in Taiwan Annales Geophysicae, 29, 749-757, 2011 Author(s): J. Y. Liu, P. K. Rajesh, I. T. Lee, and T. C. Chow Airglow imaging at mid-latitude stations often show intensity modulations associated with medium scale travelling ionospheric disturbances (MSTID), while those carried out near the equatorial regions reveal depletions caused by equatorial plasma bubbles (EPB). Two all sky cameras are used to observe plasma depletions in the 630.0 nm emission over the equatorial ionization anomaly (EIA) region, Taiwan (23° N, 121° E; 13.5° N Magnetic) during 1998–2002 and 2006–2007. The results show EPB and MSTID depletions in different solar activity conditions. Several new features of the EPB depletions such as bifurcation, secondary structure on the walls, westward tilt, etc., are discussed in this paper. Evidence of tilted depletions with secondary structures developing on the eastern wall that later evolve to appear as bifurcations, are presented for the first time. Moreover, detail investigations are carried out using International Reference Ionosphere (IRI) model as well as the electron density from Ionosonde and Global Positioning System (GPS) Occultation Experiment (GOX) onboard FORMOSAT-3/COSMIC satellite, to understand the conditions that favor the propagation of MSTID to the latitude of Taiwan.

Description: 3-D mesoscale MHD simulations of a cusp-like magnetic configuration: method and first results Annales Geophysicae, 29, 759-770, 2011 Author(s): E. Adamson and K. Nykyri We present a local mesoscale model of the magnetospheric cusp region with high resolution (up to 300 km). We discuss the construction and implementation of the initial configuration and give a detailed description of the numerical simulation. An overview of simulation results for the case of strongly northward interplanetary magnetic field (IMF) is then presented and compared with data from Cluster 2 spacecraft from 14 February 2003. Results show a cusp diamagnetic cavity (CDC) with depth normal to the magnetospheric boundary on the order of 1–2 R E and a much larger extent of ~5–9 R E tangential to the boundary, bounded by a gradual inner boundary with the magnetospheric lobe and a more distinct exterior boundary with the magnetosheath. These results are qualitatively consistent with observational data.

Description: Polar cap ion beams during periods of northward IMF: Cluster statistical results Annales Geophysicae, 29, 771-787, 2011 Author(s): R. Maggiolo, M. Echim, J. De Keyser, D. Fontaine, C. Jacquey, and I. Dandouras Above the polar caps and during prolonged periods of northward IMF, the Cluster satellites detect upward accelerated ion beams with energies up to a few keV. They are associated with converging electric field structures indicating that the acceleration is caused by a quasi-static field-aligned electric field that can extend to altitudes higher than 7 R E (Maggiolo et al., 2006; Teste et al., 2007). Using the AMDA science analysis service provided by the Centre de Données de la Physique des Plasmas, we have been able to extract about 200 events of accelerated upgoing ion beams above the polar caps from the Cluster database. Most of these observations are taken at altitudes lower than 7 R E and in the Northern Hemisphere. We investigate the statistical properties of these ion beams. We analyze their geometry, the properties of the plasma populations and of the electric field inside and around the beams, as well as their dependence on solar wind and IMF conditions. We show that ~40 % of the ion beams are collocated with a relatively hot and isotropic plasma population. The density and temperature of the isotropic population are highly variable but suggest that this plasma originates from the plasma sheet. The ion beam properties do not change significantly when the isotropic, hot background population is present. Furthermore, during one single polar cap crossing by Cluster it is possible to detect upgoing ion beams both with and without an accompanying isotropic component. The analysis of the variation of the IMF B Z component prior to the detection of the beams indicates that the delay between a northward/southward turning of IMF and the appearance/disappearance of the beams is respectively ~2 h and 20 min. The observed electrodynamic characteristics of high altitude polar cap ion beams suggest that they are closely connected to polar cap auroral arcs. We discuss the implications of these Cluster observations above the polar cap on the magnetospheric dynamics and configuration during prolonged periods of northward IMF.

Description: Rank ordering multifractal analysis of the auroral electrojet index Nonlinear Processes in Geophysics, 18, 277-285, 2011 Author(s): G. Consolini and P. De Michelis In the second half of the 90s interest grew on the complex features of the magnetospheric dynamics in response to solar wind changes. An important series of papers were published on the occurrence of chaos, turbulence and complexity. Among them, particularly interesting was the study of the bursty and fractal/multifractal character of the high latitude energy release during geomagnetic storms, which was evidenced by analyzing the features of the Auroral Electrojet (AE) indices. Recently, the multifractal features of the small time-scale increments of AE-indices have been criticized in favor of a more simple fractal behavior. This is particularly true for the scaling features of the probability density functions (PDFs) of the AE index increments. Here, after a brief review of the nature of the fractal/multifractal features of the magnetospheric response to solar wind changes, we investigate the multifractal nature of the scaling features of the AE index increments PDFs using the Rank Ordering Multifractal Analysis (ROMA) technique. The ROMA results clearly demonstrate the existence of a hierarchy of scaling indices, depending on the increment amplitude, for the data collapsing of PDFs relative to increments at different time scales. Our results confirm the previous results by Consolini et al. (1996) and the more recent results by Rypdal and Rypdal (2010).

Description: Diurnal variation of short-period (20–120 min) gravity waves in the equatorial Mesosphere and Lower Thermosphere and its relation to deep tropical convection Annales Geophysicae, 29, 623-629, 2011 Author(s): N. Venkateswara Rao, Y. Shibagaki, and T. Tsuda We study short period gravity waves (20–120 min) in the equatorial Mesosphere and Lower Thermosphere (MLT) using a Medium Frequency (MF) radar at Pameungpeuk (7.4° S, 107.4° E), Indonesia. In particular, we study local time and seasonal variation of the gravity wave variance and its relation to tropical convection. The gravity wave variance at 88 km enhances between 20:00 LT and 07:00 LT, with a peak at 02:00–03:00 LT. The enhancement is mainly observed during February–April and September–October and shows inter-annual variability. Convective activity over the same location persists from 16:00–21:00 LT with a peak activity ~18:00 LT and enhances between November–April. Time delay between the peak of convection and that of gravity wave activity ranges 1–15 h, which is consistent with theoretical calculations and previous reports based on reverse ray tracing analysis.

Description: The features of electronics structure of the multichannel scintillation module for the EMMA experiment Astrophysics and Space Sciences Transactions, 7, 171-174, 2011 Author(s): V. Volchenko, G. Volchenko, E. Akhrameev, L. Bezrukov, I. Dzaparova, T. Enqvist, L. Inzhechik, A. Izmaylov, J. Joutsenvaara, M. Khabibullin, A. Khotjantsev, P. Kuusiniemi, B. Lubsandorzhiev, O. Mineev, V. Petkov, R. Poleshuk, B. Shaibonov, J. Sarkamo, A. Shaykhiev, W. Trzaska, A. Yanin, and N. Yershov A brief description of the developed structural electric diagrams of 16-channel scintillation module for the underground EMMA experiment, the basic characteristics and parameters of the electrical diagrams of this module are presented. Multi-pixel photodiodes operating in a limited Geiger mode are used for photoreadout of the scintillator detectors in 16-channel scintillation module. The method of the automatic tuning of the photosensors gain based on the stabilization of an average counting rate of the scintillation detectors from gamma rays of a natural radioactive background is described.

Description: Observations of AGW/TID propagation across the polar cap: a case study Annales Geophysicae, 29, 1355-1363, 2011 Author(s): H. T. Cai, F. Yin, S. Y. Ma, and I. W. McCrea In this paper, we present observational evidence for the trans-polar propagation of large-scale Traveling Ionospheric Disturbances (TIDs) from their nightside source region to the dayside. On 13 February 2001, the 32 m dish of EISCAT Svalbard Radar (ESR) was directing toward the geomagnetic pole at low elevation (30°) during the interval 06:00–12:00 UT (MLT ≈ UT + 3 h), providing an excellent opportunity to monitor the ionosphere F-region over the polar cap. The TIDs were first detected by the ESR over the dayside north polar cap, propagating equatorward, and were subsequently seen by the mainland UHF radar at auroral latitudes around geomagnetic local noon. The propagation properties of the observed ionization waves suggest the presence of a moderately large-scale TIDs, propagating across the northern polar cap from the night-time auroral source during substorm conditions. Our results agree with the theoretical simulations by Balthazor and Moffett (1999) in which poleward-propagating large-scale traveling atmospheric disturbances were found to be self-consistently driven by enhancements in auroral heating.

Description: Statistical evidence for O + energization and outflow caused by wave-particle interaction in the high altitude cusp and mantle Annales Geophysicae, 29, 945-954, 2011 Author(s): M. Waara, R. Slapak, H. Nilsson, G. Stenberg, M. André, and I. A. Barghouthi We present a statistical study of the low (

Description: O + heating associated with strong wave activity in the high altitude cusp and mantle Annales Geophysicae, 29, 931-944, 2011 Author(s): R. Slapak, H. Nilsson, M. Waara, M. André, G. Stenberg, and I. A. Barghouthi We use the Cluster spacecraft to study three events with intense waves and energetic oxygen ions (O + ) in the high altitude cusp and mantle. The ion energies considered are of the order 1000 eV and higher, observed above an altitude of 8 earth radii together with high wave power at the O + gyrofrequency. We show that heating by waves can explain the observed high perpendicular energy of O + ions, using a simple gyroresonance model and 25–45% of the observed wave spectral density at the gyrofrequency. This is in contrast to a recently published study where the wave intensity was too low to explain the observed high altitude ion energies. Long lasting cases (〉10 min) of high perpendicular-to-parallel temperature ratios are sometimes associated with low wave activity, suggesting that high perpendicular-to-parallel temperature ratio is not a good indicator of local heating. Using multiple spacecraft, we show that the regions of enhanced wave activity are at least one order of magnitude larger than the gyroradius of the heated ions.

Description: Simulation of aerosol optical properties over a tropical urban site in India using a global model and its comparison with ground measurements Annales Geophysicae, 29, 955-963, 2011 Author(s): D. Goto, K. V. S. Badarinath, T. Takemura, and T. Nakajima Aerosols have great impacts on atmospheric environment, human health, and earth's climate. Therefore, information on their spatial and temporal distribution is of paramount importance. Despite numerous studies have examined the variation and trends of BC and AOD over India, only very few have focused on their spatial distribution or even correlating the observations with model simulations. In the present study, a three-dimensional aerosol transport-radiation model coupled with a general circulation model. SPRINTARS, simulated atmospheric aerosol distributions including BC and aerosol optical properties, i.e., aerosol optical thickness (AOT), Ångström Exponent (AE), and single scattering albedo (SSA). The simulated results are compared with both BC measurements by aethalometer and aerosol optical properties measured by ground-based skyradiometer and by satellite sensor, MODIS/Terra over Hyderabad, which is a tropical urban area of India, for the year 2008. The simulated AOT and AE in Hyderabad are found to be comparable to ground-based measured ones. The simulated SSA tends to be higher than the ground-based measurements. Both these comparisons of aerosol optical properties between the simulations with different emission inventories and the measurements indicate that, firstly the model uncertainties derived from aerosol emission inventory cannot explain the gaps between the simulations and the measurements and secondly the vertical transport of BC and the treatment of BC-containing particles can be the main issue in the global model to solve the gap.

Description: Energy and flux variations across thin auroral arcs Annales Geophysicae, 29, 1699-1712, 2011 Author(s): H. Dahlgren, B. Gustavsson, B. S. Lanchester, N. Ivchenko, U. Brändström, D. K. Whiter, T. Sergienko, I. Sandahl, and G. Marklund Two discrete auroral arc filaments, with widths of less than 1 km, have been analysed using multi-station, multi-monochromatic optical observations from small and medium field-of-view imagers and the EISCAT radar. The energy and flux of the precipitating electrons, volume emission rates and local electric fields in the ionosphere have been determined at high temporal (up to 30 Hz) and spatial (down to tens of metres) resolution. A new time-dependent inversion model is used to derive energy spectra from EISCAT electron density profiles. The energy and flux are also derived independently from optical emissions combined with ion-chemistry modelling, and a good agreement is found. A robust method to obtain detailed 2-D maps of the average energy and number flux of small scale aurora is presented. The arcs are stretched in the north-south direction, and the lowest energies are found on the western, leading edges of the arcs. The large ionospheric electric fields (250 mV m −1 ) found from tristatic radar measurements are evidence of strong currents associated with the region close to the optical arcs. The different data sets indicate that the arcs appear on the boundaries between regions with different average energy of diffuse precipitation, caused by pitch-angle scattering. The two thin arcs on these boundaries are found to be related to an increase in number flux (and thus increased energy flux) without an increase in energy.

Description: Storm-time total electron content and its response to penetration electric fields over South America Annales Geophysicae, 29, 1765-1778, 2011 Author(s): P. M. de Siqueira, E. R. de Paula, M. T. A. H. Muella, L. F. C. Rezende, M. A. Abdu, and W. D. Gonzalez In this work the response of the ionosphere due to the severe magnetic storm of 7–10 November 2004 is investigated by analyzing GPS Total Electron Content (TEC) maps constructed for the South America sector. In order to verify the disturbed zonal electric fields in South America during the superstorm, ionospheric vertical drift data obtained from modeling results are used in the analysis. The vertical drifts were inferred from Δ H magnetometer data (Jicamarca-Piura) following the methodology presented by Anderson et al. (2004). Also used were vertical drifts measured by the Jicamarca ISR. Data from a digisonde located at São Luís, Brazil (2.33° S, 44.2° W, dip latitude 0.25°) are presented to complement the Jicamarca equatorial data. Penetration electric fields were observed by the comparison between the equatorial vertical drifts and the Interplanetary Electric Field (IEF). The TEC maps obtained from GPS data reflect the ionospheric response over the South America low-latitude and equatorial region. They reveal unexpected plasma distributions and TEC levels during the main phase of the superstorm on 7 November, which is coincident with the local post-sunset hours. At this time an increase in the pre-reversal enhancement was expected to develop the Equatorial Ionization Anomaly (EIA) but we observed the absence of EIA. The results also reveal well known characteristics of the plasma distributions on 8, 9, and 10 November. The emphasized features are the expansion and intensification of EIA due to prompt penetration electric fields on 9 November and the inhibition of EIA during post-sunset hours on 7, 8, and 10 November. One important result is that the TEC maps provided a bi-dimensional view of the ionospheric changes offering a spatial description of the electrodynamics involved, which is an advantage over TEC measured by isolated GPS receivers.

Description: Climatology of rapid geomagnetic variations at high latitudes over two solar cycles Annales Geophysicae, 29, 1783-1792, 2011 Author(s): A. Viljanen and E. Tanskanen We investigate the characteristics of rapid geomagnetic variations at high latitudes based on the occurrence of large time derivatives of the horizontal magnetic field ( d H / dt exceeding 1 nT s −1 ). Analysis of IMAGE magnetometer data from North Europe in 1983–2010, covering more than two solar cycles, confirms and specifies several previous findings. We show that d H / dt activity is high around the midnight and early morning hours, and nearly vanishes at noon and early afternoon. This happens during all seasons, although the midnight maximum is nearly invisible during summer. As indicated by modelled ionospheric equivalent currents, large d H / dt values occur predominantly during westward ionospheric electrojets. Before and around midnight, d H / dt tends to be north-south oriented, whereas in the morning hours, its direction is more west-east directed. d H / dt tends to be more strictly north-south oriented during winter than other seasons. The seasonal occurrence of large d H / dt values is similar to the variation of the maximum amplitude of westward equivalent currents. The yearly fraction of east-west directed large d H / dt vectors at the Kilpisjärvi station (MLAT 65.88) varies from 31 to 47 % without any clear correlation with the general geomagnetic activity nor with the yearly averages of solar wind parameters.