ALBERT

Description: Ground-based observations have shown Pi1B magnetic pulsations are associated with substorm onset. These pulsations can also be observed at geosynchronous orbit, suggesting that they propagate from (or beyond) geosynchronous orbit to the ionosphere at substorm onset. Independently, investigations have shown that the initial brightening of an arc at subtorm onset is Alfvénic in nature (i.e., that the aurora during the initial brightening is wave-driven). These results raise the question of whether Pi1B pulsations might drive Alfvénic aurora at substorm onset. In this paper, data from a single event are presented that show Pi1B pulsations observed simultaneously at geosynchronous orbit, by FAST just above the ionosphere and by various ground stations. The event is observed by FAST within a few minutes of the onset of Pi1B pulsations, with an electron signature of the onset arc that is quite characteristic of Alfvénic aurora, showing that at least a portion of the initial brightening is wave-driven and associating this with the presence of Pi1B pulsations. The implication of this work is that Pi1B pulsations propagate to the ionosphere from beyond geosynchronous orbit and provide the wave power to drive Alfvénic aurora at substorm onset, at least for this single, isolated event. The luminosity associated with the Alfvénic aurora, however, may only provide a small contribution to the total brightness.

Description: We report observations of very large amplitude whistler mode waves in the Earth's nightside inner radiation belt enabled by the STEREO Time Domain Sampler. Amplitudes range from 30–110 mV/m (zero-peak), 2 to 3 orders of magnitude larger than previously observed in this region. Measurements from the peak electric field detector (TDSMax) indicate that these large-amplitude waves are prevalent throughout the plasmasphere. A detailed examination of high time resolution electric field waveforms is undertaken on a subset of these whistlers at L 〈 2, associated with pump waves from lightning flashes and the naval transmitter NPM in Hawaii, that become unstable after propagation through the ionosphere and grow to large amplitudes. Many of the waveforms undergo periodic polarization reversals near the lower hybrid and NPM naval transmitter frequencies. The reversals may be related to finite plasma temperature and gradients in density induced by ion cyclotron heating of the plasma at 200 Hz, the modulation frequency of the continuous-mode NPM naval transmitter signal. Test particle simulations using the amplitudes and durations of the waves observed herein suggest that they can interact strongly with high-energy (〉100 keV) electrons on a time scale of

Description: In this paper, we report a long-term study of medium-scale traveling ionospheric disturbance (MSTID) occurrence using all-sky images of O I 630 nm airglow emission. Our study is based on a 10.5 year data series in which 5.5 years compound a new data set. The images were obtained by an all-sky imager installed at Cachoeira Paulista (22.7°S, 45°W, 15°S magnetic latitude). Our results show the greatest occurrence of MSTIDs during solar minimum and a minor occurrence rate during descending and ascending solar activity periods. During solar maximum, we have not detected MSTID signatures on all-sky images. All detected events have occurred during geomagnetically quiet conditions. Simultaneous measurements using both all-sky images and ionograms obtained at the same site show the occurrence of spread F and a sharp rise in the ionospheric F layer at the same time as dark bands are optically registered over the zenith. It is possible that for certain weaker events, the ionosonde may resolve bands that are not sufficiently raised to cause darkening of the all-sky images. In order to investigate this possibility, we have conducted a study using only ionograms for a 1 year period (March 2000–February 2001) during solar maximum, when we have not seen any MSTID events in the all-sky images. As we conjectured, MSTIDs also occur during solar maximum, and the risings of the ionospheric F layer are not able to disturb the airglow layer during such events.

Description: We reproduce a magnetospheric reconfiguration under southward interplanetary magnetic field (IMF) conditions using numerical magnetohydrodynamic simulations. To investigate the relative displacements of the geomagnetic conjugate points, we trace both footprints of the geomagnetic field lines during the magnetospheric reconfiguration under positive and negative IMF By conditions. Several substormlike features, namely, the formation of a near-Earth neutral line, a fast earthward flow, and tailward releases of the plasmoid, occur about 1 h after a southward turning of the IMF. The surveyed field line traced from the near-Earth magnetotail was strongly distorted duskward in the north and south after the substorm onset for positive and negative IMF By, respectively. The maximum of the relative longitudinal displacement of both footprints is 4.5 and 5.5 h in magnetic local time for positive and negative IMF By, respectively. While observational studies have indicated that the IMF orientation is the main factor controlling the relative displacement of the conjugate points, the present simulation-based study with a constant IMF orientation shows for the first time that the combined effects of plasma pressure, magnetic field intensity, and the field-aligned current density distribution along the field line are likely to be major factors controlling the relative displacement of conjugate points.

Description: Motivated by recent attempts to derive geomagnetic activity from hourly mean data in long-term studies, we test the recursive Kalman filter method to obtain the regular solar variation curve of the geomagnetic field. Using a simple algorithm, we are able to assign a quiet day curve to every day separately, without the need for additional input parameter(s) to define the geomagnetically quiet days. We derive a digital counterpart AhK of the analog range index Ak at the subauroral Sodankylä station and compare it to the earlier digital estimate Ah and the local Ak index. We find that the new method outperforms the former estimate in every aspect studied and provides a robust, straightforward manner of estimating and verifying the manually scaled Ak index, based on readily available hourly values. The model is independent of sampling; thus, for shorter-term studies where high-sampling data are available, more accurate estimates can also be obtained when needed. Therefore, in contrast to other recent approaches, we do not provide a method to quantify irregular activity directly but derive the actual quiet day curves in the traditional manner. In future applications the same algorithm may be used to define a wide variety of geomagnetic indices (such as Ak, Dst, or AE).

Description: An experimental investigation of the temporal dynamics of the magnetic zenith (MZ) effect associated with ionospheric modification by high-power HF electromagnetic waves is presented. The observed electron temperature enhancement when the heater beam and the UHF radar are directed along the magnetic zenith is at least twice that observed when the heater and radar are both directed vertically. It is shown that the temperature enhancement reaches the stationary state within 10 s after the heater is turned on. Such times (∼5–10 s) are typical for the development of striations with transverse sizes of the order of several meters. Also, the temporal behavior of the ion line spectra is analyzed for the field-aligned and vertical directions of the UHF radar. A new theoretical explanation is suggested for the aspect sensitivity of the electron temperature enhancement that explains the fast manifestation of the MZ effect. Furthermore, it is shown that maximum electron heating is achieved at some intermediate inclination angle of the heater beam between the MZ and the Spitze angle. An estimate of the angle within which the maximum heating effect exists is presented.

Description: This paper for the first time reports global three-dimensional (3-D) structures of the ionospheric midlatitude trough using electron density profiles derived from the GPS radio occultation experiment on board FORMOSAT-3/COSMIC (F3/C) satellites during the solar minimum period, February 2008 to January 2009. Results show that the midlatitude trough extends from dusk to dawn in all four seasons and is most pronounced in the winter hemisphere. The troughs in the two hemispheres are asymmetric, where the trough in the Northern Hemisphere is more evident and stronger than that in the Southern Hemisphere during the equinoctial seasons. In general, the trough minimum position shows a high-low-high latitudinal variation with magnetic local time and occurs at lower latitudes under higher magnetic activity. On the other hand, the midlatitude trough structures become more complex in the Southern Hemisphere because of the nighttime plasma density enhancement of the Weddell Sea Anomaly. Our results demonstrate that the new data set of GPS radio occultation by F3/C is useful to probe the global 3-D electron density structures of the midlatitude trough.

Description: In this report we present a temporal relationship between ground Pi2 and auroral kilometric radiation (AKR). We analyzed six isolated substorm events, which were observed by the MAGDAS/CPMN ground magnetometer network and the plasma wave instrument onboard the Polar satellite. We found that the time derivative of the height-integrated AKR power and the ground Pi2 D component had the same periodicity and that the two were synchronized with each other. When the D component fluctuated with the same (opposite) polarity as the magnetic bay variation, the AKR power tended to increase (decrease) during the corresponding interval. An isolated substorm event (AE ∼ 40 nT), which occurred around 10:19 UT on 24 January1997, was selected for a detailed study. The behavior of the Pi2 event can be interpreted by the substorm current wedge (SCW) and Pi2 propagation models. It is confirmed that the midlatitude and high-latitude D component oscillations can be treated as a proxy of the SCW oscillations, whereas the H component oscillations exhibited some phase shifts by the propagation delay of the Pi2 waves. That is, the temporal relation between the time derivative of the AKR power and the ground Pi2 suggests that the height-integrated AKR power was modulated coherently with the SCW oscillations.

Description: Using data from the high-energy neutral atom (HENA) imager onboard the IMAGE satellite, we examined the relation between the SYM-H index and the ring current energy during a storm main phase. The energy range of the energetic neutral atom (ENA) flux data used here is 16–120 keV for hydrogen and

Description: Using observations from the THEMIS spacecraft, we investigate the modulation of whistler mode chorus waves in the magnetosphere by compressional Pc4–5 pulsations (i.e., pulsations with periods from tens of seconds to a few minutes) with an anticorrelation between the total electron density and the background magnetic field intensity. We find that such compressional pulsations are associated with modulations of resonant electron fluxes and chorus intensity. Changes in the total electron density, background magnetic field, and the flux and anisotropy of resonant electrons could all be responsible for triggering the excitation of chorus waves. To quantitatively investigate which parameters dominantly contribute to chorus generation, we evaluate the changes in linear growth rates of whistler mode waves due to variations in either the ratio of resonant electrons to the total electrons R(VR) or the electron anisotropy A(VR). In the majority of cases, the modulation of whistler mode wave intensity is dominated by R(VR) variations caused by compressional Pc4–5 pulsations and tends to occur at large L shells of 8–12 in the dawn sector. Only a small fraction of events are associated with A(VR) modulations and these typically occur at lower L shells (

Description: The boundary of a planetary magnetosphere is the site of mass, momentum, and energy transport. This transport produces a layer of mixed solar wind and magnetospheric plasma inside and adjacent to the boundary. In the case of Earth, the electron structure of this layer is distinctive, and has been explained by models of the layer on open magnetic field lines. In this paper we examine the electron structure of Saturn's low-latitude boundary layer (LLBL) using observations made by the Cassini spacecraft; the typical properties and variability of Saturn's LLBL are examined in a companion paper. By analyzing the relationship between the electron density and temperature measured during Cassini magnetopause crossings we demonstrate that the electron structure of Saturn's LLBL is highly variable. At some of the crossings the structure of Saturn's LLBL is similar to previously reported examples of the structure of Earth's LLBL, where the major changes in electron density and temperature clearly occur in different regions of the layer, producing a distinctive shape to the temperature-density distribution. However, at many crossings the structure of Saturn's LLBL is unlike the previously reported examples of the structure of Earth's LLBL, since they lack the same distinctive shape to the distribution. We discuss the possible explanations for these differences in the electron structure of Saturn's LLBL, and what these differences could tell us about how the solar wind interacts with a planetary magnetosphere.

Description: Fast azimuthal auroral expansion and poleward expansion are characteristic features of the expansion phase of substorms. In the first study of its kind, we have investigated the azimuthal auroral expansion and its magnetospheric counterpart using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imagers and multiple spacecraft. During the tail season in 2008–2009, we found 16 events of azimuthally expanding aurora that passed near the magnetic footprints of the multiple spacecraft operating in the near-Earth plasma sheet. In the magnetosphere, these events commonly showed fast azimuthal and earthward flows associated with intense electric fields and magnetic dipolarization. The speed of the propagating structure, which was estimated from the time difference of the depolarization observed by the multiple spacecraft, was close to the measured azimuthal plasma flow velocity. We also found that this azimuthal plasma transport was dominated by the E × B drift speed associated with the enhanced electric field. In a statistical analysis, the averaged speeds of the leading edge of the westward and eastward auroral expansion were 8.8 and 5.3 km/s, respectively. When mapped onto the equatorial magnetosphere, these speeds (267 and 162 km/s) were comparable to the averaged azimuthal plasma (E × B) flow speeds observed by the spacecraft, which were 193 (239) km/s in the westward direction and 112 (139) km/s in the eastward direction. Our events showed that E × B flows and auroral expansion predominantly propagated westward, indicating an effect of westward background convection in the Harang flow shear. From these results, we concluded that the azimuthal auroral expansion was closely related to magnetic dipolarization which expanded azimuthally at the E × B drift speed. On the basis of the abrupt formation of the fast E × B flows and their propagation away from the onset location, we suggest that the effects of the intense large-scale electric fields, which are possibly generated through substorm onset turbulence, propagate to the ionosphere along the magnetic field lines and lead to azimuthal expansion of an auroral arc.

Description: Relativistic electron fluxes of the outer radiation belt often decrease rapidly in response to solar wind disturbances. The importance of the magnetopause shadowing (MPS) effect on such electron losses has yet to be quantified. If the MPS is essential for outer radiation belt electron losses, a close relationship between the outer edge of the outer belt and the magnetopause standoff distance is expected. Using GOES and THEMIS data, we examined earthward movement of the outer edge of the outer belt during electron loss events at geosynchronous orbit and its correlation with the magnetopause standoff distance. In events with significant earthward movement, we found a good correlation. There were no clear correlations in events without significant earthward movement, however. Comparing the observational results with a test particle simulation, the observed dependence between the outer edge and the magnetopause standoff distance is consistent with the MPS effect.

Description: An experimental investigation of the physical properties of lightning on Venus was carried out by applying fast impulses to a pressurized (1–2.8 atm) CO2-N2 gas mix. The maximum current employed in the experiments was 2.8 kA. It was found that the pressure has a marked effect on the measured electric field. This effect is mostly due to negative ions created by electron attachment. This negative space charge could hamper breakdown on Venus if the clouds are negatively charged. Using a time-resolved spectroscopic technique, the instantaneous temperature of the arc was measured. This varies from 29,000 K soon after the arc appears to 14,000 K some 15 μs later. The arc's linear resistance and the power it dissipates were estimated using the Spitzer resistivity model and the measured temperatures. The linear resistance was found to be in the 17–233 Ω m−1 interval, which is comparable to the values of terrestrial lightning. The by-products of the discharge were analyzed using a mass analyzer, and it was found that the discharge decomposes the CO2 into CO and O at similar rates. Our estimate of the CO yield is 2.7 × 1018 molecules J−1.

Description: Since 1980, we have observed the thermospheric neutral wind at the Arecibo Observatory using a Fabry-Perot interferometer to measure the O(1D) 630 nm emission. Burnside and Tepley (1989) examined the first 8 years of this extended data set and found that there were no significant or systematic solar cycle influences on the magnitude or direction of the neutral wind field, nor on its horizontal gradients. Such affects have been observed previously at other locations around the globe, and their absence at Arecibo may have been due to the limited data set. Thus, we have extended the period of acquisition and analysis of our neutral wind measurements to include nearly three complete solar cycles (or approximately 30 years) and will present our results within the framework of the earlier work. While the earlier conclusion that no major systematic solar cycle influence on the neutral winds at Arecibo generally remains intact, we did find a slight increase in wind magnitude and a gradual, yet consistent rotation of the thermospheric neutral wind vector from a general southeast to a more eastward flow during 30 years of observation. We explain the magnitude and directional variations in terms of long-term changes in the density and temperature of the upper atmosphere and their possible dissimilar influences on each wind component that appear as a rotation of the neutral wind vector.

Description: Cold point mesopause is characterized by the coldest point in the temperature profile of the Earth's atmosphere. TIMED/SABER observations of cold point mesopause and its variability at diurnal and planetary wave scales are discussed in this study. For the first time, the diurnal and semidiurnal tidal modulations of mesopause are quantified on a global scale during all the four seasons, namely, winter, vernal equinox, summer, and autumnal equinox. The composite of diurnal variations of mesopause height and temperature are discussed during each season and using least squares fit, diurnal and semidiurnal tidal amplitudes and phases are obtained. Most of the features exhibited by the diurnal variation of mesopause height are consistent with the present understanding of the migrating tides. The diurnal tidal modulations of mesopause show its peak over equatorial latitude and change its phase around 20° latitude. The phase of the diurnal tidal modulation is consistent during all seasons expect for a phase shift of 4–6 h observed during boreal summer. The similarities/discrepancies between the latitudinal structure of migrating tides and the diurnal variation of mesopause height are discussed. The results reveal that the diurnal tidal modulations of mesopause height show hemispherical asymmetry, which is not reflected in mesopause temperature. The diurnal and semidiurnal amplitudes in mesopause height across the globe are comparable in magnitude and it is found that over equatorial and low latitudes, the variability of mesopause is maximum at these scales as compared to seasonal scales. Quantification of mesopause height at diurnal scales is very important as it also changes the chemistry of that region. In the present study, an attempt is also made to demonstrate the modulation of the mesopause by propagating planetary waves. The results emphatically show that propagating planetary waves do modulate the mesopause height.

Description: We present a study of Spread F occurrence over a location under the southern crest of the equatorial ionization anomaly (Cachoeira Paulista 22.7°S, 45.0°W, mag. Lat.: 16°S, dip angle: −32.3°, Brazil) during the last solar cycle, which presented an extended solar minimum activity. After analyzing hundreds of ionograms obtained with a digital ionosonde DGS 256, between 2001 and 2010, we verified high Spread F occurrence around midnight-postmidnight during June solstice (Southern Hemisphere winter) with a peak occurrence between 2006 and 2009, when the solar flux has reached very low values (

Description: In this paper we examine suprathermal He ions measured by the Suprathermal Ion Telescope instrument associated with tilted corotating interaction regions (CIRs). We use observations of the two STEREO spacecraft (S/C) for the first 2.7 years of the mission, along with ground-based measurements of the solar magnetic field during the unusually long minimum of Solar Cycle 23. Owing to the unique configuration of the STEREO S/C orbits, we are able to investigate spatial variations in the intensity of the corotating ions on time scales of less than one solar rotation. The observations reveal that the occurrence of the strong CIR events was the most frequent at the beginning of the period. The inclination of the heliospheric current sheet relative to the heliographic equator (the tilt angle) was quite high in the first stage of the mission and gradually flattened with the time, followed by a decrease in the CIR activity. By examining the differences between measurements on the two STEREO S/C, we discuss how the changes in the position of the S/C relative to the CIRs affect the energetic particle observations. We combine STEREO observations with observations from the Ultra Low Energy Isotope Spectrometer (ULEIS) instrument on the ACE S/C and argue that the main factor that controls the differences in the ion intensities is the latitudinal separation between the two STEREO S/C relative to the tilted CIRs. The position of the S/C is less important when the tilt angle is high. In this case we found that the CIR ion intensity positively correlates with the tilt angle.

Description: The spatiotemporal evolution of the daytime mesopause temperature (MT) over a tropical station, Trivandrum (8.5°N, 77°E), has been investigated during the annular solar eclipse of 15 January 2010 using a meridional scanning Multi-Wavelength Dayglow Photometer. This eclipse was a unique event by virtue of its exceptionally long duration and noontime occurrence over the present observational site. It has been observed that during the course of the eclipse, MT underwent significant enhancement (∼35 K). This increase in temperature was found to be present throughout the region of coverage of the photometer, i.e., ±200 km centered at Trivandrum in the meridional direction. It has also been found that the enhancement in MT revealed a preferential northward/southward movement before/after the main phase of the eclipse. In addition to this, a sudden launch of gravity waves with periodicity of ∼30 min was noticed during the first contact, which was amplified during the maximum phase of the eclipse. Another noteworthy observation was the amplification of a ∼2 hour wave during the course of the eclipse. These observations and their plausible mechanisms are discussed in detail in context of the daytime hydroxyl emissions and mesopause energetics and dynamics.

Description: Ground-based Global Positioning System (GPS) measurements of ionospheric total electron content (TEC) show variations consistent with atmospheric internal gravity waves caused by ocean tsunamis following two recent seismic events: the Samoa earthquake of 29 September 2009 and the Chile earthquake of 27 February 2010. Both earthquakes produced ocean tsunamis that were destructive to coastal communities near the epicenters, and both were observed in tidal gauge and buoy measurements throughout the Pacific Ocean. We observe fluctuations in TEC correlated in time, space, and wave properties with these tsunamis using the Jet Propulsion Laboratory's Global Ionospheric Mapping software. These TEC measurements were band-pass filtered to remove ionospheric TEC variations with wavelengths and periods outside the typical range for tsunamis. Observable variations in TEC appear correlated with the tsunamis in some locations (Hawaii and Japan), but not in others (Southern California or near the epicenters). Where variations are observed, the typical amplitude tends to be ∼0.1–0.2 TEC units for these events, on the order of ∼1% of the background TEC value. These observations are compared to estimates of expected tsunami-driven TEC variations produced by Embry Riddle Aeronautical University's Spectral Full Wave Model, an atmosphere-ionosphere coupled model, and are found to be in good agreement. Significant TEC variations are not always seen when a tsunami is present, but in these two events the regions where a strong ocean tsunami was observed coincided with clear TEC observations, while a lack of clear TEC observations coincided with smaller sea surface height amplitudes. There exists the potential to apply these detection techniques to real-time GPS TEC data, providing estimates of tsunami speed and amplitude that may be useful for early warning systems.

Description: Multipoint spacecraft observations provide unique opportunities to constrain the propagation and evolution of interplanetary coronal mass ejections (ICMEs) throughout the heliosphere. Using Mars Global Surveyor (MGS) data to study both ICME and solar energetic particle (SEP) events at Mars and OMNI and Geostationary Operational Environmental Satellite (GOES) data to study ICMEs and SEPs at Earth, we present a detailed study of three CMEs and flares in late November 2001. In this period, Mars trailed Earth by 56° solar longitude so that the two planets occupied interplanetary magnetic field lines separated by only ∼25°. We model the interplanetary propagation of CME events using the ENLIL version 2.6 3-D MHD code coupled with the Wang-Sheeley-Arge version 1.6 potential source surface model, using Solar and Heliospheric Observatory (SOHO) Large Angle and Spectrometric Coronagraph (LASCO) images to determine CME input parameters. We find that multipoint observations are essential to constrain the simulations of ICME propagation, as two very different ICMEs may look very similar in only one observational location. The direction and width of the CME as parameters essential to a correct estimation of arrival time and amplitude of the ICME signal. We find that these are problematic to extract from the analysis of SOHO/LASCO images commonly used for input to ICME propagation models. We further confirm that MGS magnetometer and electron reflectometer data can be used to study not only ICME events but also SEP events at Mars, with good results providing a consistent picture of the events when combined with near-Earth data.

Description: We have extended our earlier calculations of the distance to the heliospheric termination shock (HTS), which covered the period from the launch of V1 and V2 in 1977 to 2005, to the period from 2006 to 2011. During this latter period, the solar wind speed, ram pressure, and magnetic field decreased to the lowest levels in recent history, related to the sunspot minimum in 2008–2009. The HTS distance has decreased correspondingly so that V1, which was crossed by the HTS at 94 AU in late 2004, would now, in early 2011, be expected to reach the HTS at a distance of ∼80 AU, when the HTS distance would be expected to be at its minimum. Similarly, V2, which was crossed by the HTS at 84 AU in mid-2007, would, in early 2011, reach the HTS at a distance of only 74 AU. These distances, in early 2011, are ∼15% less than those at which V1 and V2 initially reached the HTS. The distance to the heliopause (HP) is more uncertain, but recent calculations place its equilibrium distance at between 1.4 and 1.6 times the HTS distance. Allowing for an additional 1 year for the HP to reach its equilibrium minimum distance relative to the HTS would mean that, assuming this distance remains a constant fraction larger than the HTS distance, the HP distance would be at its minimum distance of (1.4–1.6) × 80 AU = 112–128 AU at V1 in early 2012. At this time, V1 will be in the direction of a distance of ∼120 AU so that there is a possibility that V1 could cross the HP and enter interstellar space at the time 2012.0 ± 1 year. If the crossing does not happen during this time period, then it is unlikely that V1 will reach this defining boundary before about 2016 because of the expected outward motion of the HTS and the HP toward their more normal distances of 85–96 and ∼120–140 AU, coincident with the maximum of the new sunspot cycle.

Description: We have analyzed Cluster magnetic field and plasma data during high-altitude cusp crossing on 14 February 2003. Cluster encountered a diamagnetic cavity (DMC) during northward interplanetary magnetic field (IMF) conditions, and as IMF rotated southward, the spacecraft reencountered the cavity more at the sunward side. The DMC is characterized by a high level of magnetic field fluctuations and high-energy electrons and protons. Ultralow-frequency turbulence has been suggested as a mechanism to accelerate particles in DMC. We demonstrate in this paper for the first time that many of the low-frequency fluctuations in the cavity are back and forth motion of the DMC boundaries over the spacecraft and transient reconnection signatures. We also find examples of some isolated high-amplitude waves that could possibly be nonlinear kinetic magnetosonic modes. The lack of strong wave power at the vicinity of local ion cyclotron frequency in the DMC suggests that perhaps a mechanism other than wave-particle heating is a dominant source for ion heating in DMCs.

Description: Knowledge of multidimensional correlation functions is crucial for understanding the anisotropy of turbulence. The two-dimensional (2-D) spatial correlation functions (SCFs) obtained in previous studies of space plasma turbulence were restricted to large-length scales and covered a limited angular domain of the two-point separation vector with respect to the mean magnetic field. Here we aim to derive 2-D SCFs with smaller-length scale and nearly full angular distribution for the fluctuations of the number density and magnetic field in magnetosheath turbulence. We use the Cluster four-spacecraft measurements of the fluctuations with respect to a temporally and spatially varying background magnetic field to construct the 2-D SCFs. We find that the correlation function of the density fluctuations shows a pattern similar to that of the magnetic field fluctuations, both of which appear to be composed of two populations, whereby the major population extends along the coordinate parallel to mean magnetic field (S$\parallel$) and the minor one deviates toward the perpendicular coordinate (S$\perp$). This pattern of 2-D SCFs implies that the energy of magnetosheath turbulence seems to cascade, in the inertial range close to the ion scale, mostly transverse to the background magnetic field and meanwhile partly along the field (i.e., k$\perp$ $\gg$ k$\parallel$).

Description: Modulation of whistler mode chorus waves, which plays an important role in driving the pulsating aurora and other processes related to energetic electron dynamics, is an interesting but a long-standing unresolved problem. Here we utilize in situ observations from the THEMIS spacecraft to investigate the role of density variations in the modulation of the chorus wave amplitude, which forms a complementary study to the modulation of chorus by compressional Pc4–5 pulsations presented in a companion paper. We show that these density variations are correlated remarkably well with modulated chorus intensity and typically occur on a timescale of a few seconds to tens of seconds. Both density depletions (DD) and density enhancements (DE) are frequently correlated with increases in chorus wave amplitudes. Furthermore, density enhancements cause a lowering of the central frequencies of the generated chorus waves and vice versa. DD events are more likely to be related to quasi-periodic chorus emissions and thus may be related to the generation of the pulsating aurora. A systematic survey of both DD and DE events shows that DD events preferentially occur between premidnight and dawn, whereas DE events dominantly occur from dawn to noon. We also evaluate the growth rates of chorus waves using linear theory for both DD and DE events and show that both density depletions and enhancements can lead to an intensification of chorus wave growth. However, other potential mechanisms for chorus intensification caused by density variations such as wave trapping by density crests and troughs cannot be excluded.

Description: ELF/VLF waves have been generated via steerable HF heating of the lower ionosphere. The temperature-dependent conductivity of the lower ionospheric plasma enables HF heating (and subsequent recovery) to modulate natural current systems such as the auroral electrojet, thus generating an antenna embedded in the ionospheric plasma. We apply a realistic three-dimensional model of HF heating and ionospheric recovery, as well as ELF/VLF wave propagation in and below the ionosphere, to derive the radiation pattern into the magnetosphere as a result of steerable HF heating. It is found that modulated HF heating preferentially directs signals upward into space because of the phasing effect of the upward HF wave propagation. We find that the steering techniques such as the geometric modulation “circle sweep” enhances the total ELF/VLF power injected into the magnetosphere by 5–7 dB compared to amplitude modulated heating, with a few dB enhancement in the peak magnetic field value. Another technique known as beam painting enhances the total injected power by 1–3 dB but produces weaker peak magnetic fields due to the power being spread over a larger area. Observations on the DEMETER spacecraft are presented and compared with theoretical predictions. DEMETER observations show that the signal produced with geometric modulation can be stronger than the signal from AM under the same conditions.

Description: For nearly a century it has been known that the tendency for geomagnetic activity is, on average, higher at the equinoxes than at the solstices. Previous studies on semiannual geomagnetic activity were performed mainly for geomagnetic indices such as am, aa, U, and AL. Thus, we need to understand the seasonal variation of geomagnetic activity defined by the Dst index over the long term. It is also necessary to test the solar magnetic polarity dependence of geomagnetic activity. This paper is a statistical analysis of the geomagnetic storms defined by the Dst index. Our storm data consists of two sets of storm data for 5 years at each solar minimum during the four solar cycles (19–22) from 1962 through 1998 for two of each solar magnetic polarity. The storms are divided into two groups defined by Dst index (Dst(min) 〈 −50 nT, ∣Dst∣ 〉 70 nT; Dst(min) 〈 −50 nT, ∣Dst∣ 〉 90 nT). Monthly occurrences of these storms are compared. Storms of ∣Dst∣ 〉 90 nT and of ∣Dst∣ 〉 70 nT occurred 153 and 238 times, respectively, during the testing periods. Storms occurred more frequently during the spring and fall seasons for all solar cycle minima, regardless of solar magnetic polarity.

Description: A widely accepted explanation of the location of the inner edge of the electron plasma sheet and its dependence on electron energy is based on drift motions of individual particles. The boundary is identified as the separatrix between drift trajectories linking the tail to the dayside magnetopause (open paths) and trajectories closed around the Earth. A statistical study of the inner edge of the electron plasma sheet using THEMIS Electrostatic Analyzer plasma data from November 2007 to April 2009 enabled us to examine this model. Using a dipole magnetic field and a Volland-Stern electric field with shielding, we find that a steady state drift boundary model represents the average location of the electron plasma sheet boundary and reflects its variation with the solar wind electric field in the local time region between 21:00 and 06:00, except at high activity levels. However, the model does not reproduce the observed energy dispersion of the boundaries. We have also used the location of the inner edge of the electron plasma sheet to parameterize the potential drop of the tail convection electric field as a function of solar wind electric field (Esw) and geomagnetic activity. The range of Esw examined is small because the data were acquired near solar minimum. For the range of values tested (meaningful statistics only for Esw 〈 2 mV/m), reasonably good agreement is found between the potential drop of the tail convection electric field inferred from the location of the inner edge and the polar cap potential drop calculated from the model of Boyle et al. (1997).

Description: An earthquake of magnitude 9.0 occurred near the east coast of Honshu (Tohoku area), Japan, producing overwhelming Earth surface motions and inducing devastating tsunamis, which then traveled into the ionosphere and significantly disturbed the electron density within it (hereafter referred to as seismotraveling ionospheric disturbances (STIDs)). The total electron content (TEC) derived from nationwide GPS receiving networks in Japan and Taiwan is employed to monitor STIDs triggered by seismic and tsunami waves of the Tohoku earthquake. The STIDs first appear as a disk-shaped TEC increase about 7 min after the earthquake occurrence centered at about 200 km east of the epicenter, near the west edge of the Japan Trench. Fast propagating disturbances related to Rayleigh waves quickly travel away from the epicenter along the main island of Japan with a speed of 2.3–3.3 km/s, accompanied by sequences of concentric circular TEC wavefronts and followed by circular ripples (close to a tsunami speed of about 720–800 km/h) that travel away from the STID center. These are the most remarkable STIDs ever observed where signatures of Rayleigh waves, tsunami waves, etc., simultaneously appear in the ionosphere.

Description: The specific entropy (entropy density) S is examined for the outer electron radiation belt at geosynchronous orbit and for the energetic electron population in the Earth's magnetotail. The outer electron radiation belt is measured with the SOPA detectors on board six geosynchronous satellites and the energetic electrons of the magnetotail are measured with instrumentation on board 12 Global Positioning Satellites (GPS) with a magnetic field model used to map the GPS orbit to the magnetotail. Density n and temperature T values are determined from relativistic Maxwellian fits to the electron measurements, enabling the specific entropy S to be calculated. For low temperatures the nonrelativstic specific entropy is S = T/n2/3; for a relativistic Maxwellian distribution a relativistically correct expression for S = S(T,n) is derived and used. The outer electron radiation belt at geosynchronous orbit local midnight (n ∼ 3 × 10−4 cm−3 and T ∼ 140 keV) and the energetic-electron population in the magnetotail (n ∼ 1 × 10−4 cm−3 and T ∼ 50 keV) statistically have the same specific entropy. Hence the two populations are probably the same. This implies adiabatic transport (1) from the magnetotail to the dipole (where the magnetotail electrons are the source of the outer electron radiation belt) or (2) from the dipole to the magnetotail (where the magnetotail electrons are leakage from the radiation belt).

Description: Solar wind dynamic pressure (Pdyn) enhancements have been observed to cause large-scale auroral brightening. The mechanism for this kind of auroral brightening is still a topic of current space research. Using the global piecewise parabolic method Lagrangian remap (PPMLR)-MHD simulation model, we investigate three auroral brightening events caused by dynamic pressure enhancement under different interplanetary magnetic field (IMF) conditions: (1) Bz 〈 0 and By 〉 0 on 11 August 2000, (2) Bz 〈 0 and By 〈 0 on 8 May 2001, and (3) Bz ≥ 0 on 21 January 2005. We show that the auroral location depends on the IMF conditions. Under southward IMF conditions, when By is negative, the duskside aurora is located more equatorward at around 70° magnetic latitude (MLAT) for all magnetic local times; when By is positive, the duskside aurora can even reach beyond 80° MLAT. A smaller and more localized response is seen when the IMF Bz is nearly zero or northward, as shown in previous studies. Our simulation results are consistent with these observations, indicating that the observed aurora activities could be caused by solar wind dynamic pressure enhancements. The simulation results suggest that the enhancement of Pdyn can increase the ionospheric transpolar potential and the corresponding field-aligned currents, leading to the observed auroral brightening.

Description: On 18 February 2003 a long-duration, almost monochromatic, wave event was detected by Cluster at radial distances from the bow shock smaller than ≈10 Re. The interplanetary magnetic field orientation determined a wide and almost symmetric foreshock region around the bow shock nose, providing highly favorable conditions for a direct wave penetration into the magnetosphere. The general correspondence between the characteristics of the wave trains observed in the foreshock region and at two widely separated ground stations (namely, at low latitudes in the Northern Hemisphere and in Antarctica) confirms that the ground activity is ubiquitously determined and controlled by the upstream activity. At low latitudes, the direct propagation of the external waves through the subsolar point is sharply confined to the dayside hemisphere. In Antarctica, the wave activity is detected even on field lines stretched into the tail; here the polarization pattern suggests a significant contribution of sunward propagating waves in the postmidnight hours, consistent with a wave penetration through the magnetotail lobes. At both stations the wave energy is typically ≈5–10% of the external energy; in addition, in Antarctica the contribution of the cusp turbulence largely overcomes that one related to the penetration of the upstream waves.

Description: In this paper, we investigate the role of plasma sheet bubbles in the ion flux variations at geosynchronous orbit during substorm injections by using the Rice Convection Model with an equilibrated magnetic field model (RCM-E). The bubble is initiated in the near-Earth plasma sheet with a localized reduction in entropy parameter PV5/3 following a substorm growth phase. In the expansion phase, characteristic features of substorm injections are reproduced; that is, there is a prominent dispersionless flux increase for energetic protons (〉40 keV) and a flux decrease for lower-energy protons near midnight geosynchronous orbit while there is dispersive flux enhancement near the dusk sector. We find that the injection boundary is well coincident with the earthward boundary of the bubble, inside which the depletion of plasma content causes the magnetic field dipolarization, and in return, the magnetic field collapse energizes particles and alters the drift paths dramatically. Our results also show that a high-PV5/3 island is pushed ahead of the fast earthward propagating bubble, and a dipolarization front forms between them. Within the high-PV5/3 island, the diamagnetic effect makes the plasma pressure increase and the strength of the magnetic field decrease to a local minimum. We suggest that plasma sheet bubbles are elementary vehicles of substorm time particle injections from the main plasma sheet to the inner magnetosphere.

Description: The thermospheric response at satellite altitudes along low Earth orbit is subject to the energy deposition locally, i.e., at high altitudes, and the vertical wave propagation from the energy injection at lower altitudes. In this study, a general circulation model has been run to investigate the source of nonhydrostatic effects and the sensitivity of the vertical wind and neutral density at satellite orbits to the energy deposited at low and high altitudes. Through comparing the simulations with and without the Joule heating enhancement above 150 km altitude, the impact of the heating at low and high altitudes on the high-altitude thermosphere has been separated. The numerical simulations show that most of the nonhydrostatic effects at high altitudes (300 km) arise from sources below 150 km and propagate vertically through the acoustic wave. The heating above 150 km is responsible for a large increase of the average vertical velocity (40 m/s) and neutral density (50%) at 300 km and higher altitudes.

Description: We have developed a numerical model of a double-probe electric field sensor equipped with a photoelectron guard electrode for the particle-in-cell simulation. The model includes typical elements of modern double-probe sensors on, e.g., BepiColombo/MMO, Cluster, and THEMIS spacecraft, such as a conducting boom and a preamplifier housing called a puck. The puck is also used for the guard electrode, and its potential is negatively biased by reference to the floating spacecraft potential. We apply the proposed model to an analysis of an equilibrium plasma environment around the sensor by assuming that the sun illuminates the spacecraft from the direction perpendicular to the sensor deployment axis. As a simulation result, it is confirmed that a substantial number of spacecraft-originating photoelectrons are once emitted sunward and then fall onto the puck and sensing element positions. In order to effectively repel such photoelectrons coming from the sun direction, a potential hump for electrons, i.e., a negative potential region, should be created in a plasma region around the sunlit side of the guard electrode surface. The simulation results reveal the significance of the guard electrode potential being not only lower than the spacecraft body but also lower than the background plasma potential of the region surrounding the puck and the sensing element. One solution for realizing such an operational condition is to bias the guard potential negatively by reference to the sensor potential because the sensor is usually operated nearly at the background plasma potential.

Description: In a recent observation by the Cluster spacecraft, emissions triggered by electromagnetic ion cyclotron (EMIC) waves were discovered in the inner magnetosphere. We perform hybrid simulations to reproduce the EMIC triggered emissions. We develop a self-consistent one-dimensional hybrid code with a cylindrical geometry of the background magnetic field. We assume a parabolic magnetic field to model the dipole magnetic field in the equatorial region of the inner magnetosphere. Triggering EMIC waves are driven by a left-handed polarized external current assumed at the magnetic equator in the simulation model. Cold proton, helium, and oxygen ions, which form branches of the dispersion relation of the EMIC waves, are uniformly distributed in the simulation space. Energetic protons with a loss cone distribution function are also assumed as resonant particles. We reproduce rising tone emissions in the simulation space, finding a good agreement with the nonlinear wave growth theory. In the energetic proton velocity distribution we find formation of a proton hole, which is assumed in the nonlinear wave growth theory. A substantial amount of the energetic protons are scattered into the loss cone, while some of the resonant protons are accelerated to higher pitch angles, forming a pancake velocity distribution.

Description: We analyze the heliocentric evolution of fast interplanetary counterparts of coronal mass ejections (ICMEs) and their transient shocks to investigate how and where they decelerate in the interplanetary medium. We employ two one-dimensional hydrodynamic models, analytic and numeric, to study three fast CME events. We focus on the transferring of momentum from the ICME to the shock. The two models show that initially the fast ICME propagates at about a constant speed and drives the shock (driving stage) until it reaches a certain distance from which it decelerates and decouples from the shock (decoupling process). Then the ICME and its shock decelerate (decaying stage). This deceleration depends on the speed difference with respect to the ambient wind and tends to a negligible value when the ICME-shock approaches to the ambient wind speed. The location and duration of these propagation stages depend on the initial CME conditions and the ambient wind characteristics. We present a parametric study to compare the results by the analytic and numeric models, showing the variations of their results as a function of the initial conditions. We perform three study cases to compare the model's predictions with a set of speed measurements of ICME-shock events.

Description: We discuss results of a superposed epoch analysis of dipolarization fronts, rapid (δt 〈 30 s), high-amplitude (δBz 〉 10 nT) increases in the northward magnetic field component, observed during six Time History of Events and Macroscale Interactions during Substorms (THEMIS) conjunction events. All six fronts propagated earthward; time delays at multiple probes were used to determine their propagation velocity. We define typical magnetic and electric field and plasma parameter variations during dipolarization front crossings and estimate their characteristic gradient scales. The study reveals (1) a rapid 50% decrease in plasma density and ion pressure, (2) a factor of 2–3 increase in high-energy (30–200 keV) electron flux and electron temperature, and (3) transient enhancements of ∼5 mV/m in duskward and earthward electric field components. Gradient scales of magnetic field, plasma density, and particle flux were found to be comparable to the ion thermal gyroradius. Current densities associated with the Bz increase are, on average, 20 nA/m2, 5–7 times larger than the current density in the cross-tail current sheet. Because j · E 〉 0, the dipolarization fronts are kinetic-scale dissipative regions with Joule heating rates of 10% of the total bursty bulk flow energy.

Description: The determination of the internal magnetic field of Jupiter has been the object of many studies and publications. These models have been computed from the Pioneer, Voyager, and Ulysses measurements. Some models also use the position of the Io footprints as a constraint: the magnetic field lines mapping to the footprints must have their origins along Io's orbit. The use of this latter constraint to determine the internal magnetic field models greatly improved the modeling of the auroral emissions, in particular the radio ones, which strongly depends on the magnetic field geometry. This constraint is, however, not sufficient for allowing a completely accurate modeling. The fact that the footprint field line should map to a longitude close to Io's was not used, so that the azimuthal component of the magnetic field could not be precisely constrained. Moreover, a recent study showed the presence of a magnetic anomaly in the northern hemisphere, which has never been included in any spherical harmonic decomposition of the internal magnetic field. We compute a decomposition of the Jovian internal magnetic field into spherical harmonics, which allows for a more accurate mapping of the magnetic field lines crossing Io, Europa, and Ganymede orbits to the satellite footprints observed in UV. This model, named VIPAL, is mostly constrained by the Io footprint positions, including the longitudinal constraint, and normalized by the Voyager and Pioneer magnetic field measurements. We show that the surface magnetic fields predicted by our model are more consistent with the observed frequencies of the Jovian radio emissions than those predicted by previous models.

Description: The equatorial mass density anomaly (EMA) is a fascinating phenomenon in the equatorial upper atmosphere. In this study, we investigate the generation mechanism of the EMA using the ground-to-topside model of the atmosphere and ionosphere for aeronomy (GAIA). The GAIA model is a self-consistent global model of the atmosphere and ionosphere covering the height range from the ground surface to the exobase. It can reproduce the observed EMA structure at 300–400 km heights. Our results show that the EMA structure can extend down to 200 km height. The EMA during daytime is caused by the in situ diurnal tide and the upward propagating terdiurnal tide. About half of the magnitude of the EMA is generated by the upward propagating terdiurnal tide from the lower atmosphere. This is the first report concerning the importance of the upward propagating tide for EMA formation. The in situ diurnal tide in the thermosphere is also essential for EMA formation. The in situ diurnal tide is modified by the momentum exchange between the plasma and the neutral atmosphere. This is seen as the enhanced upward flow of the neutral atmosphere along the dip equator in the 200–400 km height region, which has a profound effect on the latitudinal distributions of the atmospheric composition, temperature, pressure, and density in the thermosphere.

Description: A test particle simulation has been performed in order to reproduce the interaction between protons and time-dependent electromagnetic fluctuations in the magnetotail current sheet. The three-dimensional model takes into account a dawn-dusk electric field component E0y, a magnetic field reversal Bx(z), and a constant component Bn along the z direction. Electromagnetic perturbations are generated by random oscillating “clouds” moving in the (x, y) plane. The simultaneous presence of stochastic time-dependent fluctuations and 3-D large-scale fields gives rise to a rich variety of phenomena that are studied by varying the size and shape of the oscillating clouds. The main findings are as follows: (1) the efficiency of the Fermi-like interaction between test particles and moving clouds is largest close to z = 0, where the large-scale magnetic field is weakest; (2) in times of 1–2 min protons can reach energy values up to 50–70 keV, which is beyond the maximum potential drop due to the presence of E0y; (3) the ion energization grows with the size of the magnetic clouds, while it is slightly influenced by the thickness of the clouds along z, confirming that the main acceleration takes place in the quasi-neutral sheet; and (4) assuming parameters of the model corresponding to those of the Earth's magnetotail, it is found that many observed features, such as the formation of a beam in velocity space and non-Gaussian velocity distributions, can be reproduced.

Description: In this paper we present the global spatial structure and seasonal variability of the COSMIC electron density response to the forced from below DE3 and DE2 tides during the period January 2008 to March 2009. The COSMIC electron density at fixed altitudes and SABER temperatures were utilized in order to define the ionospheric DE3 and DE2 tidal response to the DE3 and DE2 temperature tides coming from below. The tidal waves from both data sets have been extracted by the same data analysis method. The latitude structure of the ionospheric response for heights up to 450–500 km is distributed on both sides of the dip equator at about ±30° modified dip latitude; above 500 km the response is approximately confined over the equator. Two altitude regions of enhanced ionospheric response are found: an upper level response (above 300 km) and a bottom level one (below 250 km); the two regions are separated by a narrow altitude zone with no tidal response. The different phase structure of the ionospheric response in both altitude regions suggests that (1) the upper level response is mainly due to the DE3/DE2 modulated vertical plasma drift and (2) the bottom level response is complex and, in all likelihood, is caused by more than one mechanism.

Description: The inversion algorithm for Estimating Model Parameters from Ionospheric Reverse Engineering (EMPIRE) has been created to gain insight into ionospheric dynamics, particularly when direct measurement is unavailable. We extend the capabilities of EMPIRE here, in order to demonstrate its effectiveness on densities obtained from real data. We apply this method to real Ionospheric Data-Assimilation 4-Dimensional (IDA4D) data from storm time measurements, focusing on the midlatitude F2 layer. EMPIRE is used to estimate midlatitude field-aligned and field-perpendicular drifts. The estimated upward drifts from EMPIRE are validated against measurements obtained from the Millstone Hill incoherent scatter radar zenith antenna. The horizontal $\vec {E}$ × $\vec {B}$ drifts are compared to the assimilative mapping of ionospheric electrodynamics (AMIE) model, which estimates drifts from data sources independent of those used in IDA4D. Results show that the direction and magnitude of the $\vec {E}$ × $\vec {B}$ drifts (and therefore the electric fields) may be deduced from imaging based primarily on total electron content data, although altitude variation is not significantly discernible. We also indicate that the initial uplift of the storm-enhanced density may have been more strongly influenced by field-aligned contributions of neutral winds and diffusion than the electric fields.

Description: Long-term observations of proton cyclotron waves in the upstream region of Venus raise the question of under which general solar wind conditions these waves are generated and maintained. The waves are characterized by their occurrence at the local proton cyclotron frequency and left-hand polarization, both in the spacecraft frame. Magnetometer data of the Venus Express spacecraft for two Venus years of observations are analyzed before, during, and after the occurrence of these waves. The configuration of the upstream magnetic field and the solar wind velocity is investigated, to study if the waves are generated from a ring distribution of pickup ions in velocity space or from a parallel pickup ion beam, i.e., for quasi-parallel conditions of solar wind velocity and magnetic field when the solar wind motional electric field is weak. It is found that stable and mainly quasi-parallel magnetic field conditions for up to ∼20 min prior to wave observation are present, enabling sufficient ion pickup and wave growth to obtain observable waves in the magnetometer data. Persistent waves occur mainly under quasi-parallel conditions. This is in agreement with linear theory, which predicts efficient wave growth for instabilities driven by field-aligned planetary ion beams, already for low pickup ion density. The occurrence of highly coherent waves at 4 RV upstream toward the Sun implies that planetary neutral hydrogen is initially picked up at least 5 RV toward the Sun from a sufficiently dense Venus hydrogen exosphere.

Description: A new self-consistent and kinetic model for ring current particles in the inner magnetosphere is presented. A closed set of nonlinear time evolution equations is derived that incorporates kinetic particle dynamics and self-consistent development of the electromagnetic field. The particle transport is described by a five-dimensional collisionless drift kinetic equation, in which particle trajectories are approximated by their guiding centers under the influence of a time-dependent electromagnetic field. The time evolution of the electromagnetic field follows the Maxwell equations with the feedback from particles through electric currents. A numerical simulation code solving the system of equations in a global inner magnetosphere in three spatial dimensions (or five dimensions in phase space) is developed. It is demonstrated that the propagation of magnetohydrodynamic waves can successfully be described by the present model. It is also found that the self-consistent coupling could affect the transport of energetic particles especially at low energies as well as the intensity and spatial distribution of field-aligned currents. These preliminary results suggest the importance of the self-consistent coupling in the global development of geomagnetic storms.

Description: Earlier studies of Saturn's inner ionic radiation belts revealed that their content was surprisingly constant while their evolution appeared decoupled from dynamics of the Saturnian magnetosphere. Saturn's icy moons in combination with the neutral gas and dust that surround the planet seem to effectively restrict radial transport of energetic ions and are responsible for all these unusual characteristics. A possible process through which MeV ions may be populating the regions between the icy moons is cosmic ray albedo neutron decay (CRAND). While some circumstantial evidence suggests that this process actually occurs, the concept of CRAND has only been applied to the proton energy spectrum above ∼10 MeV; the source of ions below 10 MeV is not yet obvious. Additional hints about the nature of this source are now becoming evident by monitoring Saturn's radiation belts about half a solar cycle (from the declining phase of the solar maximum to solar minimum). Using Cassini's magnetosphere imaging instrument and low-energy magnetospheric measurement system (MIMI/LEMMS) data from June 2004 to June 2010, we detect a weak intensification of the trapped proton component that probably originates from CRAND (〉10 MeV). This anticipated enhancement, due to the solar cycle modulation of the galactic cosmic ray influx at Saturn, is closely followed by ions in the 1–10 MeV range. This observation sets constraints on the nature of those ions' source: this source should be connected (directly or indirectly) to the access of galactic cosmic rays in the Saturnian system. We also find evidence indicating that the ionic belts experience short-term variability following the occurrence of solar energetic particle events at Saturn's distance, probably associated with coronal mass ejections that propagate in the heliosphere. LEMMS data contain clear evidence of Earth-like Forbush decreases following such events. These decreases may explain the lack of an (expected) ionic belt intensification between 2004 and 2006.

Description: We here report the first dual-spacecraft detection of planetary flux ropes in the ionosphere of Mars. The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board Mars Express, can measure the magnetic field magnitude near the spacecraft. Typically, these measurements track the known crustal magnetic field strength very well; however, occasionally, MARSIS detects transient, intense magnetic fields that deviate significantly from the known crustal fields. Two such magnetic field enhancements occur in near-coincidence with flux rope detections by the Mars Global Surveyor Magnetometer and Electron Reflectometer, which provides vector magnetic field measurements, allowing us to clearly identify the enhancements as flux ropes. The flux ropes detected are quasi-stable for at least a half hour, have peak magnetic field strengths of ∼50 and 90 nT, and are ∼650–700 km in diameter. Both occur downstream of the region of strong crustal fields. In addition, MARSIS has detected 13 other magnetic enhancements over a 5 year period, which we infer to be flux ropes. These structures have peak field strengths up to 130 nT and measured horizontal dimensions of several hundred to over a thousand kilometers. They are clustered around the intense crustal fields in the southern hemisphere of Mars. The large spatial scale of these flux ropes distinguishes them from small-scale flux ropes, with diameters of tens of kilometers, that have been seen in the ionospheres of Venus and Mars. These large-scale flux ropes are believed to be caused by solar wind stretching and shearing of the Martian crustal fields.

Description: Electron measurements on board six spacecraft in geosynchronous orbit are superposed-epoch analyzed for 42 high-speed-stream-driven storms. Using pitch angle–resolved fluxes in the range 30 keV to 1.7 MeV, the evolution of the outer electron radiation belt and the suprathermal tail of the electron plasma sheet are studied. The outer electron radiation belt exhibits perpendicular-dominated anisotropies on the dayside and parallel-dominated anisotropies on the nightside consistent with shell splitting in a distorted magnetosphere. The magnitudes of the radiation-belt anisotropies are weak prior to storm onset and become very large during the storms. The magnitudes of the anisotropies lessen with time as the storm ages and the radiation belt heats, probably owing to a weakening of the magnetic field distortion as the storm ages. When a calm before the storm occurs, the dayside radiation belt approaches isotropy, probably owing to pitch angle scattering in the outer plasmasphere that fills during the calm. If no calm before the storm occurs, the dayside radiation belt is strongly perpendicular dominated. The local-time pattern of anisotropy in storms is very different for the suprathermal tail of the electron plasma sheet, which tends to be perpendicular on the nightside and isotropic elsewhere. The magnitudes of the anisotropies of the suprathermal tail are a factor of ∼10 weaker than the anisotropies of the outer electron radiation belt.

Description: A new method utilizing stochastic inversion in determining the electric field and neutral wind from monostatic beam swing incoherent scatter measurements is described. The method consists of two stages. In the first stage, beam-aligned ion velocities from a chosen F region height interval and a set of subsequent beam directions are taken as measurements. The unknowns are the two electric field components and the field-aligned ion velocity profile. The solution gives the most probable values of the unknowns with error estimates. In the second stage, the measurements consist of beam-aligned ion velocities from the E region, and the electric fields given by the first inversion problem are also used as measurements. The number of applied beam directions may be greater than in the first inversion problem. This is a feasible approach since the neutral wind usually changes more slowly than the electric field. The solution of the second inversion problem gives the most probable values of the three neutral wind components. Results of the method are shown for 11 September 2005, when the European Incoherent Scatter (EISCAT) UHF radar was running in the CP2 experiment mode, which is a four-position 6 min monostatic cycle. In addition, from each beam direction a tristatic measurement at one F region range gate was made using two additional receivers. That allowed comparison between the monostatic and tristatic electric field results, which were in excellent agreement. The calculated neutral wind components were in good accordance with previous measurements during disturbed conditions from the same site.

Description: We made an effort to understand the associations and relationships between ground level enhancement (GLE) events and solar flares for the time period of 1986–2006. Our results show that, on average, the GLE event–associated solar flare (∼0.2 × 10−4 W/m2) is much stronger than the non-GLE-associated solar flare (∼0.3 × 10−5 W/m2). The findings have also been supported by the solar flare indices that, on average, the GLE event–associated solar flare index (∼35.01) is much higher than the non-GLE-associated solar flare index (∼4.88). However, this association does not seem to precisely imply that GLEs can occur because of a solar flare, so we examined cross correlations between GLE events and simultaneous solar flares. We found that most (∼78%) of the highest correlations (r 〉 0.8) took place during an X class flare. There is no clear indication that the more the time lag, the less or more is the correlation or vice versa. Overall, 50% of the high correlations took place at higher time lag (≥65 min), and ∼36% of the high correlations took place at lower time lag (≤40 min), while the rest (∼14%) of the correlations were abruptly high and low at medium time lag (〉40 and

Description: A three-dimensional (3-D) time-dependent, numerical magnetohydrodynamic (MHD) model with asynchronous and parallel time-marching method is used to investigate the propagation of coronal mass ejections (CMEs) in the nonhomogenous background solar wind flow. The background solar wind is constructed based on the self-consistent source surface with observed line-of-sight of magnetic field and density from the source surface of 2.5 Rs to the Earth's orbit (215 Rs) and beyond. The CMEs are simulated by means of a very simple flux rope model: a high-density, high-velocity, and high-temperature magnetized plasma blob is superimposed on a steady state background solar wind with an initial launch direction. The dynamical interaction of a CME with the background solar wind flow between 2.5 and 220 Rs is investigated. The evolution of the physical parameters at the cobpoint, which is located at the shock front region magnetically connected to ACE spacecraft, is also investigated. We have chosen the well-defined halo-CME event of 4–6 April 2000 as a test case. In this validation study we find that this 3-D MHD model, with the asynchronous and parallel time-marching method, the self-consistent source surface as initial boundary conditions, and the simple flux rope as CME model, provide a relatively satisfactory comparison with the ACE spacecraft observations at the L1 point.

Description: On the basis of the GCITEM-IGGCAS model and tides from TIMED/SABER observations, the longitudinal variations in the lower thermosphere nitric oxide (NO), which is induced by nonmigrating tides, are investigated. We simulate the intra-annual variation of the NO density and find that equinoctial lower thermospheric NO density shows an obvious wave number 4 longitudinal structure both in equinox and in June solstice and a wave number 3 longitudinal structure in December solstice. These simulation results are consistent with the longitudinal variation observed by Oberheide and Forbes (2008b). The simulations support that the wave number 4 structure in NO density is mainly driven by the eastward propagating nonmigrating diurnal tide with zonal wave number 3, and the wave number 3 structure is mainly driven by the eastward propagating nonmigrating diurnal tide with zonal wave number 2. Our simulations also show that the NO density residuals and the neutral mass density residuals in the height range between 90 and 120 km agree well with each other, and the neutral mass density mainly affects the longitudinal variations of lower thermospheric NO density through modulation of the chemical production rate, e.g., through affecting the chemical reaction between excited nitrogen and molecular oxygen.

Description: Significant populations of electrons with energies of tens of keV appear in the Earth's inner central plasma sheet during the substorm expansion phase. Increasing observational evidence indicates that these injections begin at a radially narrow but azimuthally extended transition between very stretched and less stretched field lines around 8 RE. In this work we suggest that the tailward retreat of this transition region can be responsible for adiabatic acceleration of electrons which is sufficient to cause ionospheric signatures of the dispersionless injections observed by riometers. To support this mechanism we develop a novel conceptual magnetotail magnetic field model with a few adjustable parameters which can be easily constrained by observations. Our calculations show that a tailward motion of the transition region at the speed of 0.8 RE/min is required to achieve good agreement with riometer observations.

Description: In this paper, the total electron content (TEC) of the global ionosphere map (GIM) is used to detect seismoionospheric anomalies associated with the 12 January 2010 M7 Haiti earthquake, and an ionospheric model is applied to simulate the detected anomalies. The GIM temporal variation shows that the TEC over the epicenter significantly enhances on 11 January 2010, 1 day before the earthquake. The latitude-time-TEC (LTT) plots reveal three anomalies: (1) the northern crest of equatorial ionization anomaly (EIA) moves poleward, (2) the TECs at the epicenter and its conjugate increase, and (3) the TECs at two dense bands in the midlatitude ionosphere of 35°N and 60°S further enhance. The spatial analysis demonstrates that the TEC enhancement anomaly appears specifically and persistently in a small region of the northern epicenter area. The simulation well reproduces the three GIM TEC anomalies, which indicate that the dynamoelectric field of the ionospheric plasma fountain might have been perturbed by seismoelectric signals generated around the epicenter during the earthquake preparation period.

Description: We compare solar wind driving and its changes for three data sets: (1) 4861 identifications of substorm onsets from satellite global imagers (Polar UVI and IMAGE FUV); (2) a similar number of otherwise random times chosen with a similar solar wind distribution (slightly elevated driving); (3) completely random times. Multiple measures of solar wind driving were used, including interplanetary magnetic field (IMF) Bz, the Kan-Lee electric field, the Borovsky function, and dΦMP/dt (all of which estimate dayside merging). Superposed epoch analysis verifies that the mean Bz has a northward turning (or at least averages less southward) starting 20 min before onset. We argue that the delay between IMF impact on the magnetopause and tail effects appearing in the ionosphere is about that long. The northward turning is not the effect of a few extreme events. The median field shows the same result, as do all other measures of solar wind driving. We compare the rate of northward turning to that observed after random times with slightly elevated driving. The subsequent reversion to mean is essentially the same between random elevations and substorms. To further verify this, we consider in detail the distribution of changes from the statistical peak (20 min prior to onset) to onset. For Bz, the mean change after onset is +0.14 nT (i.e., IMF becomes more northward), but the standard deviation is σ = 2.8 nT. Thus large changes in either direction are common. For EKL, the change is −15 nT km/s ± 830 nT km/s. Thus either a hypothesis predicting northward turnings or one predicting southward turnings would find abundant yet random confirming examples. Indeed, applying the Lyons et al. (1997) trigger criteria (excluding only the prior requirement of 22/30 min Bz 〈 0, which is often not valid for actual substorms) to these three sets of data shows that “northward turning triggers” occur in 23% of the random data, 24% of the actual substorms, and after 27% of the random elevations. These results strongly support the idea of Morley and Freeman (2007), that substorms require initial elevated solar wind driving, but that there is no evidence for external triggering. Finally dynamic pressure, p, and velocity, v, show no meaningful variation around onset (although p averages 10% above an 11 year mean).

Description: We examine Millstone Hill incoherent scatter radar data collected over two solar cycles between 1979 and 2001 to determine average characteristics and features of storm time ion velocity and flux transport in the American sector midlatitude ionosphere. We use over 1100 radar azimuth scans identified as containing enhanced westward ion velocities associated with the subauroral polarization stream (SAPS), covering the 12–06 magnetic local time (MLT) sector and 50°–68° invariant latitude for weak to moderate disturbance levels with Dst from 50 to −200 nT. We find the magnetic latitude peak location of the SAPS flow channels decreases linearly with both Dst and MLT with a very good degree of correlation. We also examine for the first time SAPS peak westward ion fluxes, which transport material westward with magnitude between 3 × 1013 and 3 × 1014 m−2 s−1 in a manner nearly invariant to activity level. This invariance is maintained by an inverse relationship between electron density and ion velocity magnitudes with increasing Dst. Westward log ion flux and ion velocity are maximum in the dusk sector and decrease linearly with increasing MLT, smoothly varying across the dusk terminator. Finally, velocity distributions show that material in the afternoon SAPS flow is swept westward to earlier MLT values, delivering O+ flux to the cusp region. In contrast, SAPS streams in the post terminator sectors are fixed east-west in the Sun-Earth inertial frame, effectively maintaining entrained ion fluxes at the same MLT.

Description: We have analyzed Cluster magnetic field and plasma data during high-altitude cusp crossing and compared them with high-resolution MHD simulations. Cluster encountered a diamagnetic cavity (DMC) during northward interplanetary magnetic field (IMF) conditions, and as the IMF rotated southward, the spacecraft reencountered the cavity more at the sunward side of the cusp because the reconnection site had changed location. We found evidence of magnetic reconnection both during northward and southward IMF conditions. The Cluster separation was ∼5000 km, enabling for the first time measurements both inside the DMC and surrounding boundaries that allowed us to construct the structure of the DMC and put the observations of ion pitch angle distributions in context of local reconnection topology and gradients of the boundaries. The cavity is characterized by strong magnetic field fluctuations and high-energy particles. At the magnetosheath boundary the high-energy particle fluxes reduced by several orders of magnitude. Throughout the magnetosheath, the high-energy proton fluxes remained low except during brief intervals when sc4 and sc1 dropped back into the cavity due to changes in solar wind dynamic pressure. However, the high-energy O+ fluxes did not drop as much in the magnetosheath and were mostly at 60°–120° pitch angles, indicative of a trapped population in the DMC which is observed in the magnetosheath due to a large gyroradius. Significant fluxes of protons and ionized oxygen were also observed escaping from the diamagnetic cavity antiparallel to the magnetic field in a time scale more consistent with the local DMC source than with a reflected bow shock source.

Description: Variations in the dayside field-aligned current (FAC) density (J//), field-aligned parallel potential drop (Δ$\phi$//), peak precipitating electron energy (peak Ee), and precipitating electron energy flux ($\varepsilon$) as functions of solar wind (SW) and interplanetary magnetic field (IMF) are investigated with Defense Meteorological Satellite Program observations and a quasi-stationary low-latitude boundary layer (LLBL)–FAC coupling model. Region 1 (R1) J// responses to variations in SW velocity (Vsw) and density (nsw) at 8–16 magnetic local time (MLT) suggest that R1 at these local times is frequently open while R1 at 6–08 and 17–18 MLT is frequently closed. R2 is located mostly on closed field lines. In the afternoon open R1 at 12–16 MLT, an increase in nsw increases J//, decreases maximum peak Ee (proxy for Δ$\phi$//), but has little effect on maximum $\varepsilon$. In the same R1 region, an increase in Vsw increases J//, maximum peak Ee, and maximum $\varepsilon$. The dependencies of J//, maximum peak Ee, and maximum $\varepsilon$ are consistent with the Knight relation and the voltage generator at the magnetopause boundary in the afternoon open R1. In the midmorning and midafternoon, the response of J// to Vsw is higher for southward than for northward IMF. This can be attributed to the higher-velocity shear at the magnetopause boundary due to higher sunward convection in the LLBL inside the magnetopause. R1 in the closed-field lines near dawn and dusk appears to be more sensitive to merging rate (dΦ/dt = Vsw4/3 BT2/3 sin8/3($\theta$c/2)) than to SW dynamic pressure.

Description: The Kelvin-Helmholtz waves have been observed along the Earth's low-latitude magnetopause and have been suggested to play a certain role in the entry of solar wind plasma into Earth's magnetosphere. In situ observations of the KH waves (KHW) and, in particular, a nonlinear stage of the KH instability, i.e., rolled-up KH vortices (KHVs), have been reported to occur preferentially for northward interplanetary magnetic field (IMF). Using Cluster data, we present the first in situ observation of nonlinearly developed KHW during southward IMF. The analysis reveals that there is a mixture of less-developed and more-developed KHW that shows inconsistent variations in scale size and the magnetic perturbations in the context of the expected evolution of KH structures. A coherence analysis implies that the observed KHW under southward IMF appear to be irregular and intermittent. These irregular and turbulent characteristics are more noticeable than previously reported KHW events that have been detected preferentially during northward IMF. This suggests that under southward IMF KHVs become easily irregular and temporally intermittent, which might explain the preferential in situ detection of KHVs when the IMF is northward. MHD simulation of the present event shows that during southward IMF dynamically active subsolar environments can cause KHV that evolve with considerable intermittency. The MHD simulations appear to reproduce well the qualitative features of the Cluster observations.

Description: This brief report presents daytime 30 MHz radar observations of equatorial electrojet plasma irregularities in São Luís, Brazil, during the 10 January 2002, 4 September 2002, and 6 November 2001 geomagnetic storms. Modulation of equatorial electrojet plasma waves by the overshielding electric field is detected by the radar during the January and September storm events. The depressions of radar echo intensity observed by the São Luís radar during the 10 January and 4 September 2002 storms are caused by inhibition of large-, medium-, and short-scale electrojet plasma waves by the overshielding electric field. Corresponding to the depressions of the radar echoes, counter equatorial electrojets were observed by ground-based magnetometer in São Luís. The influences of disturbance dynamo, undershielding, and overshielding electric fields on the equatorial electrojet irregularities and current have been evident on the 6 November 2001 radar and magnetic field data. This study demonstrates the control of the equatorial electrojet plasma waves (growth and suppression) by the interplanetary magnetic field and also the coupling between the high-latitude and equatorial ionospheres.

Description: How the solar wind affects the location of the magnetopause has been widely studied and excellent models of the magnetopause based on in situ observations in the solar wind and at the magnetopause have been established, while the careful insight into the responses of the magnetopause to the variations in the solar wind can still provide us some new information about the processes in space plasmas. The short distance from Cluster to TC-1 on 9 March 2004, between 06:10 and 08:10 UT, gives us a good opportunity to precisely monitor the responses of the magnetopause to the variations in the solar wind. On the basis of the combined observations between Cluster, TC-1, and SuperDARN we analyze the magnetopause crossings associated with magnetopause motion or magnetic reconnection when the solar wind conditions have a series of variations. New results about the time delays for the propagation from the solar wind monitor to the magnetopause of the interplanetary magnetic fields (IMF) and of the solar wind dynamic pressure, respectively, and the intrinsic time for reconnection onset at the magnetopause are obtained. The most important feature of the event is that the dynamic pressure and the IMF in the solar wind do not arrive at the magnetopause at the same time, which will direct us to find out how the variation in the solar wind dynamic pressure is transported from the bow shock to the magnetopause. Another significant feature is that this event presents a shorter intrinsic time, ∼2 min, for reconnection onset at the dayside magnetopause than that given by the previous work of Le et al. (1993) and Russell et al. (1997).

Description: Stream particles are nanometer-scale dust particles ejected with speeds $\gtrsim$100 km s−1 from both the Jovian and Saturnian systems. Here we report the dynamical analysis of Saturnian stream particles on the basis of observations made by the Cosmic Dust Analyzer on board the Cassini spacecraft during its first three orbits around Saturn. The time span of the presented measurements covers from the beginning of orbit A to the end of orbit C (from the Saturn orbit insertion on 1 July 2004 UTC to 16 January 2005 UTC). During these orbits the Cassini spacecraft was usually located outside but not far from Saturn's magnetosphere. The Cassini observations therefore provide important information on the dynamics of stream particles just ejected from the system. As with earlier observations, two impact populations are identified. These appear as faint but continuous impacts as well as semiregular and energetic impact bursts. Faint impacts from directions close to the Saturn line of sight are recognized as recently ejected stream particles, while energetic dust bursts most probably consist of previously ejected particles that experienced significant acceleration within the solar wind. The presented measurements not only confirm the previous proposed stream particle ejection scenario but also serve as essential inputs for detailed dynamical modeling.

Description: Kinetic linear dispersion theory for electromagnetic fluctuations in a homogeneous collisionless plasma is used to study the properties of a proton Bernstein mode instability driven by a proton velocity distribution fp(v) such that ∂fp(v$\perp$)/∂v$\perp$ 〉 0 at suprathermal values of v$\perp$ and v$\parallel$ $\simeq$ 0, where $\parallel$ and $\perp$ denote directions parallel and perpendicular to the background magnetic field Bo, respectively. The model uses a three-component proton velocity distribution with fp(v) = f1(v) + f2(v$\parallel$, v$\perp$) − f3(v$\parallel$, v$\perp$), where f1(v) represents a Maxwellian thermal component. Here f2 and f3 are bi-Maxwellians with T$\perp$p 〉 T$\parallel$p and slightly different densities and temperatures to represent a suprathermal component consistent with proton perpendicular velocity distributions observed in the magnetospheric ring current. As is well established, the growth rate of the resulting instability has relative maxima near harmonics of the proton cyclotron frequency, the wave vector k satisfies 0 〈 k$\parallel$ $\ll$ k$\perp$, and wavelengths are of the order of or smaller than the proton gyroradius. The instability growth rate decreases as the electron/thermal proton temperature ratio increases and, for the dimensionless parameters chosen here, has a maximum value for the thermal proton β of about 10%.

Description: We present a multi-instrument study on the variations of optical auroras and ionospheric electron densities during an interval of a series of fast earthward flows in the magnetotail on 3 March 2009. The flow-related auroral signatures include intermittent higher-latitude (〉68° magnetic latitude) intensifications manifested in green and blue line auroras and more latitudinally extended red line auroral intensifications and expansions. During the same interval the Poker Flat incoherent scatter radar (PFISR) detected F region ionospheric electron density enhancements which, together with the red line auroral intensifications, give evidence for soft electron (

Description: The ionospheric responses to the solar eclipse of 15 January 2010 in the equatorial anomaly region have been investigated by three vertical-incidence and seven oblique-incidence ionosondes arranged along the meridian from geomagnetic latitudes 18°N to 30°N in eastern China. Though the solar eclipse occurred later in the evening, the eclipse effect on electron density and reflection height of ionospheric F2 layer was clearly observed. The study of the eclipse lag (the time lag between the occurrence of the eclipse maximum obscuration and the occurrence of the maximum depletion of foF2) with latitude indicates it increased with F2 layer altitude. Results suggest also that this eclipse enhanced the prereversal enhancement. An unusual peak occurred after the maximum reduction in foF2 and this was observed by all our ionosondes. The following F2 layer plasma density increase was considered to be caused by the increased westward electric field.

Description: Solutions to the differential equations describing the behavior of driven-dissipative systems are compared with measured exospheric temperatures (T∞) and provisional Dst indices acquired during 38 magnetic storms between mid-2002 and 2008. The only storm selection criterion was the availability of solar wind and interplanetary magnetic field data to compute driving electric fields $\varepsilon$VS. Globally averaged T∞ was inferred from measurements by accelerometers on the GRACE satellites. Statistical regression analyses indicate that the coupling coefficients for T∞, Dst, and their ratio are well represented as functions of 81 day averaged F10.7a. Using Dst as the driver, this functional relationship yielded reasonable estimates of the evolution of T∞ during the Halloween 2003 magnetic storm. Linear relations between T∞ and the total energy of the thermosphere (Eth) and between Dst and the energy of the ring current (ERC) allow estimates of the storm time energy partitioning. Empirical estimates of the energy coupling coefficients for the thermosphere (αE) and ring current (αERC) span the ranges 1.5–0.2 and 0.5–0.2 TW/mV/m, respectively. Outside of extreme solar minimum conditions, main phase increases in Eth exceed those of ERC.

Description: The magnetized plasma of near-Earth space supports the shear and fast Alfvén, ultralow-frequency (ULF; 1–100 mHz), magnetohydrodynamic wave modes. The fast mode may propagate across the magnetic field, spreading ULF wave energy throughout the magnetosphere, and couple with the shear Alfvén mode to form field line resonances (FLRs). The FLR electric field in the magnetosphere may have sufficiently large amplitudes to energize electrons and enhance radiation belt particle diffusion rates. Ozeke et al. (2009) recently described a technique that related the observed north-south ULF magnetic component at the ground, bνg, with the radial electric field component in the equatorial plane of the magnetosphere, eνeq, via the fields in the ionosphere. In this paper we use a fully coupled ULF wave model to determine the ratio eνeq/bνg for a 5 mHz FLR formed at high latitudes. We find that Ozeke et al. (2009) underestimated the ULF wave magnetic field on the ground which varies with ionosphere Hall conductance. This difference is found to be caused by assuming a decoupled wave mode model for the ionosphere fields. Any relationship that involves ULF wavefields in the ionosphere must include the effects of ULF wave mode mixing.

Description: Pickup ions formed from ionized neutral exospheres in flowing plasmas have phase space distributions that reflect their source's spatial distributions. Phase space distributions of the ions are derived from the Vlasov equation with a delta function source using three-dimensional neutral exospheres. The ExB drift produced by plasma motion picks up the ions while the effects of magnetic field draping, mass loading, wave particle scattering, and Coulomb collisions near a planetary body are ignored. Previously, one-dimensional exospheres were treated, resulting in closed form pickup ion distributions that explicitly depend on the ratio rg/H, where rg is the ion gyroradius and H is the neutral scale height at the exobase. In general, the pickup ion distributions, based on three-dimensional neutral exospheres, cannot be written in closed form, but can be computed numerically. They continue to reflect their source's spatial distributions in an implicit way. These ion distributions and their moments are applied to several bodies, including He+ and Na+ at the Moon, H2+ and CH4+ at Titan, and H+ at Venus. The best places to use these distributions are upstream of the Moon's surface, the ionopause of Titan, and the bow shock of Venus.

Description: We identified 1875 wave events in magnetic field data from geosynchronous orbit. Most of these events were transverse with respect to the background magnetic field, left-hand polarized, and were observed in the post-noon magnetic local time sector at frequencies just below the helium gyrofrequency. Combined, these observations strongly suggest that most of these events are Electromagnetic Ion Cyclotron (EMIC) waves. Average wave amplitudes are presented, binned by frequency, geomagnetic activity and magnetic local time. The amplitude increases with increasing geomagnetic activity; increased activity also narrows the local time sector in which the waves are observed. A superposed epoch analysis of solar wind parameters and geomagnetic activity indices shows that 12 hours before wave onset the AE and Kp index increased, indicating storm and substorm activity that injects hot ion populations needed to drive the EMIC instability and providing ample time for those populations to drift into the post-noon local time sector. Just before wave onset a sudden enhancement in the AE index and the solar wind dynamic pressure are observed, indicating that a final perturbation of the magnetosphere is needed to excite EMIC wave growth. EMIC waves are thought to cause loss of relativistic particles in the radiation belt. Large solar wind densities have been associated with low flux of relativistic particles during the recovery phase of geomagnetic storms. We show that EMIC waves are preferentially generated during intervals of large solar wind density, indicating that such conditions drive EMIC waves which in turn cause enhanced loss of relativistic particles.

Description: This paper presents the results from a numerical study of the effects of multiple ion species on the development of small-scale, intense electromagnetic waves and density structures that are frequently observed in the auroral ionosphere in the vicinity of discrete auroral arcs. The study is based on a multifluid MHD model describing nonlinear coupling between shear and slow ultra-low-frequency MHD waves in cold, low-altitude plasma containing several ion species. Simulations reveal that these waves can be generated by the ionospheric feedback instability in the downward current channels adjacent to the upward currents, causing aurora, when heavy ionospheric ions (in particular, O2+) provide the “matching impedance” condition between the ionosphere and the magnetosphere. The ponderomotive force associated with these waves moves ions along the ambient magnetic field from the ionosphere. Different ion species exhibit different dynamics, depending on their masses and initial distributions. The strongest variations in density of −14% and +23% of the background values occur in the F region and are produced by the dynamics of O+ ions. These density variations can be detected by ground radars and low-orbiting satellites in the auroral zone.

Description: Using the statistical CRRES measurements of the electric field intensities of lower band chorus (LBC) and upper band chorus (UBC) around L = 6 under geomagnetically moderate conditions, we evaluate the variations in modeled magnetic field spectral intensity and the resultant changes in resonant scattering rates of plasma sheet electrons caused by different choices of the wave normal distribution. UBC scattering rates inferred from electric field measurements show a common trend of decreasing scattering with increasing peak wave normal angle, $\theta$m, for the plasma sheet electrons at all resonant pitch angles. This trend is mainly due to the lower power of magnetic field as derived from the electric field measurements for oblique waves. The LBC resonant diffusion inferred from electric field measurements shows a considerable increase in scattering rates with increasing $\theta$m for ∼1 keV electrons at all resonant pitch angles and for 3–30 keV electrons over certain ranges of pitch angles, which is contrary to the decrease in wave magnetic field amplitude and results mainly from the decrease in resonant energy and redistribution of the majority of wave power at large wave normal angles for increased peak wave normal angle. LBC-induced scattering rates of 3–10 keV electrons decrease with increasing $\theta$m at low pitch angles, consistent with the decrease in wave magnetic field amplitude when $\theta$m increases. Our investigation demonstrates that the knowledge of the wave normal distribution of LBC and UBC is essential for an accurate quantification of the net resonant scattering rates and loss timescales of the plasma sheet electrons for an improved global simulation of diffuse auroral precipitation and the evolution of plasma sheet electron pitch angle distribution if only measurements of wave electric field intensity are available. In contrast, the diffuse auroral scattering rates calculated from magnetic field measurements are much less sensitive to the assumption on wave normal angle distribution. While UBC scattering with constant magnetic field power is roughly insensitive to the assumed wave normal distribution, LBC scattering with constant magnetic field power becomes more dependent on the assumed wave normal angle distribution, especially for ∼1 keV electrons.

Description: Observations of lightning-induced electron precipitation (LEP) events at three geographic regions show characteristics which systematically vary with both longitude and hemisphere. These observations are quantitatively interpreted using a novel atmospheric interaction model designed to predict the characteristics of LEP events at any longitude and midlatitude L-shell by accounting for the effects of precipitating electrons which are backscattered from the atmosphere. The model of atmospheric backscatter (ABS) calculates atmospheric backscatter responses for individual monoenergetic electron beams with a single incident pitch angle using a Monte Carlo model of atmospheric interactions. The ABS model also includes an asymmetric (non-ideal dipole) geomagnetic field model in calculations of the pitch angle of backscattered electrons entering the conjugate hemisphere. Using a realistic distribution of precipitating electrons, the results of this backscatter calculation at three separate longitudes are compared with VLF remote sensing data collected on nearly north-south great circle paths (GCPs). Results predicted by the model and confirmed by data indicate that all four primary LEP characteristics exhibit longitudinal and hemispheric dependence which can be explained in terms of precipitating electrons backscattered from the atmosphere. By combining these effects with previously calculated radiation belt electron loss rates due to lightning at a single location it is possible to estimate the global loss of radiation belt electrons due to lightning.

Description: It has been over 20 years since Roble and Dickinson [1989] first concluded that global change will occur in the upper atmosphere (above 50 km) as a result of increased greenhouse gas concentrations within this region. Long-term changes to Earth's atmosphere are becoming more and more relevant to the future of our world, and it is vital that we quantify and understand changes occurring at all levels within the coupled atmospheric system. The increasing concentration of greenhouse gases, stratospheric ozone depletion, slowly varying solar and geomagnetic activity, secular change of Earth's magnetic field, and changing dynamics propagating up from the troposphere are some of the possible causes of long-term changes in the stratosphere, mesosphere, thermosphere and ionosphere.

Description: The storm time evolution of equatorially mirroring H+ ions in the inner magnetosphere has been statistically examined by using data from the Polar satellite. We focused on two energy ranges of H+ observed by Polar; 31–80 keV and 125–173 keV, which are referred to as low- and high-energy components, respectively. The following two phases were defined; the developing phase (pre-storm time to near the most disturbed time) and the declining phase (near the most disturbed time to post-storm time), which is 3 days before (after) the equatorial crossing of Polar during the storm time. We obtained the following results: (1) Low-energy H+ tends to increase during the developing, and to decrease during the declining at all magnetic local times (MLTs) except for the pre-noon sector. (2) The low-energy H+ is anti-correlated with the magnetic field, probably indicating that the low-energy H+ reduces the equatorial magnetic field due to a diamagnetic effect. (3) High-energy H+ tends to increase on the dayside during the declining phase. (4) The high-energy H+ is poorly correlated with the magnetic field. High-energy H+ behaves significantly different from the low-energy H+, and that some process other than betatron acceleration, diffusion and substorm-associated injection could have been responsible for the variation of the high-energy H+.

Description: Observations of tweeks with higher harmonics (n 〉 1) at low latitude stations Allahabad and Nainital, in the Indian sector, during the total solar eclipse on 22 July 2009, are presented. Allahabad and Nainital stations were in 100% and 85% of the totality paths. Observations suggest that about 30–40% obscuration of solar disc can lead to the tweeks occurrence which otherwise occur only in nighttime. A total of 148 tweeks at Allahabad and 20 tweeks at Nainital were recorded with some of them up to 3rd harmonics. The World Wide Lightning Location Network data indicated that tweeks observed were generated by lightning's located in the partial eclipse area of Asia-Oceania region. The changes in D-region ionospheric VLF reflection height and electron density (∼22–23 cm−3) during eclipse have been estimated from the first cut-off frequency of the tweeks. The reflection height increased from ∼89 km from the first occurrence of tweek to about 91–92 km at the totality and then decreased to ∼87 km at the end of the eclipse, suggesting a change of about 5 km in the reflection height during eclipse. The reflection heights are lower by 2–3 km as compared to normal nighttime tweek reflection heights. The above increase in the reflection height indicate that the partial nighttime condition is created during eclipse, as the main D-region ionizing radiation Lyman α is blocked but solar soft X-ray and EUV radiations originating from the limb solar corona are not totally blocked which produce some of ionization in the D-region.

Description: One of the signatures of magnetospheric substorms is the precipitation of high energy particles into the high latitude ionosphere. In this paper, we introduce a new method of tracking substorm particle precipitation using GPS Total Electron Content (TEC) and provide some preliminary observations of precipitation signatures from application of this method. Using TEC measurements from several GPS receivers, we examined particle precipitation signatures associated with two separate substorm events (4 October 2008 and 29 October 2008) and monitored the expansion of the high energy precipitation regions with a higher temporal and spatial resolution than previously available. For each event we have observed TEC signatures associated with substorm particle precipitation along 20 to 25 separate GPS raypaths from up to 7 GPS receivers located in the Canadian Arctic. This is in addition to particle injection signatures found in CLUSTER satellite data and precipitation signatures in ground based riometer data. Signature timing on different raypaths from different stations indicates a mainly northward (tailward) expansion of the precipitation (injection) region with a smaller westward (azimuthal) component for the events studied. By applying a triangulation method, we also calculated propagation velocity of the precipitation boundary in regions covered by our GPS receivers. For each substorm, expansion velocity ranged from 0.3–2 km/s northward and 0–1 km/s westward, and tended to decrease in magnitude at higher latitudes.

Description: Observations have shown that electron phase-space holes (electron holes) possess regular magnetic structures. In this paper, two-dimensional (2D) electromagnetic particle-in-cell (PIC) simulations are performed in the (x, y) plane to study magnetic structures associated with electron holes under different plasma conditions. In the simulations, the background magnetic field (B0 = B0${\stackrel{\rightharpoonup}{\bf{e}}}$x) is along the x direction. The combined actions between the transverse instability and stabilization by the background magnetic field lead to the generation of the electric field Ey. Then electrons suffer the electric field drift and produce the current in the z direction, which leads to the fluctuating magnetic field along the x and y directions. Meanwhile, the motion of the electron holes along the x direction and the existence of the electric field Ey generate the fluctuating magnetic field along the z direction. In very weakly magnetized plasma (Ωe $\ll$ ωpe, where Ωe and ωpe are the electron gyrofrequency and electron plasma frequency, respectively.), the transverse instability is very strong and the magnetic structures associated with electron holes disappear quickly. When Ωe is comparable to ωpe, the parallel cut of the fluctuating magnetic field δBx and δBz has unipolar structures in the electron holes, while the parallel cut of fluctuating magnetic field δBy has bipolar structures. In strongly magnetized plasma (Ωe 〉 ωpe), electrostatic whistler waves with streaked structures of Ey are excited. The fluctuating magnetic field δBx and δBz also have streaked structures. The relevance between our simulation results and the magnetic structures associated with electron holes observed in the plasma sheet is also discussed.

Description: Measurements from ground-based magnetometers and riometers at auroral latitudes have demonstrated that energetic (∼30–300 keV) electron precipitation can be modulated in the presence of magnetic field oscillations at ultralow frequencies. It has previously been proposed that an ultralow-frequency (ULF) wave would modulate field and plasma properties near the equatorial plane, thus modifying the growth rates of whistler-mode waves. In turn, the resulting whistler-mode waves would mediate the pitch angle scattering of electrons resulting in ionospheric precipitation. In this paper, we investigate this hypothesis by quantifying the changes to the linear growth rate expected due to a slow change in the local magnetic field strength for parameters typical of the equatorial region around 6.6RE radial distance. To constrain our study, we determine the largest possible ULF wave amplitudes from measurements of the magnetic field at geosynchronous orbit. Using nearly ten years of observations from two satellites, we demonstrate that the variation in magnetic field strength due to oscillations at 2 mHz does not exceed ±10% of the background field. Modifications to the plasma density and temperature anisotropy are estimated using idealized models. For low temperature anisotropy, there is little change in the whistler-mode growth rates even for the largest ULF wave amplitude. Only for large temperature anisotropies can whistler-mode growth rates be modulated sufficiently to account for the changes in electron precipitation measured by riometers at auroral latitudes.

Description: We present the first three-dimensionally resolved observations of polar mesosphere winter echoes obtained with a 25 beam-experiment covering a volume of about 50 km in diameter (horizontal distance) at altitudes between 65 and 85 km. This allows us to resolve the classical space time ambiguity of single beam observations and reveals that the echoing structure was tilted in the East–West direction but showed no considerable tilt in the North–South direction. The Doppler shifts derived from the 24 off-zenith beam directions are consistent with the mean background wind measured independently by a co-located MF-radar. The time development of the 3-D echo-pattern is consistent with scattering structures that follow the constant phase lines of a medium frequency gravity wave that is propagating against the mean flow. Wave parameters derived from these observations are independently confirmed by the analysis of co-located wind measurements with the aforementioned MF-radar. Overall, the observed echo morphology in time and space is reminiscent of gravity wave breaking which is known to lead to a maximum of turbulence activity that moves with the phase of the wave.

Description: It has been suggested that the equilibrium structure of the slot region, which separates the inner and outer radiation belts, forms as the result of a balance between inward radial diffusion and pitch angle scattering of relativistic electrons by interactions with three types of whistler mode waves: plasmaspheric hiss, lightening-generated whistlers, and ground-based Very Low Frequency (VLF) transmitters. In this study, using the time-dependent 3D Versatile Electron Radiation Belt (VERB) code, we examine how effectively the slot can be formed by a combination of radial diffusion and pitch angle diffusion, together with Coulomb scattering, and compare the simulations with the CRRES MEA 1 MeV electron observations to examine the viability of the various scattering mechanisms. The results show that the overall time evolution of the observed two-zone structure is in a good agreement with our model simulations, which suggests a balance between inward radial diffusion due to Ultra Low Frequency (ULF) electromagnetic fluctuations and pitch angle scattering due to plasmaspheric hiss and lightning-generated whistlers. However, when inward radial diffusion due to the electrostatic fluctuations is included, agreement between the observed and simulated fluxes becomes weaker, suggesting that it is important to understand and quantify the radial diffusion rates in the slot region.

Description: In order to characterize plasma (0.03–45 keV) properties in response to interplanetary (IP) shock impact at the geosynchronous orbit, we have examined 95 shock events from 1997 to 2004. These shock events have been categorized into two groups: shock fronts associated with southward interplanetary magnetic field (IMF; 47 cases) and with northward IMF (48 cases). Our results show that under southward IMF, the plasma becomes denser and hotter following the IP shock arrival. The proton (0.1–45 keV) and electron (0.03–45 keV) number densities have peaks of 1.8 and of 2.5 cm−3 at the duskside, respectively (the typical tail plasma sheet density is about 0.7 cm−3). After the IP shock impact, the plasma (proton and electron) temperature anisotropy increases remarkably at the noon sector, decreases toward dawn and dusk, and minimizes at midnight, suggesting that both electromagnetic ion cyclotron and whistler waves can be stimulated mainly at the dayside magnetosphere. In addition, there are more oxygen ions injecting into the inner magnetosphere, and the density of ionospheric oxygen ions is comparable to proton density. However, for IP shocks associated with northward IMF, the plasma density and temperature increases are insignificant, while slight enhancements of the plasma temperature anisotropy are distributed globally.

Description: This paper focuses on the sun-synchronous diurnal (DW1) and semidiurnal (SW2) tidal components of neutral exosphere temperature derived from contemporaneous drag measurements made from the CHAMP and GRACE satellites. Densities are converted to exosphere temperatures using the parametric relationship that exists between density and temperature in the NRLMSISe00 empirical model. Daily, seasonal and solar-cycle dependencies of DW1 and SW2 in exosphere temperature are elucidated, and similarities and differences with NRLMSISe00 are detailed. In addition, using TIMED/SABER measurements between 80–110 km and viscous tidal theory, it is demonstrated that the measured seasonal-latitudinal variation of the semidiurnal exosphere temperature amplitude is dominated by the part excited in-situ in the thermosphere, as opposed to those tidal components that propagate upwards from the lower atmosphere.

Description: We study the frequency and time variations of Jovian hectometric emissions (HOM) recorded by the Radio and Plasma Wave Science (RPWS) experiment onboard the Cassini spacecraft during its Jupiter flyby. The capabilities of the RPWS experiment enable us to analyze the intensity extinction of HOM radiation, the so-called attenuation band. Using about 7 weeks of RPWS data obtained around the closest approach, a statistical analysis investigates the spectral variations of this phenomenon with respect to the spacecraft magnetic latitude and central meridian longitude. We show that the ‘trace’ of the attenuation band is usually not a full sinusoid as reported in previous studies and only parts of the curve are observed. Also, the intensity extinction can occur when the spacecraft is not in the planetary magnetic equator plane. This specific feature appears or vanishes when the observer (e.g., Cassini or Galileo spacecraft) is far from or close to the planet, respectively. The regular and systematic observations of the attenuation band suggest that the plasma medium at the origin of these features is steady and stable. The Io torus may be considered to be the most probable plasma medium where HOM emission is refracted through its raypath propagation. Furthermore, intensity extinction at frequencies higher than 3 MHz and up to 5 MHz implies the presence of particular electronic density irregularities in the Io torus. Volcanic activity, particularly in the northern hemisphere of the Io satellite, may be the source of such Io torus plasma irregularities.

Description: A comparison of MeV electron measurements at geosynchronous orbit, GEO, with solar wind shows that the MeV electron prediction model developed for GEO using data from the declining phase of solar cycle 22 (1995–1996) works well for the declining phase of solar cycle 23 (2006–2008), indicating that the MeV electron flux has a predictable and systematic response to the solar wind. The same comparison for solar maximum (2000–2003) shows that the model works less well partly because it does not match the high flux cutoff seen in the data and partly because it does not reproduce the sudden drops in flux that occur when the magnetopause is close to GEO. The model also reproduces the nonlinear correlation of the solar wind speed with the log of the MeV electron flux seen at GEO. An examination of 15 yr of solar wind and the MeV electron data shows that geomagnetic activity driven by a southward orientation of the interplanetary magnetic field, IMF, is a necessary condition for MeV electron enhancements at GEO and that high-speed solar wind are not necessary. The reason that high-speed solar wind is almost always associated with the enhancement of MeV electrons is mainly because high-speed solar wind almost always has some southward components of the IMF.

Description: The coupling of a super-Alfvénic plasma expansion in a magnetized background plasma is examined. Such coupling plays an important role in several high-energy, quasi-neutral, plasma configurations; the focus here is on High Altitude Nuclear Explosions (HANEs). Fully 3-D Kinetic Ion Simulation Modeling (KISM) reveals, for some initial conditions, strong coupling of the debris to the magnetized background ionosphere even though all collision processes between the ions have been neglected. The interaction dynamics are found to be altered dramatically for small changes in initial conditions. A slight increase in the ion charge density of the background plasma allows the debris ions to decouple and slip through the magnetized background. These decoupled ions in the expanding plasma then follow trajectories typical of single particle motion. The salient features of this process, guided by 1-D simulations, lead to two thresholds for the onset of decoupling. The first threshold depends on the ratio of the charge density of the expanding plasma to that of the background plasma. The second threshold is evident when the expanding plasma has a finite pulse length comparable to the gyroradius of the energized background ions.

Description: This work reports for the first time on bifurcations of the main auroral ring at Saturn observed with the UVIS instrument onboard Cassini. The observation sequence starts with an intensification on the main oval, close to noon, which is possibly associated with dayside reconnection. Consecutive bifurcations appear with the onset of dayside reconnection, between 11 and 18 magnetic local time, while the area poleward of the main emission expands to lower latitudes. The bifurcations depart with time from the main ring of emission, which is related to the open-closed field line boundary. The augmentation of the area poleward of the main emission following its expansion is balanced by the area occupied by the bifurcations, suggesting that these auroral features represent the amount of newly open flux and could be related to consecutive reconnection events at the flank of the magnetopause. The observations show that the open flux along the sequence increases when bifurcations appear. Magnetopause reconnection can lead to significant augmentation of the open flux within a couple of days and each reconnection event opens ∼10% of the flux contained within the polar cap. Additionally, the observations imply an overall length of the reconnection line of ∼4 hours of local time and suggest that dayside reconnection at Saturn can occur at several positions on the magnetopause consecutively or simultaneously.

Description: Short-time (

Description: Physics-based global magnetosphere modeling requires large computational resources. It is still impractical to resolve the computational domain to the point where numerical errors become negligible. One possible way of reducing numerical diffusion is the “Boris correction”: the semirelativistic magnetohydrodynamics equations are solved with an artificially reduced speed of light. Here we introduce a new alternative approach, an Implicit Scheme with Limited Numerical Dissipation (ISLND). The fully implicit time stepping provides stability, and the wave speeds are limited in the dissipative numerical fluxes only. This limiting only affects the numerical scheme, and it does not modify the equations being solved. This approach can be employed for most total variation diminishing schemes. The differences between the Boris and ISLND schemes are demonstrated in simple numerical tests. We also perform several simulations for two magnetic storms using the global magnetosphere, the ionosphere electrodynamics, and the inner magnetosphere models of the Space Weather Modeling Framework, and we compare the Boris scheme with the limited numerical dissipation method and also with the unmodified base scheme at various grid resolutions. We find that for these particular simulations the Boris scheme and the ISLND scheme produce comparable results, both being significantly less diffusive than the unmodified scheme.

Description: Satellite drag data indicate that the thermosphere was lower in density, and therefore cooler, during the protracted solar minimum period of 2007–2009 than at any other time in the past 47 years. Measurements indicate that solar EUV irradiance was also lower than during the previous solar minimum. However, secular change due to increasing levels of CO2 and other greenhouse gases, which cool the upper atmosphere, also plays a role in thermospheric climate, and changes in geomagnetic activity could also contribute to the lower density. Recent work used solar EUV measurements from the Solar EUV Monitor (SEM) on the Solar and Heliospheric Observatory, and the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model, finding good agreement between the density changes from 1996 to 2008 and the changes in solar EUV. Since there is some uncertainty in the long-term calibration of SEM measurements, here we perform model calculations using the MgII core-to-wing ratio as a solar EUV proxy index. We also quantify the contributions of increased CO2 and decreased geomagnetic activity to the changes. In these simulations, CO2 and geomagnetic activity play small but significant roles, and the primary cause of the low temperatures and densities remains the unusually low levels of solar EUV irradiance.

Description: To clarify the characteristics and generation process of black auroras, we investigated 13 black auroral events using simultaneous imaging and particle data from the Reimei satellite obtained between November 2005 and October 2006. The formation and motion of black auroras were determined from successive monochromatic auroral images around the satellite's magnetic footprints, while the auroral intensities at the footprints were compared with precipitating electrons. We found that a number of small-scale deficiencies were embedded in precipitating electrons from the central plasma sheet with energies greater than 2–7 keV and that each deficiency corresponded exactly to black arcs and black patches at the magnetic footprint. Therefore black arcs and black patches are not associated with a field-aligned potential (such as a divergent potential structure) but probably originate from pitch angle scattering. In the black auroral region, low-energy (2–5 keV) inverted V–type downward electrons (spanning channels that are several tens of kilometers wide) often appear to overlap with high-energy (several keV) plasma sheet electrons. Drifting black patches were also observed. We estimated the speed and direction of the drift by minimum mean squared error analysis.

Description: We analyze the evolution of the interplanetary magnetic field spatial structure by examining the inner heliospheric autocorrelation function, using Helios 1 and Helios 2 in situ observations. We focus on the evolution of the integral length scale (λ) anisotropy associated with the turbulent magnetic fluctuations, with respect to the aging of fluid parcels traveling away from the Sun, and according to whether the measured λ is principally parallel (λ$\parallel$) or perpendicular (λ$\perp$) to the direction of a suitably defined local ensemble average magnetic field B0. We analyze a set of 1065 24-hour long intervals (covering full missions). For each interval, we compute the magnetic autocorrelation function, using classical single-spacecraft techniques, and estimate λ with help of two different proxies for both Helios data sets. We find that close to the Sun, λ$\parallel$ 〈 λ$\perp$. This supports a slab-like spectral model, where the population of fluctuations having wave vector k parallel to B0 is much larger than the one with k-vector perpendicular. A population favoring perpendicular k-vectors would be considered quasi-two dimensional (2D). Moving toward 1 AU, we find a progressive isotropization of λ and a trend to reach an inverted abundance, consistent with the well-known result at 1 AU that λ$\parallel$ 〉 λ$\perp$, usually interpreted as a dominant quasi-2D picture over the slab picture. Thus, our results are consistent with driving modes having wave vectors parallel to B0 near Sun, and a progressive dynamical spectral transfer of energy to modes with perpendicular wave vectors as the solar wind parcels age while moving from the Sun to 1 AU.

Description: A detailed overview of long-term secular trends in temperature of the mesosphere and lower thermosphere considered to be induced by increase in greenhouse gases has been provided by Beig et al. (2003). Since then, quite a few new results have been emerged as some of the data series have become sufficiently large enough to provide results with improved confidence. Our understanding on the nature of temperature trends in the mesosphere/lower thermosphere (MLT) region is relatively better now. In the mesosphere, some of the results confirmed the earlier findings, and some new results obtained by satellite and lidar data over the tropical region have indicated a relatively weaker cooling trend as compared to the past but nevertheless strengthened the conclusion about the cooling trends. However, in the mesopause region, some of the new results now indicate a break in trend and tendency of negative signal where earlier no trend feature was noticed. This slice of no trend feature in between two cooling regimes was puzzling the modeling community, who were in search of a convincing explanation. This paper briefly outlines the progress made over the recent past in the field of MLT region secular temperature trends attributed mainly to growth of greenhouse gases near the Earth's surface.

Description: The total current intensity (Jtotal) of the equivalent Sq current system along the 210° magnetic meridian shows the following variations: (1) solar activity variations, (2) seasonal variations, and (3) day-to-day variations. These variations arise from different physical mechanisms. The main objective of the present paper is to determine the relative amount contributed by each variation to the Jtotal. First, the empirical Sq field model by Yamazaki et al. (2011) is analyzed to examine the impact of solar activity and seasonal variations. The results show that Jtotal changes by ±33% with the change of solar radiation activity in one solar cycle and by ±17% with the change of season. Next, observation data are analyzed to examine day-to-day variations. The daily values of Jtotal from 2000 to 2002 are derived after removal of the solar activity and seasonal contributions. The results show that Jtotal changes by ±14% from day to day. Therefore, we conclude that variations in Jtotal are mainly controlled by solar radiation activity, while the impact of seasonal effects is about half of the solar activity contribution and the impact of day-to-day effects is also about half of the solar activity contribution.

Description: We investigated how the rise rate and decay rate of solar flares affect the thermosphere and ionosphere responses to them. Model simulations and data analysis were conducted for two flares of similar magnitude (X6.2 and X5.4) that had the same location on the solar limb, but the X6.2 flare had longer rise and decay times. Simulated total electron content (TEC) enhancements from the X6.2 and X5.4 flares were ∼6 total electron content units (TECU) and ∼2 TECU, and the simulated neutral density enhancements were ∼15%–20% and ∼5%, respectively, in reasonable agreement with observations. Additional model simulations showed that for idealized flares with the same magnitude and location, the thermosphere and ionosphere responses changed significantly as a function of rise and decay rates. The “Neupert Effect,” which predicts that a faster flare rise rate leads to a larger EUV enhancement during the impulsive phase, caused a larger maximum ion production enhancement. In addition, model simulations showed that increased E × B plasma transport due to conductivity increases during the flares caused a significant equatorial anomaly feature in the electron density enhancement in the F region but a relatively weaker equatorial anomaly feature in TEC enhancement, owing to dominant contributions by photochemical production and loss processes. The latitude dependence of the thermosphere response correlated well with the solar zenith angle effect, whereas the latitude dependence of the ionosphere response was more complex, owing to plasma transport and the winter anomaly.

Description: Recent studies showed that, regardless of the orientation of the Interplanetary Magnetic Field (IMF), ULF wave activity in the solar wind can substantially enhance the convection in the high latitude ionosphere, suggesting that ULF fluctuations may also be an important contributor to the coupling of the solar wind to the magnetosphere-ionosphere system. We conduct a statistical study to understand the effect of ULF power in the IMF on the cross polar cap potential, primarily focusing on northward IMF. We have analyzed the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) calculations of the polar cap potential, a IMF ULF index that is defined as the logarithm of Pc5 ULF power in IMF, and solar wind velocity and dynamic pressure for 249 days in 2003. We find that, separated from the effects of solar wind speed and dynamic pressure, the average cross polar cap potentials show a roughly linear dependence on the ULF index, with a partial correlation coefficient of 0.19. Highly structured convection flow patterns with a number of localized vortices are often observed under fluctuating northward IMF. For such a convection configuration, it is hard to estimate properly the cross polar cap potential drop, as the enhanced flows around the vortices that may be associated with IMF fluctuations do not necessarily yield a large potential drop. Thus, despite the relatively small correlation coefficient, the linear trend we found gives support to the significant role of IMF ULF fluctuations on the coupling of the solar wind to the magnetosphere-ionosphere system.

Description: We use a new magnetohydrodynamic (MHD) model to study the effects of thermal-electron heating in Titan's ionosphere. This model improves the previously used multispecies MHD model by solving both the electron and ion pressure equations instead of a single plasma pressure equation. This improvement enables a more accurate evaluation of ion and electron temperatures inside Titan's ionosphere. The model is first applied to an idealized case, and the results are compared in detail with those of the single-pressure MHD model to illustrate the effects of the improvement. Simulation results show that the dayside ionosphere thermal pressure is larger than the upstream pressure during normal conditions, when Titan is located in the dusk region; thus Saturn's magnetic field is shielded by the highly conducting ionosphere, similar to the interaction of Venus during solar maximum conditions. This model is also applied to a special flyby of Titan, the T34 flyby, which occurred near the dusk region. It is shown that better agreement with the magnetometer data can be achieved using the two-fluid MHD model with the inclusion of the effects of thermal electron heating. The model results clearly demonstrate the importance of thermal-electron heating in Titan's ionosphere.

Description: With the storm of 7–8 September 2002 as a study case, we demonstrate that an ionospheric model driven by a suitable storm time convection electric field can reproduce the F region dayside density enhancements associated with the ionospheric storm positive phase. The ionospheric model in this case is the Utah State University Time Dependent Ionospheric Model (TDIM); the electric field model is the University of Michigan's Hot Electron and Ion Drift Integrator (HEIDI). Extensive ground truth is available throughout the study period from two independent sources: ground-based vertical TEC and ionosonde stations; our simulation results are in good agreement with these observations. We address the question of what is the source of the high-density plasma that is seen during the positive storm phase and show that in this case a magnetospheric electric field with an eastward component that penetrates to midlatitudes increases local production on the dayside to a degree that is sufficient to account for the storm time density increases that have been observed.

Description: The Earth's magnetic field changes in orientation and strength over time. We study the response of the magnetosphere-ionosphere-thermosphere system to a 25% reduction in magnetic field intensity, using the coupled magnetosphere-ionosphere-thermosphere (CMIT) model. Simulations were performed with a dipole moment of 8 × 1022 A m2, close to the present-day value, and a dipole moment of 6 × 1022 A m2, both under the same solar wind conditions, intermediate solar activity (F10.7 = 150), and for March equinox and June solstice. The 25% reduction in field strength causes the magnetosphere to shrink and the polar cap to expand, in agreement with theory. The Pedersen and the Hall ionospheric conductances increase by 50%–60% and 60%–65%, respectively. This causes a ∼9%–12% decrease in electric potential and a ∼20% increase in field-aligned currents during equinox. Ion E × B drift velocities are enhanced by ∼10%–15%. The Joule heating also increases, by 13%–30%, depending on the season. Changes in the neutral temperature structure are caused partly by changes in Joule heating and partly by changes in the neutral wind. The neutral wind itself is also affected by changes in neutral temperature and by changes in ion velocities. The changes in the neutral wind, together with changes in the vertical component of the E × B drift, affect the height of the ionospheric F2 layer. Changes in electron density are related to changes in the O/N2 ratio. The global mean increase in neutral temperature causes the thermosphere to expand, resulting in a global mean uplift of the ionosphere. These effects are generally smaller during solstice.