ALBERT

Description: Abstract The injection region's formation, scale size, and propagation direction have been debated throughout the years, with new questions arising with increased plasma sheet observations by missions like Cluster and THEMIS. How do temporally and spatially small‐scale injections relate to the larger injections historically observed at geosynchronous orbit? How to account for opposing propagation directions—earthward, tailward, and azimuthal—observed by different studies? To address these questions, we used a combination of multisatellite and ground‐based observations to knit together a cohesive story explaining injection formation, propagation, and differing spatial scales and timescales. We used a case study to put statistics into context. First, fast earthward flows with embedded small‐scale dipolarizing flux bundles transport both magnetic flux and energetic particles earthward, resulting in minutes‐long injection signatures. Next, a large‐scale injection propagates azimuthally and poleward/tailward, observed in situ as enhanced flux and on the ground in the riometer signal. The large‐scale dipolarization propagates in a similar direction and speed as the large‐scale electron injection. We suggest small‐scale injections result from earthward‐propagating, small‐scale dipolarizing flux bundles, which rapidly contribute to the large‐scale dipolarization. We suggest the large‐scale dipolarization is the source of the large‐scale electron injection region, such that as dipolarization expands, so does the injection. The 〉90‐keV ion flux increased and decreased with the plasma flow, which died at the satellites as global dipolarization engulfed them. We suggest the ion injection region at these energies in the plasma sheet is better organized by the plasma flow.

Description: Abstract We analyze the spatial variation in the response of the surface geomagnetic field (or the equivalent ionospheric current) to variations in the solar wind. Specifically, we regress a reanalysis of surface external and induced magnetic field (SEIMF) variations onto measurements of the solar wind. The regression is performed in monthly sets, independently for 559 regularly spaced locations covering the entire northern polar region above 50° magnetic latitude. At each location, we find the lag applied to the solar wind data that maximizes the correlation with the SEIMF. The resulting spatial maps of these independent lags and regression coefficients provide a model of the localized SEIMF response to variations in the solar wind, which we call “Spatial Information from Distributed Exogenous Regression.” We find that the lag and regression coefficients vary systematically with ionospheric region, season, and solar wind driver. In the polar cap region the SEIMF is best described by the By component of the interplanetary magnetic field (50–75% of total variance explained) at a lag ∼20–25 min. Conversely, in the auroral zone the SEIMF is best described by the solar wind ϵ function (60–80% of total variance explained), with a lag that varies with season and magnetic local time (MLT), from ∼15–20 min for dayside and afternoon MLT (except in Oct–Dec) to typically 30–40 min for nightside and morning MLT and even longer (60–65 min) around midnight MLT.

Description: Abstract Chorus emissions composed of coherent whistler mode waves are responsible for pitch angle scattering of energetic electrons. This scattering is closely related to energetic electron precipitation into the atmosphere, contributing to pulsating auroras. Conventionally, energetic electrons are considered to satisfy the cyclotron resonance condition over the energy range of a few to tens of kiloelectron volts and are scattered toward the loss cone by waves. However, previous simulation studies indicate that low pitch angle electrons tend to be scattered away from the loss cone by coherent whistler mode waves. We examine the mechanism of anomalous trapping at low pitch angles, deriving a particle equation with low pitch angle assumptions. An additional term that is conventionally neglected represents the Lorentz force caused by the wave magnetic field and the parallel particle velocity. Therefore, due to the large v‖×Bw Lorentz force, low pitch angle electrons satisfying the cyclotron resonant condition are scattered away from the loss cone and effectively trapped by waves. We perform test particle simulations in a one‐dimensional dipole magnetic field with a whistler mode wave model and reproduce the anomalous trapping of electrons. The simulation results show that the majority of electrons at high and moderate pitch angles are scattered toward low pitch angle regions while low pitch angle electrons are strongly scattered toward high pitch angle regions. Consequently, a coherent chorus element produces a bump in the electron pitch angle distribution.

Description: Abstract This paper describes a novel technique that allows separation and quantification of different sources of convection in the high‐latitude ionosphere. To represent the ionospheric convection electric field, we use the Spherical Elementary Convection Systems representation. We demonstrate how this technique can separate and quantify the contributions from different magnetospheric source regions to the overall ionospheric convection pattern. The technique is in particular useful for distinguishing the contributions of high‐latitude reconnection associated with lobe cells from the low‐latitude reconnection associated with Dungey two‐cell circulation. The results from the current paper are utilized in a companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026641) to quantify how the dipole tilt angle influences lobe convection cells. We also describe a relation bridging other representations of the ionospheric convection electric field or potential to the Spherical Elementary Convection Systems description, enabling a similar separation of convection sources from existing models.

Description: Abstract Using the bow shock crossing events from four spacecraft: IMP 8, Geotail, Magion‐4, and Cluster 1, a new three‐dimensional asymmetric bow shock model is constructed. The model is parameterized by the solar wind dynamic pressure, the interplanetary magnetic field, magnetosonic Mach number, solar wind β, and the Earth's dipole tilt angle. It is shown that the shape and size of bow shock are both affected by the dipole tilt angle. The dipole tilt angle causes asymmetries in the meridional plane: (1) the bow shock subsolar standoff distance and the north‐south asymmetry increase with the dipole tilt angle; (2) as the dipole tilt angle increases, the shock flaring angle in the equatorial plane is slightly reduced, while in the meridional plane the flaring angle obviously decreases in Southern Hemisphere and keeps almost unchanged in the Northern Hemisphere. The flaring angle in the Northern Hemisphere is larger than in the Southern Hemisphere; (3) the effects of negative dipole tilt angle on shock flaring are just the opposite of those for positive tilt, and the effects of dipole tilt angle on the shape of the bow shock are north‐south symmetric. The model results are also validated by comparing with one previous empirical model and with observational crossings, and it is demonstrated that the new model is able to predict the observed crossings more accurately and can better describe the rotational asymmetry and north‐south asymmetry of the Earth's bow shock.

Description: Abstract With simultaneous ionospheric measurements from ROCSAT‐1 Satellite and ground ionosondes/GPS receivers, three cases of concurrent plasma blobs and bubbles in the same magnetic meridian were observed around 22:30 LT in Asian‐Oceanian sector during solar maximum. Two cases were observed Equatorial Spread F (ESF) over Vanimo station (geog. 2.7°S, 141.3°E; geom. 11.2°S, 146.2°W) and plasma blobs around 8.0°S (geom.) on June 1 and October 6, 2003. The other case observed ESF over Hainan station (geog. 19.5°N, 109.1°E; geom. 9.1°N, 179.1°W) and plasma blob near the dip equator on March 8, 2004. Plasma blobs were all observed near 600 km height near the equator. ESF and amplitude scintillations from the ground stations were observed near the same magnetic meridian, indicating the existence of bubbles. Considering that both plasma bubbles and blobs are field‐aligned elongated structures, magnetic field line mapping shows that in the two cases at Vanimo, blobs were above bubbles, providing direct observational evidence for blob formation in the intermediate stage of plasma bubble evolution; in the case at Hainan the blob and bubble were likely at similar height, and it could be generated by gravity wave.

Description: Abstract The magnetic field records of the magnetometer networks in the American, East Asian‐Australian, and European‐African sectors were employed in this present work. We used them to investigate equatorial electrojet (EEJ), counter electrojet (CEJ), tidal variability in EEJ strength and ionospheric current during the 2005/2006 and 2008/2009 sudden stratospheric warming (SSW) events. In addition to the well‐investigated tidal variability in EEJ strength over the American and East Asian sectors, we investigated that of the African sector for the first time. Interestingly, the tidal components in EEJ strength during both SSW events clearly exhibit marked longitudinal differences with high, moderate, and low amplitudes in the American, East Asian, and African sectors, respectively. An exception found around day 71 in the African sector after the 2008/2009 SSW event had higher solar diurnal tidal component as compared to that of the Asian sector. Over the American sector, solar and lunar semidiurnal tides were strongly associated with CEJ current during both SSW events, whereas at the African and East Asian sectors such variabilities are not evident. A solar diurnal tidal component was strongly related to a reduction in the EEJ strength over the East Asian sector. In addition, a prolonged period of CEJ occurrence that begins during the SSW precondition and ends when the SSW was evolving characterized the African sector during both SSW events. There is a steady shift in phase at later hours when both SSW events are evolving.

Description: Abstract The existence of an additional stratification in the daytime equatorial ionospheric F region (the F3 layer) was known since the 1940s. However, its characteristics and the underlying physical mechanism have been uncovered only recently. In this paper, we present and discuss the F3 layer characteristics observed by six ionosondes distributed over equatorial and low latitudes (−20° to +25° dip latitudes) in the Brazilian longitude sector during the strongest geomagnetic storm (DstMin = −223 nT) of solar cycle 24, the St. Patrick's Day storm of 17 March 2015. Two eastward prompt penetration electric field (PPEF) events, as seen in equatorial electrojet, occurred during the main phase of the storm on 17 March 2015, a strong one (~100 nT) at around ~1200 UT and a weak one (~50 nT) at around ~1725 UT. Local time variations in the F3 layer occurrence and ionospheric base height (h′F), peak height (hmF), and peak electron density (Nmax) are investigated. Notably, the F3 layer occurred at all six locations, more distinctly during the stronger PPEF event. The large latitudinal extend in the occurrence of the F3 layer in opposite hemispheres (−20° to +25° dip latitudes) covering the equatorial ionization anomaly crests observed for the first time is interpreted in terms of the combined effect of the super plasma fountain generated by the eastward PPEF and storm time equatorward neutral wind.

Description: Abstract We investigate the statistical characteristics of the ion upflow occurrence in association with ion and electron heatings in the polar ionosphere using the European Incoherent Scatter Svalbard Radar during the period of 2000–2015. The ion upflow events are classified as four types: with ion temperature increase (Type 1), with electron temperature increase (Type 2), with both ion and electron temperature increases (Type 3), and without any temperature increase (Type 4). These four types of upflow events are statistically analyzed with various geophysical conditions. We found that the overall occurrence of ion upflow is highest for Type 3 and then followed by Type 2, Type 1, and Type 4. This result indicates that the ion upflow is highly associated with soft particle precipitation induced electron heating, which becomes more effective with simultaneous friction induced ion heating. The statistical characteristics of ion upflow is summarized as follows: (1) It is most highly distributed in the daytime with a double peak structure but a deep minimum at dusk, (2) the highest occurrence appears at about 350‐ to 450‐km altitude for most of local time but extended to higher altitude near the magnetic local noon, (3) the ions mostly reach only up to about 200 km above their initiated altitudes, (4) it tends to increase with magnetic activity, particularly during the daytime, but (5) decreases and distributed at higher altitude with increasing solar activity, and (6) finally, the maximum occurrence appears in December solstice but the minimum in June solstice for most of local times.

Description: Abstract Two remarkable ionospheric irregularities named equatorial plasma bubble (EPB) and medium‐scale traveling ionospheric disturbance (MSTID) are verified by using multi‐instrumental observations, for example, the ground‐based GPS networks, ionosonde stations, and Swarm satellites, when the tropical cyclones Tembin and Hagibis approached Hong Kong on 26 August 2012 and 15 June 2014, respectively. The low‐latitude plasma bubble over an area of 105–120 °E in longitude was observed during 12:30–17:00 universal time (UT) on 26 August 2012. GPS observations from magnetically conjugate locations indicate that the nighttime bubble during 20:30–01:00 local time (LT = UT + 8h) on 26 August should be formed from the magnetically equatorial region rather than drifted from the west (eastward drift) or generated locally. Different from the EPB, during another cyclone on 15 June 2014, the northwest‐southeast aligned nighttime MSTID was verified in midlatitude regions at a mean horizontal velocity of 156 m/s southwestward propagation during 12:30–17:30 UT when Hagibis was near the mainland coast. By comparing the ionospheric observations during the two cyclones, differences are identified: small‐scale irregularities associated with plasma bubble cause obvious perturbations of the rate of total electron content index with the value of ~3.0 TECU/min; while the MSTID in midlatitude only cause relatively slight rate of total electron content index disturbances with the value of ~1.5 TECU/min. In addition, the magnetic conjugacy of EPB and MSTID in two hemispheres during the passage of tropical cyclones has been also discussed in this study.

Description: Abstract Traditionally, the ionosphere determination just uses American Global Positioning System and Russian GLObal NAvigation Satellite System dual‐frequency data and has a low precision particularly on oceans. With the rapid development of Chinese BeiDou and European Galileo systems, they are playing an increasingly important role for modeling global ionosphere. Meanwhile, satellite altimetry provides valuable and precise ionosphere delay over the oceans. Through introducing priori ionosphere values from an advanced empirical ionosphere model, combining the advantages of Global Navigation Satellite System (GNSS) and satellite altimetry technologies, the precision of global ionosphere estimation can be further improved. To assess the improvement, we collect satellite altimetry data from Jason‐2/3 and more than 300 global GNSS stations, the data are processed in 2014 and 2018 when the Sun is in a high and a low activity conditions. The results suggest that the ionosphere determination based on multitechnique fusion in a solar‐geomagnetic reference frame is well suitable to represent the ionosphere and its structure. The determined ionosphere achieves a better global consistency, and its formal accuracy is significantly reduced. By comparing with the International GNSS Service products, evaluating by satellite altimetry measurements, and independently validating with ionosonde techniques, it is proved that the ionosphere results are further improved through employing additional available data, especially for the ionosphere over the oceans.

Description: Abstract Cluster data from late July to early October were used to study the distribution of field‐aligned electron (FAE) events around the two cusps. An FAE event was defined as electron parallel flux 〉3 × 108 (cm2 s)−1. The total number of FAE events around the two cusps was basically identical, but downward FAE events prevailed in the south and upward FAE events in the north. In the southern cusp, the peak of the FAE events distribution versus altitude was about 1.3 RE higher and the peak of the FAE events distribution versus invariant latitude (ILAT) was about 4° ILAT lower. Only the downward FAEs around the southern cusp had a second ILAT peak, which was located about 11° higher than the main peak. The normalized number of FAEs showed nearly the same features as the unnormalized number of the FAEs events. These results indicated a north‐south asymmetry of the FAE distribution around the two cusps. Some causes for the asymmetry are discussed, the main ones being the asymmetry of the magnetospheric configuration resulting from geomagnetic dipolar tilt and solar wind flows, the interplanetary magnetic field asymmetry related to the magnetosphere, and the difference of ionospheric conductivity in the two hemispheres. Various solar wind‐magnetosphere interaction processes, such as quasi‐viscous interaction and reconnection, are responsible for the asymmetry, too. The second distribution peak (at higher ILAT) of the downward FAE events around the southern cusp corresponded to high solar wind speed and may be associated with the northward interplanetary magnetic field Bz field‐aligned current at low altitude. This requires further studies, however.

Description: Abstract The present study shows that the ionospheric Hall polarization can deform the high‐latitude ionospheric convection field, which is widely considered to be a manifestation of the convection field in the magnetosphere. We perform the Hall polarization field separation with a potential solver by changing the conductance distribution step‐by‐step from a uniform one to a more realistic one. We adopt dawn‐dusk and north‐south symmetric distributions of conductance and region 1 (R1) field‐aligned current (FAC). The pair of the primary field of the R1 system and each gradient of Off‐diagonal component of conductance tensor (Hall conductance) generates the Hall polarization field and consequently causes potential deformations as follows. (a) The equatorward gradient causes clockwise rotation. (b) The gradient across the terminator, together with the effect of the equatorward gradient, causes the dawn‐dusk asymmetry. (c) The high conductance band in the auroral region causes kink‐type deformations. In particular, a nested structure at the equatorward edge of the band in the midnight sector well resembles the Harang Reversal. Result (a) can explain the clockwise bias inexplicable by the IMF‐By effect alone, the combination of (a) and (b) can explain the clearness and unclearness in the round or crescent shapes of the dawn‐dusk cells depending on the IMF‐By polarity, and (c) suggests that the ionosphere may not need the upward‐FAC for the formation of the Harang Reversal. We suggest that the final structure of the ionospheric potential is established by the combined effects of the magnetospheric requirements (external causes) and ionospheric polarization (internal effect).

Description: Abstract We present a continuing investigation of mass‐/charge‐dependent interactions between energetic ions (greater than tens of kiloelectron volts) and planetary magnetopauses and of the escape of the ions across the boundary. Previous studies at Earth using Magnetospheric Multiscale mission data are refined and advanced showing profound behavior differences between light (H, He) and singly charged heavy ions (O+). We highlight a distinctive feature of oxygen ions: an angular distribution bifurcation providing clear indication of entrainment along the magnetopause in Speiser‐like orbits during relatively stable magnetic conditions. This signature, interpreted using a simple kinetic model, suggests that these ions tend to be carried substantial distances along the boundary (even with boundary‐normal magnetic fields) in a fashion that impedes their full dayside escape. While large fluctuations and waves can likely sometimes disrupt the observed ordering, the following picture emerges. Energetic particles with gyroradii much smaller than the magnetopause thickness (e.g., electrons and absent boundary‐normal magnetic fields) and ions with gyroradii much larger than the thickness (e.g., O+) are impeded from fully escaping across the boundary. However, energetic ions with intermediate‐sized gyroradii commensurate with the thickness (e.g., H+, He++, and O6+) can be effectively scattered within the boundary causing them to escape much more readily, with and without boundary‐normal fields. This picture is supported by observations from the Juno spacecraft at the near‐dawn meridian side of Jupiter's magnetopause. There it is observed that energetic electrons and heavy ions are more strongly contained by the magnetopause than are the energetic protons and helium ions.

Description: Abstract The loss cone electron distribution function can be unstable to the excitation of whistler instability, which can be effective in pitch angle diffusion, thus rapidly filling up the loss cone and removing the free energy source. The present paper carries out a combined quasi‐linear analysis and one‐dimensional particle‐in‐cell simulation in order to investigate the dynamical consequences of the excitation of whistler instability. Thermal ring distribution can be considered as a simple substitution for the actual loss cone distribution. It is found according to both the reduced quasi‐linear theory and the particle‐in‐cell simulation that while the whistler instability is effective in pitch angle diffusion of the initial loss cone distribution, the complete isotropization is not achieved such that substantial loss cone persists in the saturation stage. This finding may explain the fundamental question of why weak loss cone feature persists in the magnetosphere and, more importantly, why charged particles trapped in dipole field do not steadily undergo pitch angle scattering and be lost eventually.

Description: Abstract We report, for the first time, strong evidences that a fast Fermi mechanism is taking place at the Mars bow shock. The MAVEN spacecraft observations from the Solar Wind Electron Analyzer instrument show electron flux spikes with energies up to ∼1.5 keV. These spikes are associated with sunward propagating electrons and appear when the interplanetary field line threading the spacecraft is connected near the Martian bow shock tangency point. The observed loss cone distribution is a salient feature of these backstreaming electrons as the phase space density peaks on a ring centered along the magnetic field direction. Moreover, the data show no evidence of any effect due to a hypothetical cross‐shock electric potential on the observed angular distributions. Although similar distributions are seen at the terrestrial bow shock, the quantitative analysis of the measurements strongly indicates that the electrons are produced at the shock foot and escape upstream before exploring the entire shock structure.

Description: Abstract Subauroral polarization stream (SAPS) is latitudinally narrow flow channels of large westward plasma drifts in the subauroral ionosphere. In this study, the global structure and dynamic evolution of SAPS are investigated by using the Coupled Magnetosphere‐Ionosphere‐Thermosphere model with ring current extension, namely, the Lyon‐Fedder‐Mobarry‐Thermosphere Ionosphere Electrodynamics General Circulation Model‐Rice Convection Model, to simulate the 2013 St. Patrick's Day storm event. This is the first time that the global distribution and temporal evolution of SAPS are investigated using first‐principle models. The model shows a strong westward ion drift channel formed equatorward of the auroral electron precipitation boundary on the duskside, which is identified as the SAPS structure. The simulated ion drift velocity and auroral electron precipitation sampled along the trajectory of the Defense Meteorological Satellite Program F18 satellite are in good agreement with the satellite measurements. SAPS initiate in the predusk sector when the interplanetary magnetic field turns southward. SAPS latitude generally decreases with magnetic local time from dusk to midnight. The SAPS channel shows wedge, inverse wedge, and crescent morphologies during the storm and becomes discontinuous when the interplanetary magnetic field is weakly southward. The SAPS mean latitude has a correlation coefficient of 0.77 with the Dst index. The mean latitude moves equatorward, and the flow channel broadens in the storm main phase. The simulation results illustrate both the global distribution and highly dynamic behavior of SAPS that are not readily apparent from the observation data.

Description: Abstract Pi2 oscillations (40–150 s) in the nightside upper ionosphere are studied using magnetic field data acquired by multiple Swarm spacecraft in low‐Earth orbit and at the low‐latitude Bohyun (BOH, L = 1.3) station on 22 October 2014. Four Pi2 events were identified from the BOH data near midnight (magnetic local time = 1.5 hr), while Swarm‐A, Swarm‐B, and Swarm‐C spacecraft were orbiting in the premidnight (magnetic local time = 21–22 hr) meridian from 70° to −60° in magnetic latitude at ∼450‐ to 500‐km altitudes. Unlike previous low‐Earth orbit studies, which used a single point observation, the latitudinal structure of the amplitude and phase of ionospheric magnetic field perturbations can be determined by simultaneous multipoint observations along the latitude at a constant radial distance. We observed that the horizontal H component of BOH data is well correlated with the compressional (Bz) component of ionospheric magnetic fields when Swarm spacecraft were at |magnetic latitude|  〈 30° with or without an accompanying ionospheric field perturbation in the radial (Bx) component, depending on the latitude of the spacecraft. It is found that the phase and amplitude relationship between Bx and Bz along the latitude is consistent with the model ionospheric field perturbations at 500‐km altitude, which are associated with a plasmaspheric resonance excited in a dipole numerical simulation. This indicates that the latitudinal variation of the ionospheric Pi2 pulsations in both Bx and Bz components is the consequence of the spatial mode structure in the north‐south direction of trapped fast mode waves inside the plasmasphere.

Description: Abstract Convection under the due northward interplanetary magnetic field (IMF) is reproduced by the global simulation. The resulting magnetosphere is closed except in the XZ plane and separated from the solar wind by the separatrix generated from cusp nulls. Inside the separatrix, there exist three plasma regimes of the cusp high‐pressure region, the low‐latitude boundary layer (LLBL) and the plasma sheet. In the ionosphere, the northward Bz (NBZ) current and the reverse cell occur in higher latitudes than 80°, and the fun‐shaped arc‐like field‐aligned current and the main oval occur in lower latitudes than 80°. Magnetic field lines in the antisunward flow region of the reverse cell are connected to the LLBL that is accelerated to supersonic flow by the cusp pressure. Circulation on the reverse cell in the ionosphere is as a whole constructed to the interchange cycle in the magnetosphere. Convection is looked upon as the process to discharge stress generated by the dayside cusp reconnection. Magnetic stress generated by the reconnection is first converted to thermal energy in the cusp. This thermal energy is drained through three possible routes: release of plasma downtail through the LLBL, dissipation as electromagnetic energy through formation of the dynamo, and evacuation down to the ionosphere through the plasma sheet.

Description: Abstract A hybrid gyrofluid‐kinetic electron model is adapted and used to simulate poloidal standing modes for different electron temperatures and azimuthal mode numbers. As in previous studies of toroidal standing modes, mirror force effects lead to increased parallel potential drops, monoenergetic electron energization, and wave energy dissipation as the ambient electron temperature is increased. A similar trend is also observed when the electron temperature is held fixed and the azimuthal mode number increased—owing to the narrowing of the azimuthal flux tube width, which necessitates more electron energization to carry the increased parallel current density. In both cases, the increase in electron energization eventually leads to more rapid decreases in the parallel current with time because of the dissipation of wave energy.

Description: Abstract We study the release of various elements from Callisto's surface into its exosphere by plasma sputtering. The cold Jovian plasma is simulated with a 3‐D plasma‐planetary interaction hybrid model, which produces 2‐D surface precipitation maps for magnetospheric H+, O+, O++, and S++. For the hot Jovian plasma, we assume isotropic precipitation onto the complete spherical surface. Two scenarios are investigated: one where no ionospheric shielding takes place and accordingly full plasma penetration is implemented (no‐ionosphere scenario) and one where an ionosphere lets virtually none of the cold plasma but all of the hot plasma reach Callisto's surface (ionosphere scenario). In the 3‐D exosphere model, neutral particles are sputtered from the surface and followed on their individual trajectories. The 3‐D density profiles show that whereas in the no‐ionosphere scenario the ram direction is favored, the ionosphere scenario produces almost uniform density profiles. In addition, the density profiles in the ionosphere scenario are reduced by a factor of ∼2.5 with respect to the no‐ionosphere scenario. We find that the Neutral Gas and Ion Mass Spectrometer, which is part of the Particle Environment Package on board the JUpiter ICy moons Explorer mission, will be able to detect the different sputter populations from Callisto's icy surface and the major sputter populations from Callisto's nonicy surface. The chemical composition of Callisto's exosphere can be directly linked to the chemical composition of its surface and will offer us information not only on Callisto's formation scenario but also on the building blocks of the Jupiter system.

Description: Abstract We present a new method for determining the main relevant features of the local magnetic field configuration, based entirely on the knowledge of the magnetic field gradient using four‐spacecraft measurements. The method, named “Magnetic Configuration Analysis” (MCA), estimates the spatial scales on which the magnetic field varies locally. While it directly derives from the well‐known Magnetic Directional Derivative (MDD) and Magnetic Rotational Analysis (MRA) procedures (Shi et al., 2005, doi:10.1029/2005GL022454; Shen et al., 2007, doi:10.1029/2005JA011584), MCA was specifically designed to address the actual magnetic field geometry. By applying MCA to multi‐spacecraft data from the MMS satellites, we perform both case and statistical analyses of local magnetic field shape and dimensionality at very high cadence and small scales. We apply this technique to different near‐Earth environments and define a classification scheme for the type of configuration observed. While our case studies allow us to benchmark the method with those used in past works, our statistical analysis unveils the typical shape of magnetic configurations and their statistical distributions. We show that small‐scale magnetic configurations are generally elongated, displaying forms of cigar and blade shapes, but occasionally being planar in shape like thin pancakes (mostly inside current sheets). Magnetic configurations, however, rarely show isotropy in their magnetic variance. The planar nature of magnetic configurations and, most importantly, their scale lengths strongly depend on the plasma β parameter. Finally, the most invariant direction is statistically aligned with the electric current, reminiscent of the importance of electromagnetic forces in shaping the local magnetic configuration.

Description: Abstract We performed the first systematic analysis of pickup ion (PUI) cutoff speed variations, across compression regions and due to fast fluctuations in solar wind (SW) speed and magnetic field strength. This study is motivated by the need to remove or correct for systematic effects on the determination of the interstellar flow longitude based on the longitudinal variation of the PUI cutoff. Using 2007–2014 STEREO A PLASTIC observations, we identified SW compression regions and accumulated the contained PUI velocity distributions in a superposed epoch analysis. The shift of the cutoff in velocity, interpreted as PUI energization, varies systematically across the compression region and increases approximately linearly with the speed gradient of the compression. Additionally, the shift remains positive into the negative speed gradient at the beginning of the rarefaction region. A similar response is found when PUI distributions are sorted according to the strength of fast fluctuations in SW speed, density, and magnetic field strength. These parameters remain high in the first part of the rarefaction region, suggesting a possible PUI energization through compressive turbulence. Based on these results, we removed the strongest compression regions from the interstellar flow analysis, finding no significant change in direction or uncertainty. Thus, we have revealed the influence of adiabatic compression and compressive turbulence, increasing the PUI cutoff energy, and we have demonstrated that the determination of the interstellar inflow direction via analysis of PUI distributions is robust for a multiyear data set, even in the presence of SW interaction regions.

Description: Abstract Recent studies of Pc5‐band (150–600 s) ultralow frequency waves found that foreshock disturbances can be a driver of dayside compressional waves and field line resonance, which are two typical Pc5 wave modes in the dayside magnetosphere. However, it is difficult to find spatial structure of dayside Pc5 waves using a small number of satellites or ground magnetometers. This study determines 2‐D structure of dayside Pc5 waves and their driver by utilizing coordinated observations by the THEMIS satellites and the all‐sky imager at South Pole during two series of Pc5 waves on 29 June 2008. These Pc5 waves were found to be field line resonances (FLRs) and driven by foreshock disturbances. The ground‐based all‐sky imager at South Pole shows that periodic poleward moving arcs occurred simultaneously with the FLRs near the satellite footprints over ~3° latitude and had the same frequencies as FLRs. This indicates that they are the auroral signature of the FLRs. The azimuthal distribution of the FLRs in the magnetosphere and their north‐south width in the ionosphere were further determined in the 2‐D images. In the first case, the FLRs distribute symmetrically in the prenoon and postnoon regions with out‐of‐phase oscillation as the odd toroidal mode in the equatorial plane. In the second case, the azimuthal wavelengths of the 350–500 s and 300–450 s period waves were ~8.0 and ~5.2 Re in the equatorial plane. It also shows a fine azimuthal structure embedded in the large‐scale arcs, indicating that a high azimuthal wave number (m ~ 140) mode wave coupled with the low‐wave number FLRs.

Description: Abstract Structures of sudden enhancements/depressions and associated interhemispheric asymmetry in low‐latitude total electron content (TEC) during the main phase (MP) of geomagnetic storms have remained unpredictable majorly due to oscillating equatorial vertical E×B drifts and resultant redistribution of plasma in low latitudes in a given seasonal background. Robust analysis of 7 major and 30 moderate ionospheric storms during the years 2000–2018 is performed with comprehensive literature review encompassing various sources of asymmetry in magnetosphere‐ionosphere coupling. Taking advantage of simultaneous long‐term observations of E×B drift from Jicamarca, H component from magnetometers, and global ionospheric map vertical TEC (VTEC) and TEC observations across the dip equator from the South American sector, simultaneous formation of peaks and valleys in VTEC and associated asymmetry are studied. Additionally, a three‐layer neural network‐based E×B drift model is developed using delta‐H observations that provide drift estimates in the absence of Jicamarca drifts. The main results establish simultaneous high‐magnitude short‐lived (1–2 hr) enhancements and depression in VTEC during the MP in daytime in both hemispheres with varying differences of −30 to 100 TECU with respect to quiet time mean and along with prominent existence of interhemispheric asymmetry in TEC during the MP regardless of seasons. Maximum VTEC in the northern and southern low latitudes is found to occur at different times during storms. Large difference of VTEC is found ranging between 10 and 30 TECU between the near conjugate locations of the hemispheres. Coincident global episodic peaks marked by steep VTEC falls show dominance of episodic eastward and westward penetration electric fields in the low‐latitude daytime ionosphere.

Description: Abstract The Van Allen Probes have observed both symmetric and asymmetric bipolar electric field structures in the Earth's inner magnetosphere. In general, the symmetric bipolar structures are identified as electron phase space holes, whereas the asymmetric structures are interpreted as electron acoustic double layers (EADLs). The generation mechanism of these EADLs is not entirely understood yet. We have modelled the EADLs observed on November 13, 2012 by Van Allen Probe‐B. We performed a fluid simulation of the EADLs and tracked their formation and evolution in the simulation. We found that the localized depletion and enhancement in the electron populations act as a perturbation to excite the symmetric bipolar electron acoustic solitary waves (EASWs), which later evolve into the EADLs. The Ponderomotive force is found to be the main driver behind transformation of the symmetric EASWs to EADLs via formation of the electron‐acoustic shocks.

Description: Abstract An all‐sky imager at El Leoncito Observatory (−31.8°, 69.3°W, 18.2° magnetic latitude) is used to study 630.0‐nm airglow emissions related to medium‐scale traveling ionospheric disturbances (MSTIDs). On the night of 6 December 2007 an unusual event consisting of bright bands propagating northwestward was observed. Enhancements in total electron content from ground‐based Global Positioning System receivers were observed collocated with the bright airglow bands. A regional Global Positioning System‐derived total electron content map matches the direction of motion, scale size, and location of these bright bands. Model results including F region coupling with E region structures reproduce the characteristics of the bright bands. Specific conditions in the E region must exist in order to observe these unusual MSTIDs consisting of propagating bright bands only.

Description: Abstract Numerical experiments performed with the National Center for Atmospheric Research thermosphere‐ionosphere‐electrodynamics general circulation model forced with an observationally based diurnal and semidiurnal tidal spectrum are utilized to investigate the physical processes by which the dissipation of vertically propagating tides act to alter the zonally and diurnally averaged (“zonal‐mean”) dynamical state of the thermosphere. Below 150 km the largest contributors to the zonal‐mean zonal wind distribution are the pressure gradient, Coriolis, and the tidally driven momentum flux divergence terms, the latter being about half the former two. Ion drag plays a smaller role in the lower thermosphere but becomes increasingly important at higher altitudes (i.e., above ∼150 km). We also find that the tidally driven heat flux divergence term contributes to the generation of zonal‐mean zonal winds through its coupling into the pressure gradient term. Tidally induced zonally and diurnally averaged zonal wind changes achieve values of up to 30 m/s in the lower thermosphere, of which about a third can be traced to the heat flux divergence. Tidal amplitudes used to force the thermosphere‐ionosphere‐electrodynamics general circulation model lower boundary represent conservative estimates, since they are based on forcing by a multiyear 60‐day mean tidal climatology, which significantly underestimates their amplitudes at any given time. Eliassen‐Palm Fluxes further support the conclusion that the heat flux divergence has a statistically significant direct impact on the zonal‐mean zonal wind balance in the lower thermosphere.

Description: Abstract Field‐aligned currents (FACs) are a key component of the magnetosphere‐ionosphere system, providing the transfer of energy and momentum between the distant collision‐free magnetospheric plasma and the collisional ionosphere. ESA's Swarm mission offers a unique opportunity to explore the FACs low altitude end by a broad range of single‐ and multi‐spacecraft techniques. The present technical report demonstrates the application of dual‐ and three‐satellite FAC estimation methods based on the least‐squares approach, with the goal to enhance the use of Swarm data in exploring the high latitude current structures, beyond the official FAC product presently available to the end‐user. The dual‐satellite method presents some clear advantages since it provides stabler solutions, can be applied on a more general spacecraft configuration, and offers a robust error estimation scheme. Consequently, the method provides significantly more data near the singularity where the satellites' orbits intersect, allows FAC estimation with configurations that involves the upper Swarm satellite, or to fine tune the constellation geometry to the problem at hand. Similarly, the three‐satellite method, meant to be applied when Swarm forms a close configuration, brings additional valuable information, associated to a different (larger) scale. Moreover, it can estimate the FAC density with high time resolution when instantaneous measurements are used. The performance of the methods are thoroughly analyzed both on Swarm events as well as on simulated data, and the results are compared with other available methods. Particular emphasis is put on how different FAC estimation methods complement each other and provide consistent results.

Description: Abstract The Jovian Auroral Distributions Experiment Ion sensor (JADE‐I) on Juno is a plasma instrument that measures the energy‐per‐charge (E/Q) distribution of 0.01 to 46.2 keV/q ions over a mass‐per‐charge (M/Q) range of 1 – 64 amu/q. However, distinguishing O+ and S2+ from JADE‐I's measurements is a challenging task due to similarities in their M/Q (≈ 16 amu/q). Because of this, O+ and S2+ have not been fully resolved in the in‐situ measurements made by plasma instruments at Jupiter (e.g., Voyager PLS and Galileo PLS) and their relative ratios has been studied using physical chemistry models and UV remote observations. To resolve this ambiguity, a ray‐tracing simulation combined with carbon foil effects is developed and used to obtain instrument response functions for H+, O+, O2+, O3+, Na+, S+, S2+, and S3+. The simulation results indicate that JADE‐I can resolve the $M/Q$ ambiguity between O+ and S2+ due to a significant difference in their charge state modification process and a presence of a large electric potential difference (≈ 8 kV) between its carbon foils and MCPs. A forward model based on instrument response functions and eight convected kappa distributions is then used to obtain ion properties at the equatorial plasma sheet (≈ 36 jovian radii) in the pre‐dawn sector of magnetosphere. The number density ratio between O+ and S2+ for the selected plasma sheet crossings ranges from 0.2 to 0.7 (mean value 0.37 ± 0.12) and the number density ratio between total oxygen ions to total sulfur ions ranges from 0.2 to 0.6 (0.41 ±0.09).

Description: Abstract We examine the effect on drift‐resonant particle dynamics of a strongly peaked internally driven poloidal mode FLR (with specified frequency ω in the Pc5 range and azimuthal mode number m〉〉1). Using an analytic MHD model in a dipole field to describe the ULF wave mode, we use the bounce‐averaged formalism of Northrop (1963) to obtain equations of motion for charged particles in the wave frame, and find an analytic solution for the case of a temporally constant ULF wave amplitude profile. Focusing on equatorially mirroring electrons in this study, we demonstrate that, for sufficiently peaked radial profiles, multiple drift resonances appear that are associated with the FLR peak. These are in addition to the well‐known zeroth order drift resonance location, occurring when the unperturbed drift speed φ0 satisfies the resonance condition (mφ‐ω = 0). The additional resonances arise because the strongly peaked FLR wave field components provide sufficiently strong perturbations to the azimuthal drift speed to cause multiple zero‐crossings in the resonance condition. These additional resonances have trapping periods much lower than that of the zeroth order resonance, and considerably complicate the electron dynamics. Further properties of these resonances and their measurable effect on electron dynamics are discussed. For example, their effect on observations of energetic electron flux on board satellites in the vicinity of an FLR is calculated, and shown to significantly distort the typical signature associated with drift resonance (modulations in electron flux at the wave frequency with a 180 degree phase change across the resonant energy).

Description: Abstract Although sudden stratospheric warming (SSW) is mainly a northern high‐latitude phenomena, there are several reports of a concomitant global dynamical response throughout the mesosphere and lower thermosphere. Published reports based on model simulations so far attributed such variabilities to changes in global circulation; however, there is no clear explanation of how all these regions are physically connected during SSW events. The present investigation uses wind observations from two ground‐based specular meteor radars over northern high latitudes and midlatitudes and global winds from a high‐altitude meteorological analysis system to characterize global mesospheric circulation anomalies for major SSW events during 2010 and 2013. During these events radar observations and the reanalysis winds exhibited strong southward winds over the two northern midlatitude and high‐latitude stations. By removing seasonal variability from the high‐altitude meteorological analyses, we show that these southward wind anomalies are indeed part of a larger global‐scale circulation, which gets set up during SSW and extend from the Northern pole to low‐latitude regions of Southern Hemisphere in the mesosphere and lower thermosphere altitudes. These results also offer a possible explanation of how low‐latitude ionospheric electrodynamics are influenced by the changes in the circulation set in during SSW at high latitudes.

Description: Abstract Head‐on collisions between negative and positive streamers have been proposed as a mechanism behind X‐ray emissions by laboratory spark discharges. Recent simulations using plasma fluid and particle in cell models of a single head‐on collision of two streamers of opposite polarities in ground pressure air predicted an insignificant number of thermal runaway electrons 〉1 keV and hence weak undetectable X‐ray emissions. Because the current available models of a single streamer collision failed to explain the observations, we first use a Monte Carlo model coupled with multiple static dielectric ellipsoids immersed in a subbreakdown ambient electric field as a description of multiple streamer environment and we investigate the ability of multiple streamer‐streamer head‐on collisions to accelerate runaway electrons 〉1 keV up to energies ∼200–300 keV instead of just one single head‐on collision. The results of simulations show that the streamer head‐on collision mechanism fails to accelerate electrons; instead, they decelerate in the positive streamer channel. In a second part, we use a streamer plasma fluid model to simulate a new streamer‐electron acceleration mechanism based on a collision of a large negative streamer with a small neutral plasma patch in different Laplacian electric fields |E0|= (35, 40, 45) kV/cm, respectively. We observe the formation of a secondary short propagating negative streamer with a strong peak electric field 〉250 up to 378 kV/cm over a time duration of ∼0.16 ns at the moment of the collision. The mechanism produces up to 106 runaway electrons with an upper energy limit of 24 keV.

Description: Abstract The neutral exosphere of Mars extends far upstream beyond the bow shock, and as a result, solar wind protons can charge exchange with this neutral exosphere to produce energetic neutral atoms. Energetic neutral atoms produced directly upstream of Mars will precipitate into the Martian dayside atmosphere, where some fraction can undergo a charge stripping reaction and can be observed as “penetrating protons.” Clear, quasiperiodic modulations in penetrating proton densities are observed during certain Mars Atmosphere and Volatile EvolutioN (MAVEN) periapsis passes, and we show that these modulations occur during radial interplanetary magnetic field conditions. During such times, the region sunward of Mars is defined by quasi‐parallel shock conditions, generating foreshock structures characterized by enhancements in magnetic field strength, enhancements in proton density, and deceleration and deflection of the solar wind flow. These structures are observed at time cadences equal to the modulation of penetrating proton densities at periapsis. Particle tracing simulations show that the convection of these structures with the solar wind leads to localized enhancements in the rate of charge exchange upstream of the shock, producing the observed temporal variations in penetrating proton densities at periapsis. The observation of modulated penetrating proton densities at periapsis can thus be used to infer the existence of radial interplanetary magnetic field conditions upstream of the bow shock at Mars at times when MAVEN does not sample the upstream solar wind.

Description: Abstract Satellite observations indicate that multiple components often exist in the plasma sheet, particularly during impulsive fast flow events. In this paper, we perform a kinetic analysis of the energy transport of plasma sheet ion flow using a model of two‐component plasma sheet ion flows (Background convection flow represented by subscript “b” and Fast flow represented by subscript “f”) and compare the energy transport calculated by kinetic approach Qk with those obtained from magnetohydrodynamic (MHD) approach QMHD. The ratio of Qk/QMHD is always larger than unity and is positively proportional to the ratios of Vf/Vb and Tf/Tb. The maximum values of Qk/QMHD occur in the low‐speed ranges (i.e., small density ratio Nf/N). When Nf/N exceeds 0.4, the ratio of Qk/QMHD is almost the same for a wide parameter ranges of Vf, Tf, and Vb. Heat flux is important in low‐speed range and is neglectable in the high‐speed range. The adiabatic polytropic index 5/3 cannot correctly describe energy transport rate. A density ratio Nf/N of 0.3% of high‐speed ion flow can make the effective polytropic index obviously deviate from adiabatic polytropic index (5/3). The above theoretic results can well explain previously reported satellite in situ observations.

Description: Abstract It has previously been shown that in the high‐latitude thermosphere, sudden changes in plasma velocity (such as those due to changes in interplanetary magnetic field) are not immediately propagated into the neutral gas via the ion‐drag force. This is due to the neutral particles (O, O2, and N2) constituting the bulk mass of the thermospheric altitude range and thus holding on to residual inertia from a previous level of geomagnetic forcing. This means that consistent forcing (or dragging) from the ionospheric plasma is required, over a period of time, long enough for the neutrals to reach an equilibrium with regard to ion drag. Furthermore, mesoscale variations in the plasma convection morphology, solar pressure gradients, and other forces indicate that the thermosphere‐ionosphere coupling mechanism will also vary in strength across small spatial scales. Using data from the Super Dual Auroral Radar Network and a Scanning Doppler Imager, a geomagnetically active event was identified, which showed plasma flows clearly imparting momentum to the neutrals. A cross‐correlation analysis determined that the average time for the neutral winds to accelerate fully into the direction of ion drag was 75 min, but crucially, this time varied by up to 30 min (between 67 and 97 min) within a 1,000‐km field of view at an altitude of around 250 km. It is clear from this that the mesoscale structure of both the plasma and neutrals has a significant effect on ion‐neutral coupling strength and thus energy transfer in the thermosphere.

Description: Abstract The Kelvin‐Helmholtz instability is a known mechanism for penetration of solar wind matter into the magnetosphere. Using three‐dimensional, resistive magnetohydrodynamic simulations, the double midlatitude reconnection (DMLR) process was shown to efficiently exchange solar wind matter into the magnetosphere, through mixing and reconnection. Here we compute test particle orbits through DMLR configurations. In the instantaneous electromagnetic fields, charged particle trajectories are integrated using the guiding center approximation. The mechanisms involved in the electron particle orbits and their kinetic energy evolutions are studied in detail, to identify specific signatures of the DMLR through particle characteristics. The charged particle orbits are influenced mainly by magnetic curvature drifts. We identify complex, temporarily trapped trajectories where the combined electric field and (reconnected) magnetic field variations realize local cavities where particles gain energy before escaping. By comparing the orbits in strongly deformed fields due to the Kelvin‐Helmholtz instability development, with the textbook mirror‐drift orbits resulting from our initial configuration, we identify effects due to current sheets formed in the DMLR process. We do this in various representative stages during the DMLR development.

Description: Abstract A double‐peak structure in the peak height of ionospheric F2 layer around ±10o geomagnetic latitudes similar to the equatorial ionization anomaly was recently reported. This unique feature was referred as the equatorial height anomaly (EHA). In the present paper, a simulation study is carried out using the data‐driven artificial neural network‐based two‐dimensional ionospheric model (ANNIM‐2D) and the physics‐based thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) to understand the local time and latitudinal variation of EHA during the main phase of St. Patrick's Day geomagnetic storm. Both the ANNIM‐2D and TIEGCM consistently show pronounced EHA during the main phase of the geomagnetic storm. Further, the local time of EHA development on the storm day is much earlier (nearly 2 hr) than the quiet time over Brazilian sector (90°W). The TIEGCM simulation revealed that the storm time enhancement of the equatorial fountain associated with the enhanced equatorial zonal electric field is the main controlling factor for the pronounced EHA during the main phase. The storm time meridional neutral winds positively contribute to the development of EHA. This study revealed the direct manifestation of the storm time‐enhanced plasma fountain on the EHA.

Description: Abstract Electron cyclotron harmonic (ECH) and whistler chorus waves are recognized as the two mechanisms responsible for the resonant wave‐particle interactions necessary to precipitate plasma sheet electrons into the ionosphere, producing the diffuse Aurora. Previous work has demonstrated ECH waves dominate electron scattering at L shells 〉8, while whistler chorus dominates scattering at L shells L 〈 8. However, we find from Time History of Events and Macroscale (THEMIS) Interactions during Substorms observations of fast flows at L = 12 that oblique whistler chorus emissions play the dominant role in scattering electrons. Previous works have identified whistler‐mode waves within fast flows that are produced by an electron temperature anisotropy Te,⊥/Te,||〉 1, consistent with electron betatron acceleration. Here, however, we find whistler chorus emissions throughout an interval of fast flows where Te,⊥/Te,||〈 1. Parallel electron beams account for the enhanced parallel electron temperature and serve as the instability mechanism for the whistler chorus. The parallel electron beams and associated cigar‐shaped distributions are consistent with Fermi acceleration at dipolarizations in fast flows. We demonstrate that the scattering efficiency of the whistler chorus exceeds that of ECH waves, which THEMIS also detects during the fast flows. The obliquity of the whistler waves permits efficient scattering of lower‐energy electrons into the diffuse aurora. We conclude that Fermi acceleration of electrons provides one important free‐energy source for the wave‐particle interactions responsible for coupling plasma sheet electrons into the diffuse aurora during substorm conditions.

Description: Abstract We simulated the effects of the 21 August 2017 total solar eclipse on the ionosphere‐thermosphere system with the Global Ionosphere Thermosphere Model (GITM). The simulations demonstrate that the horizontal neutral wind modifies the eclipse‐induced reduction in total electron content (TEC), spreading it equatorward and westward of the eclipse path. The neutral wind also affects the neutral temperature and mass density responses through advection and the vertical wind modifies them further through adiabatic heating/cooling and compositional changes. The neutral temperature response lags behind totality by about 35 min, indicating an imbalance between heating and cooling processes during the eclipse, while the ion and electron temperature responses have almost no lag, indicating they are in quasi steady state. Simulated ion temperature and vertical drift responses are weaker than observed by the Millstone Hill Incoherent Scatter Radar, while simulated reductions in electron density and temperature are stronger. The model misses the observed posteclipse enhancement in electron density, which could be due to the lack of a plasmasphere in GITM. The simulated TEC response appears too weak compared to Global Positioning System TEC measurements, but this might be because the model does not include electron content above 550‐km altitude. The simulated response in the neutral wind after the eclipse is too weak compared to Fabry Perot interferometer observations in Cariri, Brazil, which suggests that GITM recovers too quickly after the eclipse. This could be related to GITM heating processes being too strong and electron densities being too high at low latitudes.

Description: Abstract We describe a new method of constructing an equilibrium magnetopause surface based on the pressure balance between external and internal pressure sources. Details are given for every step of the procedure, including the initial pressure balance condition (which determines magnetopause morphology), the related underlying assumptions and the final optimized magnetopause surface. Our method produces a final equilibrium magnetopause surface that satisfies the local balance between the solar wind dynamic pressure and planetary magnetic pressure with an error no greater than 1%. We also discuss the contribution of hot plasma pressure and equatorial ring currents, and their effects on magnetopause morphology. For each optimized magnetopause boundary, we give coefficients for fitted polynomial approximations to allow simple and accurate reproductions for practical applications. We specifically discuss the potential application to the modeling of Saturn's magnetopause and describe how the equilibrium magnetopause model can be used to estimate the compressibility of the boundary.

Description: Abstract Substorms are a highly variable process, which can occur as an isolated event or as part of a sequence of multiple substorms (compound substorms). In this study we identify how the low‐energy population of the ring current and subsequent energization varies for isolated substorms compared to the first substorm of a compound event. Using observations of H+ and O+ ions (1 eV to 50 keV) from the Helium Oxygen Proton Electron instrument onboard Van Allen Probe A, we determine the energy content of the ring current in L‐MLT space. We observe that the ring current energy content is significantly enhanced during compound substorms as compared to isolated substorms by ∼20–30%. Furthermore, we observe a significantly larger magnitude of energization (by ∼40–50%) following the onset of compound substorms relative to isolated substorms. Analysis suggests that the differences predominantly arise due to a sustained enhancement in dayside driving associated with compound substorms compared to isolated substorms. The strong solar wind driving prior to onset results in important differences in the time history of the magnetosphere, generating significantly different ring current conditions and responses to substorms. The observations reveal information about the substorm injected population and the transport of the plasma in the inner magnetosphere.

Description: Abstract Mars regional and global dust storms are able to impact the lower/upper atmospheres through dust aerosol radiative heating and cooling and atmospheric circulation. Here we present the first attempt to globally investigate how the dust impact transfers from the neutral upper atmosphere to the ionosphere and the induced magnetosphere above 100 km altitude. This is achieved by running a multifluid magnetohydrodynamic model under nondusty and dusty atmospheric conditions for the 2017 late‐winter regional storm and the 1971‐1972 global storm. Our results show that the dayside main ionospheric layer (below ∼250 km altitude) undergoes an overall upwelling, where photochemical reactions dominate. The peak electron density remains unchanged, and the peak altitude shift is in accordance with the upper atmospheric expansion (∼5 km and ∼15 km for the regional and global storms, respectively). Controlled by the day‐to‐night transport, the nightside ionosphere responds to the dust storms in a close connection with what happens on the dayside but not apparently with the ambient atmospheric change. At higher altitudes, dust‐induced perturbations propagate upward from the ionosphere to the magnetosphere and extend from the dayside to the nightside, within a broad region bounded by the induced magnetospheric boundary. It is found that the global dust storm is able to dramatically enhance the CO loss by a factor of ∼3, which amounts to an increase of ∼20% or more for total carbon loss (in the forms of neutrals and ions). Strong dust storms are a potentially important factor in atmospheric evolution at Mars.

Description: Abstract Terrestrial gamma‐ray flashes (TGFs) are high‐energy photon bursts originating from the Earth's atmosphere. In this study, using first‐principles Monte Carlo simulations, we quantify the effects of Compton scattering on the temporal and spectral properties of TGFs induced by a tilted source geometry. Modeling results indicate that the source orientation is a critical parameter in TGF analysis but has been significantly underestimated in previous studies. Offset distance between the lightning source and satellite location cannot be used as a single parameter characterizing Compton scattering effects. In the tilted geometry, Compton scattering effects are more pronounced in the falling part of TGF pulses and can lead to an increase of the falling part of TGF pulses by several tens of microseconds. Moreover, by performing curve‐fitting analysis on simulated TGF light curves, we explain how the symmetric and asymmetric pulses measured by the Gamma‐Ray Burst Monitor on Fermi satellite are consistent with the Compton scattering effects. Fermi‐measured TGF pulses can be fully explained using Gaussian‐distributed TGF sources with an average duration of ∼206 μs.

Description: Abstract We present an overview of the ionospheric conditions during the launch of the Investigation of Cusp Irregularities 3 (ICI‐3) sounding rocket. ICI‐3 was launched from Ny‐Ålesund, Svalbard, at 7:21.31 UT on 3 December 2011. The objective of ICI‐3 was to intersect the reversed flow event (RFE), which is thought to be an important source for the rapid development of ionospheric irregularities in the cusp ionosphere. The interplanetary magnetic field was characterized by strongly negative Bz and weakly negative By. The EISCAT Svalbard radar (ESR) 32‐m beam was operating in a fast azimuth sweep mode between 180° (south) and 300° (northwest) at an elevation angle of 30°. The ESR observed a series of RFEs as westward flow channels that were opposed to the large‐scale eastward plasma flow in the prenoon sector. ICI‐3 intersected the first RFE in the ESR field of view and observed flow structures that were consistent with the ESR observations. Furthermore, ICI‐3 revealed finer‐scale flow structures inside the RFE. The high‐resolution electron density data show intense fluctuations at all scales throughout the RFE. The ionospheric pierce point of the GPS satellite PRN30, which was tracked at Hornsund, intersected the RFE at the same time. The GPS scintillation data show moderate phase scintillations and weak amplitude scintillations. A comparison of the power spectra reveals a good match between the ground‐based GPS carrier phase measurements and the spectral slope of the in situ electron density data in the lower frequency range. It demonstrates the possibility of modelling GPS scintillations from high‐resolution in situ electron density data.

Description: Abstract The Gamma‐ray Burst Monitor (GBM) onboard the Fermi spacecraft has observed many tens of sufficiently bright events, which are suitable for individual analysis. In our previous study, we fit individual, bright terrestrial gamma‐ray flashes (TGFs) with Relativistic Runaway Electron Avalanche (RREA) models for the first time. For relativistic‐feedback‐based models, the TGF‐producing electrons, which are seeded internally by a positive feedback effect, are usually accelerated in a large‐scale field with fully developed RREAs. Alternatively, lightning leader models may apply to either a large‐scale thunderstorm fields with fully developed RREAs or to inhomogeneous fields in front of lightning leaders where RREAs only develop partially. The predictions of the latter, inhomogeneous models for the TGF‐beaming geometry show some differences from estimations of the relativistic feedback models in homogeneous fields. In this work, we analyze a large sample of 66 bright Fermi GBM TGFs in the framework of lightning leader models, making comparisons with previous results from the homogeneous‐field RREA models. In most cases, the spectral analysis does not strongly favor one mechanism over the other, with 59% of the TGF events being best fit with the fully developed RREA mechanism, which corresponds to high‐potential leader models. The majority of the GBM‐measured TGFs can be best fit if the source altitude is below 15 km and 70% of events best fit by leader models cannot be satisfactorily modeled unless a tilted photon beam is used. For several spectrally soft TGFs, the tilted beam low‐potential leader model can best fit the data.

Description: Abstract The boundary stability problem is solved for a dipole magnetosphere wrapped by an azimuthal solar wind flow. A simplified equation is obtained describing small‐scale fast magnetosonic (FMS) waves in a dipole magnetosphere. The structure of surface FMS waves along magnetic field lines is represented as an expansion into a set of standing (between the magnetoconjugate ionospheres) waves. Over the azimuthal coordinate, along which the plasma is homogeneous, the wave field is decomposed into a set of azimuthal harmonics. It is shown that, in solar wind flows with velocities actually observable near the Earth's magnetosphere, only surface waves corresponding to several first harmonics of these expansions are unstable. The frequency ranges of unstable oscillations do not overlap for harmonics with different wave numbers. As the plasma flow velocity increases, harmonics with higher and higher wave numbers become unstable. The characteristic frequencies of unstable surface waves cover the lowest‐frequency, Pc5‐Pc6 range of geomagnetic pulsations observed in the magnetosphere. The frequency spectrum of these oscillations corresponds to geomagnetic pulsations at discrete “magic frequencies” occasionally observed in the Earth's magnetosphere.

Description: Abstract In our recent paper, Yi, Reid, Xue, Younger, Spargo, et al. (2017, hereinafter Y17A), we presented a first analysis of 9‐ and 6.75‐day periodic oscillations observed in the Earth's neutral mesospheric density during 2005 and 2006. These observations reveal a direct coupling between the Sun's upper atmosphere and the Earth's mesosphere. Tsurutani et al. (2019, hereinafter T19) have commented on T19 by way of suggesting possible physical mechanisms for Y17A's observations (Y17A do not address possible mechanisms in their paper).

Description: Abstract While Yi et al. (2017a, http://doi:10.1002/2017JA024446) described the observational results showing high‐speed solar wind stream (HSS) impacts on the mesosphere over Antarctica, the specific physical mechanism behind them was not discussed. We discussed here how magnetospheric wave‐particle interactions and energetic ~20 to 50 keV electron precipitation into the auroral zone atmosphere (diffuse auroras) during HSS intervals can cause the observed effects in the mesosphere.

Description: Abstract The Magnetospheric Multiscale (MMS) mission has given us unprecedented access to high cadence particle and field data of magnetic reconnection at Earth's magnetopause. MMS first passed very near an X‐line on 16 October 2015, the Burch event, and has since observed multiple X‐line crossings. Subsequent 3D particle‐in‐cell (PIC) modeling efforts of and comparison with the Burch event have revealed a host of novel physical insights concerning magnetic reconnection, turbulence induced particle mixing, and secondary instabilities. In this study, we employ the Gkeyll simulation framework to study the Burch event with different classes of extended, multi‐fluid magnetohydrodynamics (MHD), including models that incorporate important kinetic effects, such as the electron pressure tensor, with physics‐based closure relations designed to capture linear Landau damping. Such fluid modeling approaches are able to capture different levels of kinetic physics in global simulations and are generally less costly than fully kinetic PIC. We focus on the additional physics one can capture with increasing levels of fluid closure refinement via comparison with MMS data and existing PIC simulations. In particular, we find that the ten‐moment model well captures the agyrotropic structure of the pressure tensor in the vicinity of the X‐line and the magnitude of anisotropic electron heating observed in MMS and PIC simulations. However, the ten‐moment model has difficulty resolving the lower hybrid drift instability, which has been observed to plays a fundamental role in heating and mixing electrons in the current layer.

Description: Abstract We present several examples of magnetospheric ULF pulsations associated with stream interaction regions (SIRs), and demonstrate that the observed magnetospheric pulsations were also present in the solar wind number density. The distance of the solar wind monitor ranged from just upstream of Earth's bow shock to 261 RE, with a propagation time delay of up to 90 minutes. The number density oscillations far upstream of Earth are offset from similar oscillations observed within the magnetosphere by the advection timescale, suggesting that the periodic dynamic pressure enhancements were time stationary structures, passively advecting with the ambient solar wind. The density structures are larger than Earth's magnetosphere, and slowly altered the dynamic pressure enveloping Earth, leading to a quasi‐static and globally coherent “forced‐breathing” of Earth's dayside magnetospheric cavity. The impact of these periodic solar wind density structures were observed in both magnetospheric magnetic field and energetic particle data. We further show that the structures were initially smaller amplitude, spatially larger, structures in the upstream slow solar wind, and that the higher speed wind compressed and amplified these preexisting structures leading to a series of quasi‐periodic density structures with periods typically near 20 minutes. Similar periodic density structures have been observed previously at L1 and in remote images, but never in the context of solar wind shocks and discontinuities. The existence of periodic density structures within SIRs may play an important role in magnetospheric particle acceleration, loss, and transport, particularly for outer zone electrons that are highly responsive to ULF wave activity.

Description: Abstract Directional discontinuities (DDs) are common structures in the interplanetary space. Correctly determining the normal of a DD is important to understand the changes of phase fronts of magnetic fields adjacent to a discontinuity as well as helpful for the applications in many regimes of space physics research. In this work, we propose a new scheme to estimate the normal directions of DDs by finding the smallest standard deviation of normal magnetic fields derived from the cross‐product of magnetic fields on both sides of the discontinuity, based on the idea that the phase fronts of the adjacent magnetic fields are closely parallel to the DD plane. By comparing with the normal direction determined from Cluster multiple spacecraft, we show that our scheme can provide the same accuracy as that from the multi‐spacecraft estimation. Moreover, our scheme gives a consistent result of normal estimations at different Cluster spacecraft. We notice that in some cases, the normal directions derived from the minimum variance analysis (MVA) have large differences from those of multi‐spacecraft method and our scheme implying significant influence of the kinetic effect of particles in the transition region of the discontinuity. A few events of STEREO B observations are further studied to show that our scheme can be applied to DDs in single spacecraft measurements with even small rotation of magnetic fields across the discontinuities. With the help of an accurate normal estimation, we can understand the variations of magnetic field phase fronts in the vicinity of discontinuities.

Description: Abstract We present an analysis of one‐year data of dust impacts observed on two of the Earth‐orbiting Magnetospheric Multiscale mission (MMS) spacecraft. The dust impact signals were identified in observations of the electric field probes and were registered simultaneously by monopole and dipole configurations of the instrument. This unique set‐up allows us to reliably identify changes in the spacecraft potential as candidates for dust impacts. We present a detailed study of the properties of the pulses generated by the dust impacts and show the influence of the local plasma environment (spacecraft location in the Earth magnetosphere) on signals generated by dust impacts and their detection. We discuss the credibility of impact identification and possible sources of signal misinterpretation. We find a total of 784 observed events that we can interpret as dust impacts and that we use to derive a dust flux. We show that MMS1 registered 0.7 and MMS3 0.8 dust impact like events per hour. This corresponds to dust flux of 2.5‐6 × 10‐5 m‐2 s‐1.

Description: Abstract Interplanetary coronal mass ejections (ICMEs) are at the center of the research on geomagnetic activity. Sheaths, highly fluctuating structures, which can be found in front of fast ICMEs are some of the least‐known geoeffective solar transients. Using Morlet transforms, we analyzed the magnetic fluctuations in a list of 42 well‐identified and isolated magnetic clouds driving a sheath and shock (Masías‐Meza et al., 2016). We studied the fluctuations inside sheaths by defining two quantities: the power and the anisotropy. With a simple statistical approach we found that sheaths, in particular those driven by a fast magnetic cloud, encountering a highly turbulent solar wind, and forming a high Alfvén Mach number shock have high levels of turbulent energy (∼×10 compared with the solar wind) as well as a low anisotropy (∼ halved compared with the solar wind) of their fluctuations. On the other hand, the effect of the shock angle and plasma Beta in the solar wind are less straightforward : if the shock is quasi‐parallel or the Beta in the solar wind is high, both the turbulent energy in the sheaths and the anisotropy of the fluctuations are reduced; but for quasi‐perpendicular shocks or low beta solar wind the turbulent energy and anisotropy can take any value.

Description: Abstract The feasibility of detection of electromagnetic response in the upper ionosphere to ground large‐scale extremely low frequency (ELF) transmitters (e.g., submarine communication systems) by low‐orbiting satellites is discussed. Several times when the DEMETER satellite (660 km) was in the vicinity of the ELF transmitter on the Kola Peninsula, the electric and magnetic sensors operating in a burst mode detected a narrowband 82‐Hz emission. The same emission associated with the ELF transmitter was observed by a ground‐based magnetometer. We modeled the rate of the ELF wave energy leakage into the upper ionosphere from an oscillating 82‐Hz linear current with an infinite length suspended above a high‐resistive ground. A realistic altitudinal profile of the plasma parameters has been reconstructed with the use of the IRI ionospheric model. The modeled amplitudes and polarization of electromagnetic response of the upper ionosphere are in reasonable agreement with the properties of emission recorded by the satellite.

Description: Abstract We present an event that was detected by Fermi Gamma‐ray Burst Monitor (GBM) on 04 February 2014 as the spacecraft was flying over Madagascar. We interpret the three pulses during this event (herein known as 140204581) as the following: the first pulse as a Terrestrial Gamma‐ray Flash (TGF), the second as a 2 ms long Terrestrial Electron Beam (TEB) 0.5 ms after the TGF, and the last pulse as the TEB mirror pulse 90 ms after the TEB. The nature of these events were confirmed using both the World Wide Lightning Location Network (WWLLN) and the Earth Networks Total Lightning Network (ENTLN), which detected the same simultaneous sferic underneath the spacecraft and in the magnetic footprint. Several models were fit to the data, and results show that the vertical narrow beam model was found to be inconsistent with the data.

Description: Abstract The global navigation satellite systems (GNSS) provides high‐quality total‐electron‐content (TEC) measurements that are used routinely for ionospheric diagnostics. Large‐scale TEC structure is a critical input for maintaining global ionospheric models. However, residual stochastic TEC structure is typically discarded as a phase‐scintillation‐induced error. In this paper we show that the phase‐scintillation errors are a negligibly small fraction of the stochastic TEC component for most GNSS operating conditions. With three‐frequency GPS satellite measurements two independent TEC measurements can be compared to bound the scintillation‐induced errors. Data analysis and simulations show that scintillation‐induced errors are a small fraction of one TEC unit as long as the lower contributing frequency S4 index is less than 0.5. To the extent that scintillation‐induced errors are negligible, spectral analysis of stochastic TEC provides a direct measure of path‐integrated structure. Irregularity parameter estimation can be used to estimate power‐law parameters, which in turn can be interpreted with a new global ionospheric structure model. We present a summary analysis of month‐long series of continuous measurements at Poker Flat, Alaska. The analysis confirms the generally accepted single power‐law model for high latitude structure. A small fraction of the measurements indicate an outer‐scale transition at approximately 17 km. The general pattern of the structure occurrence is consistent with auroral‐zone activity.

Description: Abstract We compare for the first time two conjugate events showing simultaneous VLF wave observations between the same ground‐station and spacecraft, at different geomagnetic conditions and on opposite sides of the magnetosphere. Waves were observed at Kannuslehto [MLAT=64.4N, L=5.46], Finland, and Arase (ERG) in the inner magnetosphere. Case 1 on 28 March 2017, shows quasi‐periodic (QP) emissions and chorus simultaneously observed on the post‐midnight side during the recovery phase of a storm, with sustained high solar‐wind speed and AE index. Case 2 on 30 November 2017 shows clear one‐to‐one correspondence of QP elements on the noon side during geomagnetic quiet time (Dst 〉 10 nT and AE 〈 100 nT). We present the characteristics of both cases, focusing on coherence and spatial extent of the waves, electron density and magnetic field variations. We report that the magnetic field gradient plays a role in the changes of spectral features of the waves.

Description: Abstract A combination of statistical studies and 18 case studies have been used to investigate the structure of the induced Martian magnetosphere. The different plasma and magnetic pressure forces on the dayside of the induced magnetosphere of Mars have been studied using 3.5 years of Mars Atmosphere and Volatile Evolution (MAVEN) and Mars Express (MEX) observations. We present estimates of typical values for the dominant pressure terms, i.e., the thermal pressures of the ionosphere and the magnetosheath, the magnetic pressure of the magnetic pile‐up region, and the solar wind dynamic pressure. For 18 typical orbits the altitudes and relative distances of the pressure balance boundaries, the photoelectron boundary (PEB), the ion composition boundary (ICB), and the induced magnetosphere boundary (IMB) are estimated. The Magnetic Pile‐up Boundary (MPB) is discussed but not further studied since earlier characterisations of the MPB do not agree with our results. This study focuses on the transition region between the ionosphere and the magnetosheath on the dayside of Mars. We show that earlier definitions of the PEB, ICB, and IMB do not characterise the transition region well, mainly because each boundary is based on measurements from only one or two instruments. In order to characterise the transition region correctly changes in magnetic field strength and fluctuations, dominant ion species, electron and ion densities and energy distributions, need to be considered. This article confirms a complex interaction between Mars and the solar wind and can explain why previous studies have had difficulties to describe the force balance.

Description: Abstract Comprehensive pattern of ionospheric troughs' location in winter for all local times, longitudes, high (HSA), and low (LSA) solar activity, in both hemispheres, is at first investigated. Statistical analysis based on a large dataset of Interkosmos‐19, CHAMP, and Kosmos‐900 satellites was performed for quiet geomagnetic conditions Kp = 1–3. Three troughs were considered: high‐latitude trough (HLT) located inside the auroral oval, main ionospheric trough (MIT) located equatorward from the auroral oval, that is, at subauroral latitudes, and mid‐latitude ring ionospheric trough (RIT). The main purpose was to study the formation of the troughs' diurnal pattern in different conditions. The main problem was to distinguish MIT from RIT and MIT from HLT. For this purpose, early morning hours (04–06 LT), late morning hours (07–10 LT), day, evening, and night conditions were examined. In the early morning sector, RIT was separated from MIT and eliminated from the dataset. In the late morning sector, MIT and HLT were first clearly divided, although only for HSA. During the day, in the Northern Hemisphere under all conditions, HLT is mainly observed, in the Southern Hemisphere at poorly lit longitudes only the daytime MIT, and at well‐lit longitudes only the HLT is observed. The division of MIT and HLT was carried out according to the (statistical) position of the equatorial boundary of the auroral oval precipitation. At night, longitudinal variations in the MIT position determine the asymmetry of the hemispheres. Thus, the occurrence and position of MIT and HLT depend on the hemisphere, longitude, and solar activity.

Description: Abstract Substorm‐type evolution of the Earth's magnetosphere is investigated by mining more than two decades (1995‐2017) of spaceborne magnetometer data from multiple missions including the first two years (2016‐2017) of the Magnetospheric MultiScale mission. This investigation reveals interesting features of plasma evolution distinct from ideal MHD behavior: X‐lines, thin current sheets and regions with the tailward gradient of the equatorial magnetic field Bz. X‐lines are found to form mainly beyond 20 RE, but for strong driving, with the solar wind electric field exceeding ~ 5mV/m, they may come closer. For substorms with weaker driving, X‐lines may be preceded by redistribution of the magnetic flux in the tailward Bz gradient regions, similar to the magnetic flux release instability discovered earlier in PIC and MHD simulations as a precursor mechanism of the reconnection onset. Current sheets in the growth phase may be as thin as 0.2 RE, comparable to the thermal ions gyroradius, and at the same time, as long as 15 RE. Such an aspect ratio is inconsistent with the isotropic force balance for observed magnetic field configurations. These findings can help resolve kinetic mechanisms of substorm dipolarizations and adjust kinetic generalizations of global MHD models of the magnetosphere. They can also guide and complement micro‐scale analysis of non‐ideal effects.

Description: Abstract A moderate C1.1 class confined flare is investigated here, which occurred on 2013 September 24 at 22:56~UT, in an arch filament system close to a regular, unipolar sunspot. Spectro‐polarimetric observations from the Tenerife Infrared Polarimeter at the 70 cm German Vacuum Tower Telescope were combined with data from the Helioseismic Magnetic Imager and the Atmospheric Imaging Assembly to identify the processes that triggered the flare. The legs of this arch filament were anchored in the leading sunspot and the network flux region of opposite polarity. The flare was driven by small‐scale, flux cancellation at the weak neutral line underlying the arch filament which resulted in two small flaring events within an hour of the C1.1 flare. Flux cancellation was facilitated by the moat flow from the leading sunspot wherein small‐scale magnetic fragments stream towards patches of pre‐existing flux. The cancellation of flux led to the destabilization of the arch filament which was seen as an increase in the twist along the arch filament. The horizontal fields across the weak neutral line decay faster which cannot prevent the filament from rising that results in a two‐ribbon flare at the neutral line. The arch filament unwinds as it rises, but is confined by the higher, overlying fields between the two polarities of the active region that decay much more slowly.

Description: Abstract Here we present some unique observations of reconnection at a quasi‐perpendicular bow shock as an interplanetary directional discontinuity (DD) is crossing it simultaneously with the Magnetospheric Multiscale (MMS) mission. There are no burst data, but available data show indications of ongoing reconnection at the shock southward of MMS: a bifurcated current sheet with signatures of Hall magnetic and electric fields, normal magnetic fields indicating a magnetic connection between the two reconnecting regions, field‐aligned currents and electric fields, E·J〉0 indicating a conversion of magnetic to kinetic energy, and {and sub‐spin resolution ion energy‐time spectrograms indicating ions being accelerated away from the X‐line. The DD is also observed by four upstream spacecraft (ACE, WIND, Geotail, and ARTEMIS P1) and one downstream in the magnetosheath (Cluster 4), but none of them resolve signatures of ongoing reconnection. We therefore suggest that reconnection was temporarily triggered as the DD was compressed by the shock. Reconnection at the bow shock is inevitably asymmetric with both the density and magnetic field strength being higher on one side of the X‐line (magnetosheath side) than on the other side where the plasma flow also is supersonic (solar wind side). This is different from the asymmetry exhibited at the more commonly studied case of asymmetric reconnection at the magnetopause. Asymmetric reconnection of the bow shock type has never been studied before, and the data discussed here present some first indications of the properties of the reconnection region for this type of reconnection.

Description: Abstract The atmospheric effects of precipitating electrons are not fully understood, and uncertainties are large for electrons with energies greater than ~30 keV. These electrons are underrepresented in modeling studies today, primarily because valid measurements of their precipitating spectral energy fluxes are lacking. This paper compares simulations from the Whole Atmosphere Community Climate Model (WACCM) that incorporated two different estimates of precipitating electron fluxes for electrons with energies greater than 30 keV. The estimates are both based on data from the Polar Orbiting Environmental Satellite Medium Energy Proton and Electron Detector (MEPED) instruments, but differ in several significant ways. Most importantly, only one of the estimates includes both the 0° and 90° telescopes from the MEPED instrument. Comparisons are presented between the WACCM results and satellite observations poleward of 30°S during the austral winter of 2003, a period of significant energetic electron precipitation (EEP). Both of the model simulations forced with precipitating electrons with energies 〉30 keV match the observed descent of reactive odd nitrogen better than a baseline simulation that included auroral electrons, but no higher energy electrons. However, the simulation that included both telescopes shows substantially better agreement with observations, particularly at mid‐latitudes. The results indicate that including energies 〉30 keV and the full range of pitch angles to calculate precipitating electron fluxes is necessary for improving simulations of the atmospheric effects of EEP.

Description: Abstract Terrestrial gamma‐ray flashes (TGFs) are bright bursts of gamma rays produced by thunderstorms, typically observed by spacecraft in the low‐Earth orbit. Unfortunately, it has been difficult to disentangle the source altitude and the width and direction of the gamma‐ray beam using single point spacecraft measurements, which has hampered attempts to constrain TGF models. Polarimetry of astrophysical sources has been of interest for many decades which raises the question: Do TGFs and X‐rays from lightning have observable polarization, and if so, what would this polarization tell us about their source? REAM Monte Carlo code has been modified to record the linear polarization of X‐rays and gamma rays as a function of source altitude and beam geometry. It is found that polarization degree of a 20 km narrow beam of TGF is substantially different from a 15 km wide beam which could be used to constrain the source geometry of TGFs. However, due to the low fluence of these events in space, detecting this level of polarization would be challenging. It is also found that low‐altitude TGFs (source at 3.5 km) produce polarizations up to about 8%, however detectors need to be very close to the source region. Furthermore, very low‐altitude ground‐level TGFs and X‐rays showed a maximum polarization of 13% on the ground, of which the TGF's fluence was large enough for polarimetry. In addition, polarization reached its maximum further away from the z‐axis as the TGF's beam broadened. The dominant mechanism of the polarization was found to be Compton scattering.

Description: Abstract The effects of the 13‐14 March 1989 great magnetic storm on the East Asian ionosphere have been re‐investigated using ground‐based and satellite measurements as well as theoretical simulation. Within introduction of new data like the Chinese ionosondes and DMSP F8&F9 and low‐equatorial magnetometer data, we are able to track the ionospheric response at both the bottomside and topside ionosphere from middle to low latitude to obtain an overall understanding of storm‐time ionospheric change. Through the comparative study of different longitude bands, we found that the East‐Asian ionosphere was characterized by a strong westward electron density gradient persisting over a day at both the bottomside and topside ionosphere at mid‐low latitudes during the main and recovery phases of the storm. This feature was not studied in the previous literature for this event at this area. We then examine the effect through a numerical simulation work from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) under Apex and Dipole geomagnetic fields. It is seen that the model well reproduces the zonal gradients during this event under realistic geomagnetic field model. The conditions favor for this structure requires a hemispheric asymmetry response of storm thermosphere as well as background condition and also the storm development which requires further investigation. This study shows that even very close stations would manifest totally different storm behaviors during the superstorm event suggesting a great challenge in the space weather prediction.

Description: Abstract The up‐looking total electron contents (TECs) from the GRACE, SWARM‐A, TerraSAR‐X and MetOp‐A satellites and in situ electron density (Ne) from SWARM‐A were utilized to investigate the topside ionospheric conditions during the 7‐8 September 2017 geomagnetic storm. The rate of TEC index (ROTI) and rate of density index (RODI), which are derivative indices of TEC and Ne respectively, were also used to characterize the topside ionospheric irregularities. The main results of this study are as follows: (1) There were significant enhancements seen in the up‐looking TEC during the first main phase of the storm. (2) The up‐looking TEC didn’t show similar unusual enhancement as observed by ground‐based TEC in the Asian‐Australian sector during the recovery phase of the storm on 11th September. (3) Prominent TEC hemispheric asymmetry at the middle and high latitudes was observed at both day and night sectors. (4) Long‐duration recovery of topside TEC with respect to the pre‐storm condition was also detected in this event. (5) Nighttime ROTI enhancements were presented in a wide latitudinal range from the equator to the poles during the main phases of the storm. (6) The ionospheric electric field disturbances associated with IMF‐Bz fluctuations probably played a very important role in triggering ionospheric irregularities during the relatively weak geomagnetic activity on 7th September, which implies that ionospheric irregularities do not necessarily occur under the severe geomagnetic conditions only.

Description: Abstract This work reports auroral spots event observed by the SSUSI instruments on board the DMSP spacecraft between 22‐23 July 2009 during the recovery phase of a moderate magnetic storm. The spots were observed between 18:00 and 02:00 magnetic local time and stayed at ~60° magnetic latitude. They lasted for ~10 hours and corotated with ~64% of the Earth's rotational speed. In situ observations indicate that the isolated auroral spots were produced by energetic ions at energies between 10 keV and 240 keV, with significantly anisotropic electron (30 keV – 300 keV) precipitations. It is expected that the energetic ions originate from the ring current and can be scattered by the EMIC waves through cyclotron resonance. The energetic electrons can be precipitated by the non‐resonant interaction between the electrons and EMIC waves, which is suggested by previous works.

Description: Abstract Based on the electron density, field‐aligned currents (FACs) and ion drift velocity data from the Swarm‐A satellite during 2013‐2018, we investigate the features of the high latitude trough in the Southern Hemisphere. The results show that the high latitude trough in the Southern Hemisphere is a persistent post‐midnight feature in winter. It is observed mainly in the eastern longitudes under low solar activity conditions and in a smaller longitudinal range under high solar activity conditions. The high latitude trough moves to higher latitudes at later local times, which is more obvious under low solar activity conditions. We also find that the features of FACs and ion drift velocity distribution in the high latitude trough region depend on longitude. In the longitude sector of 0°–70°E, the high latitude trough positions are always located at the poleward boundary of the downward FACs region, and collocated with the convection reversal boundary in the post‐midnight region. In the longitude sector of 80°–130°E, the high latitude trough positions are located at the poleward boundary of the downward FACs region in the midnight region and at the strong westward or antisunward ion flow region in the post‐midnight region. It is suggested that the high latitude trough formation and evolution is possibly associated with downward FACs, flow stagnation in the convection reversal boundary and westward or antisunward ion flow.

Description: Abstract Auroral emission at 427.8‐nm from N2+ ions is caused by precipitation of energetic electrons, or by resonant scattering of sunlight by auroral N2+ ions. The latter often causes impressive purple aurora at high altitudes. However, statistical characteristics of auroral 427.8‐nm emission have not been well studied. In this paper we report occurrence characteristics of high 427.8‐nm emission intensities (more than 100 R) at subauroral latitudes, based on measurements by a filter‐tilting photometer over 14 years (2005‐2018) at Athabasca, Canada (magnetic latitude: ~62o). We divided the dataset into solar elevation angles (θs) more than and less than ‐24o (shadow height of sunlight: 600 km) to separate the 427.8‐nm emissions caused by resonant scattering of sunlight and those excited by auroral electrons, respectively. The occurrence rate of 427.8‐nm emissions of more than 100 R is 10.6 % and 7.65 % for θs more than and less than ‐24o, respectively, confirming that resonant scattering of sunlight by N2+ ions is a cause of the strong 427.8‐nm emissions of more than 100 R in the sunlit ionosphere. The occurrence rate is high in the post‐midnight sector, and increases with increasing geomagnetic activity, solar‐wind speed, and density. The occurrence rate is lowest in winter. A high occurrence rate was observed in 2015‐2018, during the declining phase of the 11‐year solar activity. Superposed epoch analysis indicates that the 427.8‐nm emission exceeds 100 R when solar wind speed increases and solar wind density concurrently decreases, though the standard deviation of the data is rather large.

Description: Abstract We present the first and direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes (PMWE) simultaneously observed by the conjugate observation with Arase satellite and high‐power atmospheric radars in both hemispheres, namely, the Program of the Antarctic Syowa Mesosphere, Stratosphere, and Troposphere/Incoherent Scatter Radar (PANSY) at Syowa Station (SYO; ‐69.00°S, 39.58°E), Antarctica, and the Middle Atmosphere Alomar Radar System (MAARSY) at Andøya (AND; 69.30°N, 16.04°E), Norway. The PMWE were observed during 03‐07 UT on March 21, 2017, just after the arrival of corotating interaction region (CIR) in front of high‐speed solar wind stream. An isolated substorm occurred at 04 UT during this interval. Electromagnetic ion cyclotron (EMIC) waves and whistler‐mode chorus waves were simultaneously observed near the magnetic equator and showed similar temporal variations to that of the PMWE. These results indicate that chorus waves as well as EMIC waves are drivers of precipitation of energetic electrons, including relativistic electrons, which make PMWE detectable at 55‐80 km altitude. Cosmic noise absorption (CNA) measured with a 38.2‐MHz imaging riometer and low‐altitude echoes at 55‐70 km measured with an MF radar at SYO also support the relativistic electron precipitation. We suggest a possible scenario in which the various phenomena observed in near‐Earth space, such as magnetospheric plasma waves (EMIC waves and chorus waves), pulsating auroras, CNA, and PMWE, can be explained by the interaction between the high‐speed solar wind containing CIRs and the magnetosphere.

Description: Abstract The origin of interplanetary magnetic field that is frozen‐in to the solar wind plasma flow is clearly magnetic flux from the Sun's corona. However, the filamented structure of magnetic fields observed in the solar wind cannot be accounted for quite so simply. Given the 2 days or more for solar wind to travel from the Sun to 1 AU, some argue that many current sheets are present due to turbulence and other in‐transit effects in the dynamic plasma outflow. Alternatively, it is postulated that a “flux tube texture” of the solar wind exists as fossil structure of the corona. In this paper we examine the possible influence of magnetic reconnection occurring close to the Sun or in the solar wind on the character of current sheets observed by Magnetospheric Multiscale at 1 AU. Photospheric convection is used to perturb a magnetic carpet‐like configuration, which has well‐segmented open flux tubes defined by topological elements of the magnetic field. Flux tube boundaries in the model are defined by magnetic sepratrix surfaces which are a preferential location for strong currents and magnetic reconnection. Reconnection is associated with signatures in the magnetic field and plasma that may advect with the solar wind all the way to 1 AU. Aided by three‐dimensional coronal modeling and two‐dimensional simulation examples of reconnection layers, we examine properties of current sheets observed by Magnetospheric Multiscale and how these solar wind boundaries may relate to reconnection operating earlier in the solar wind or corona.

Description: Abstract Thanks to the Cassini spacecraft onboard instruments, it has been known that Titan's ionospheric chemistry is complex and the molecular growth is initiated through the photolysis of the most abundant species directly in the upper atmosphere. Among the pool of chemical compounds formed by the photolysis, N‐bearing species are involved in the haze formation, but the chemical incorporation pathways need to be better constrained. In this work, we performed low‐pressure extreme ultraviolet (EUV) photochemical laboratory experiments. The APSIS reactor was filled with a N2/CH4 (90/10%) gas mixture relevant to the upper atmosphere of Titan. The cell was irradiated by using a EUV photon source at 73.6 nm, which has been difficult to produce in the laboratory for previous studies. The photoproducts (both neutral and ionic species) were monitored in situ with a quadrupole mass spectrometer. The chemical pathways are explained by confronting experimental observations and numerical predictions of the photoproducts. The most interesting result in this work is that methanimine was the only stable N‐bearing neutral molecule detected during the experiments, and it relies on N production. This experimental result is in agreement with the relatively high abundance predicted by 1‐D photochemical models of Titan's atmosphere and comforts methanimine as an intermediate toward the formation of complex N‐bearing organic molecules. This experiment is only testing one part of the overall chemical scheme for Titan's upper atmosphere due to the selective wavelength but demonstrates the capability to probe the chemical pathways occurring in Titan's atmosphere by minimizing bias coming from wall surface reactions.

Description: Abstract Waves around the lower hybrid frequency are frequently observed at Earth's magnetopause and readily reach very large amplitudes. Determining the properties of lower hybrid waves is crucial because they are thought to contribute to electron and ion heating, cross‐field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest‐resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments, the dispersion relation is constructed and the wave‐normal angle is estimated to be close to 90° to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single‐spacecraft method, and four‐spacecraft timing analyses. These results show that single‐spacecraft methods can accurately determine lower hybrid wave properties.

Description: Abstract This paper presents a new algorithm for detecting high‐speed flow channels in the polar cap. The algorithm was applied to Super Dual Auroral Radar Network data, specifically to data from the new Longyearbyen radar. This radar is located at 78.2°N, 16.0°E geographical coordinates looking north‐east, and is therefore at an ideal location to measure flow channels in the high‐latitude polar cap. The algorithm detected 〉500 events over 1 year of observations, and within this paper two case studies are considered in more detail. A flow channel on “old‐open field lines” located on the dawn flank was directly driven under quiet conditions over 13 min. This flow channel contributed to a significant fraction (60%) of the cross polar cap potential and was located on the edge of a polar cap arc. Another case study follows the development of a flow channel on newly opened field lines within the cusp. This flow channel is a spontaneously driven event forming under strong solar wind driving and is intermittently excited over the course of almost an hour. As they provide a high fraction of the cross polar cap potential, these small‐scale structures are vital for understanding the transport of magnetic flux over the polar cap.

Description: Abstract We present a statistical study of magnetotail flows that change direction from earthward to tailward using Cluster spacecraft. More precisely, we study 318 events of particle flux enhancements in the O+ data for which the pitch angle continuously changes with time, either from 0° to 180° or from 180° to 0°. These structures are called “Pitch Angle Slope Structures” (PASSes). PASSes for which the pitch angle changes from 0° to 180° are observed in the Northern Hemisphere while those for which the pitch angle changes from 180° to 0° are observed in the Southern Hemisphere. These flux enhancements result in a reversal of the flow direction from earthward to tailward regardless of the hemisphere where they are observed. Sometimes, several PASSes can be observed consecutively which can therefore result in oscillatory velocity signatures in the earth‐tail direction. The PASS occurrence rate increases from 1.8% to 3.7% as the AE index increases from ∼0 to ∼600 nT. Also, simultaneously to PASSes, there is typically a decrease in the magnetic field magnitude due to a decrease (increase) of the sunward component of the magnetic field in the Northern (Southern) Hemisphere. Finally, based on the 115 (out of 318) PASSes that show energy‐dispersed structures, the distance to the source from the spacecraft is estimated to be typically 〈25RE along the magnetic field line. This study is important as it sheds light on one of the causes of tailward velocities in Earth's magnetotail.

Description: Abstract To understand the generation and propagation processes of electromagnetic ion cyclotron (EMIC) waves under different geomagnetic conditions in the inner magnetosphere, we performed a statistical study of EMIC wave properties observed by the Van Allen Probes (RBSP) from February 2013 to December 2016. We divided EMIC waves into two groups: those associated with, and those occurring without injections observed by the Geostationary Operational Environmental Satellites (GOES‐13 and ‐15). We found that the EMIC wave polarization sense ε increased and the normalized frequency X decreased with increasing |MLAT|. Inside the plasmasphere, He+ EMIC waves were predominantly observed with left‐hand polarization (ε〈‐0.3) and higher wave normal angles (θk=30‐40° ). Those associated with injections showed the most intense wave power at 14‐16 MLT, compared to periods without injections when these waves exhibit a similar wave power but on the dayside. H+ EMIC waves were predominantly observed outside the plasmasphere on the dayside, and showed a mixture of left‐hand and linear polarizations (ε=‐0.3‐0.0) with lower wave normal angles (θk=20‐30° ) regardless of injections. Moreover, H+ EMIC waves were accompanied by a solar wind dynamic pressure enhancement (ΔPsw=0.5 nPa). From these observations, we suggest that hot injected plasma contributes to the generation of intense He+ EMIC waves in the afternoon sector. A mixture of expanding cold plasmaspheric ions and coexisting hot ring current ions acts as the free energy source for He+ EMIC waves on the dayside during quiet times. Solar wind dynamic pressure enhancements are likely the major driver of H+ EMIC waves outside the plasmasphere.

Description: Abstract We study the response of the outer Van Allen radiation belt during an intense magnetic storm on February 15‐22, 2014. Four interplanetary coronal mass ejections (ICMEs) arrived at Earth, of which the three last ones were interacting. Using data from the Van Allen Probes, we report the first detailed investigation of electron fluxes from source (tens of keV) to core (MeV) energies and possible loss and acceleration mechanisms as a response to substructures (shock, sheath and ejecta, and regions of shock‐compressed ejecta) in multiple interacting ICMEs. After an initial enhancement induced by a shock compression of the magnetosphere, core fluxes strongly depleted and stayed low for four days. This sustained depletion can be related to a sequence of ICME substructures and their conditions that influenced the Earth's magnetosphere. In particular, the main depletions occurred during a high‐dynamic pressure sheath and shock‐compressed southward ejecta fields. These structures compressed/eroded the magnetopause close to geostationary orbit and induced intense and diverse wave activity in the inner magnetosphere (ULF Pc5, EMIC and hiss) facilitating both effective magnetopause shadowing and precipitation losses. Seed and source electrons in turn experienced stronger variations throughout the studied interval. The core fluxes recovered during the last ICME that made a glancing blow to Earth. This period was characterized by a concurrent lack of losses and sustained acceleration by chorus and Pc5 waves. Our study highlights that the seemingly complex behavior of the outer belt during interacting ICMEs can be understood by the knowledge of electron dynamics during different substructures.

Description: Abstract We show that a THEMIS (Time History of Events and Macroscale Interactions during Substorms) white‐light all‐sky imager (ASI) can estimate Pedersen conductance with an uncertainty of 3 mho or 40%. Using a series of case studies over a wide range of geomagnetic activity, we compare estimates of Pedersen conductance from the backscatter spectrum of the Poker Flat Advanced Modular Incoherent Scatter Radar (ISR) with auroral intensities. We limit this comparison to an area bounding the radar measurements and within a limited area close to, (but off) imager zenith. We confirm a linear relationship between conductance and the square root of auroral intensity predicted from a simple theoretical approximation. Hence we extend a previous empirical result found for green‐line emissions to the case of white‐light off‐zenith emissions. The difference between the radar conductance and the best‐fit relationship has a mean of ‐0.76 ± 4.8 mho, and a relative mean difference of 21% ± 78%. The uncertainties are reduced to ‐0.72 ± 3.3 mho and 0% ± 40% by averaging conductance over 10 minutes, which we attribute to the time that auroral features take to move across the imager field being greater than the 1 minute resolution of the radar data. Our results demonstrate and calibrate the use of THEMIS ASIs for estimating Pedersen conductance. This technique allows the extension of estimates of Pedersen conductance from ISRs to derive continental‐scale estimates on scales of ~1‐10 minutes and ~100 km2. It thus complements estimates from low‐altitude satellites, satellite auroral imagers, and ground‐based magnetometers.

Description: Abstract Saturn's aurora represents the ionospheric response to plasma processes occurring in the planet's entire magnetosphere. Short‐lived ~ 1 h quasiperiodic high‐energy electron injections, frequently observed in in‐situ particle and radio measurements, should therefore entail an associated flashing auroral signature. This study uses high time‐resolution UV auroral imagery from the Cassini spacecraft to demonstrate the continuous occurrence of such flashes in Saturn's northern hemisphere and investigate their properties. We find that their recurrence periods of order 1 hr and preferential occurrence near dusk match well with previous observations of electron injections and related auroral hiss features. A large spread in UV auroral emission power, reaching more than 50% of the total auroral power, is observed independent of the flash locations. Based on an event observed both by the Hubble Space Telescope and the Cassini spacecraft, we propose that these auroral flashes are not associated with low‐frequency waves and instead directly caused by recurrent small‐scale magnetodisc reconnection on closed field lines. We suggest that such reconnection processes accelerate plasma planetward of the reconnection site towards the ionosphere inducing transient auroral spots while the magnetic field rapidly changes from a bent‐back to a more dipolar configuration. This manifests as a sawtooth‐shaped discontinuity observed in magnetic field data and indicates a release of magnetospheric energy through plasmoid release.

Description: Abstract Surface oxidation of Langmuir probes is an important issue for probe measurements in space environments. Followed by our previous work about the oxidation effect on the collection of ambient plasma electrons and ions, here we present its effect on photoemission from the probe surface of various materials. Photoemission is either a contamination for traditional Langmuir probes or a necessity for electric field probes in low‐density plasma. Our results show that all materials after oxidation have a varying degree of reduction in photoemission. The photoemission of Copper, Gold and Niobium drops most significantly followed by DAG213 (a resin based graphite coating), TiN (Titanium Nitride) and Rhenium. Iridium, DAG213, and AquaDAG (graphite coating) remain the largest photoemission after oxidation, making them appropriate coating candidates for electric field probes. Both DAG materials show a large photoemission enhancement after the oxidation products are cleaned off the surface. A long exposure test shows that the photoemission from Iridium slowly degrades. Due to the high surface conductivity of oxidized Iridium shown in previous work, it is suggested that Iridium can be oxidized before flight to minimize the photoemission when being used as a coating for Langmuir probes. Overall, Iridium is found to be a coating material appropriate for both electric field probes and Langmuir probes.

Description: Abstract The conductivity of the ionospheric E‐region is known to cause effective dissipation of plasma structures in the F‐region. We use 3.5 years of 16 Hz sampling rate electron density measurements from the Swarm advanced dataset to investigate seasonal dependencies of plasma structure dissipation. Using a novel algorithm to infer plasma structure dissipation through detection of spectral breaks in density fluctuation power spectra, we analyze 100,000 spectra based on data from Swarm A in both the northern and southern polar caps. For the first time, we can present long‐term development of small‐scale plasma structure diffusion in the high‐latitude ionospheric F‐region. We discuss possible reasons for these variations. This report presents evidence for the E‐region as an important factor in the seasonal variation of F‐region plasma irregularity amplitudes.

Description: Abstract To understand the spatial features of low‐latitude Pi2 (6.6 − 25 mHz) pulsations, a comprehensive study is carried out for the first time using magnetic field measurements from a global network of low‐latitude ground stations (mlat:±2° − 51°) and the Swarm multi‐satellites located simultaneously at day and night local times (LTs). We have investigated one‐year data from 2014 and found 15 Pi2 events with coherent oscillations at satellite and ground. The Pi2 oscillations in the compressional, toroidal, and poloidal components at satellite and H, D, and Z components at ground are investigated by estimating its coherence, amplitude, and cross‐phase with respect to midnight ground H variations. The analogous pairs of magnetic field components (satellite compressional with ground H and satellite toroidal with ground D) above and below the ionosphere are found to have identical phase during night and opposite phase during day, indicating the magnetospheric and ionospheric sources for nighttime and daytime Pi2s respectively. During nighttime, Pi2 oscillations identified in the poloidal component are found to oscillate in‐phase (out‐of‐phase) in the southern (northern) hemisphere. At ground the phase and amplitude of H showed significant change near the dawn terminator, whereas H oscillates mostly in‐phase with respect to midnight ground H at other LTs. The oscillations in D component have phase reversal near midnight, dawn, dusk, and noon meridians with opposite hemispheres having opposite phase. These Pi2 characteristics observed globally at ground and at the topside ionosphere suggest that the sources for nighttime and daytime low‐latitude Pi2s are oscillating field‐aligned currents and ionospheric currents, respectively.

Description: Abstract Radial transport is an important dynamical process in Saturn's internally‐driven magnetosphere. Radial transport is presumed to occur by a centrifugally‐driven interchange instability, determined by the gradient of flux tube content and flux tube entropy. Plasma produced in the inner magnetosphere must be transported radially outward. The outward motion of the plasma stretches the magnetic field lines, leading to magnetic reconnection. Reconnection allows the mass to be transported radially while allowing the flux to circulate back to the inner magnetosphere. Both radial transport of mass and magnetic flux in Saturn's magnetosphere have been estimated based on Cassani Plasma Spectrometer (CAPS) data provided by Wilson et al. [2017] and suggesting the radial transport rate of plasma of around 55 kg s^1. The net magnetic flux transport should be zero, but the data suggest a net outward magnetic flux transport indicating the existence of different possible transport mechanisms in Saturn's magnetodisc.

Description: Abstract A shock in the solar wind was observed shortly after 05:00 by heliospheric spacecraft stationed at L1. Behind the shock, protons and electrons in the tens to hundreds of keV energy range were found to be enhanced several‐fold. The shock reached the Earth precisely at 06:00 and was detected by particle sensors aboard GOES and ARTEMIS, and by ground‐based magnetometers. In addition, the arrival of the shock was seen promptly by the magnetometer investigation aboard MMS, which at the time was located 16 RE down the geomagnetic tail. However, the arrival of the energetic particles at MMS was delayed for ~ 23 minutes, and first arrived moving earthward up the geomagnetic tail. Timing considerations between the solar wind speed and the energetic particles indicated that the solar particles first gained access to field lines connected to MMS around 115 RE downstream. Once the shock and its entrained solar particles passed the access region, the solar particles were no longer seen.

Description: Abstract Collisionless shocks are ubiquitous throughout the known Universe. They mainly convert the energy of the directed ion flow into heating. Upon crossing the shock front, the ion distribution becomes non‐gyrotropic. Relaxation to gyrotropy then occurs mainly via kinematic collisionless gyrophase mixing and interaction with waves. The theory of collisionless relaxation predicts that the downstream pressure of each ion species varies quasi‐periodically with the distance from the shock transition layer and the amplitude of the variations gradually decrease. The oscillations due to each species has its own spatial period and damping scale. Pressure balance requires that the variations in the total plasma pressure should cause anti‐correlating variations in the magnetic pressure. This process should occur at all Mach numbers, but its observation is difficult at moderate/high Mach numbers. In contrast, such magnetic oscillations have been observed at low Mach number cases of the Venusian bow shock and interplanetary shocks. In this paper, simultaneous in‐situ magnetic field and plasma measurements from the THEMIS‐B and C spacecraft are used to study, for the first time, the anti‐correlated total ion and magnetic pressure spatial variations at low‐Mach number shocks. It is found that kinematic collisionless relaxation is the dominant process in the formation of the downstream ion distribution and in shaping the downstream magnetic profile of the observed shocks, confirming fundamental theoretical results. Comparison with the results from numerical models allows the role of the different ion species to be investigated and confirms the role heavy ions play in forming the downstream magnetic profile.

Description: Abstract Sub‐Auroral Polarization Streams (SAPS) prefer geomagnetically disturbed conditions and strongly correlate with geomagnetic indexes. However, the temporal evolution of SAPS and its relationship with dynamic and structured ring current and particle injection are still not well understood. In this study, we performed detailed analysis of temporal evolution of SAPS during a moderate storm on May 18, 2013 using conjugate observations of SAPS from the Van Allen Probes (VAP) and the Super Dual Auroral Radar Network (SuperDARN) . The large‐scale SAPS (LS‐SAPS) formed during the main phase of this storm and decayed due to the northward turning of the interplanetary magnetic field (IMF). A meso‐scale (~ several hundreds km zonally) enhancement of SAPS was observed by SuperDARN at 0456 UT. In the conjugate magnetosphere, a large SAPS electric field ~8 mV/m) pointing radially outward, a local magnetic field dip and a dispersionless ion injection were observed simultaneously by VAP‐A at L shell=3.5 and MLT=20. The particle injection observed by VAP‐A is likely associated with the particle injection observed by the Geostationary Operational Environmental Satellite (GOES) 15 near 20 MLT. Magnetic perturbations observed by the ground magnetometers and flow reversals observed by SuperDARN reveal that this meso‐scale enhancement of SAPS developed near the Harang reversal and before the substorm onset. The observed complex signatures in both space and ground can be explained by a two‐loop current wedge (2LCW) generated by the perturbed plasma pressure gradient and the diamagnetic effect of the structured ring current following particle injection.

Description: Abstract We present recent high time resolution observations from an oblique (43° ) shock crossing from the Magnetospheric Multi‐Scale (MMS) mission. Short‐duration bursts between 10 ms and 100 ms of Ion acoustic waves are observed in this event alongside a persistent reflected ion population. High time resolution (150 ms) particle measurements show strongly varying ion distributions between successive measurements, implying they are bursty and impulsive by nature. Such signatures are consistent with ion bursts that are impulsively reflected at various points within the shock. We find that, after instability analysis using a Fried‐Conte dispersion solver, the insertion of dispersive ion bursts into an already stable ion distribution can lead to wave growth in the ion acoustic mode for short durations of time. We find that impulsively reflected ions are a plausible mechanism for ion acoustic wave growth in the terrestrial bow shock and, furthermore, suggest that wave growth can lead to a small but measurable momentum exchange between the solar wind ions and the reflected population.

Description: Abstract Magnetosheath is a region surrounded by the bow shock and the magnetopause. According to statistical analysis, the boundaries can be treated as non‐confocal paraboloids of rotations. It is not easy to meet boundary conditions simultaneously on both sides. This problem is treated here and its solution is presented. A linear combination of harmonics that satisfy the condition on the magnetopause are selected in order to meet the condition on the bow shock. The method allows to calculate surface currents on the magnetopause. It is applied for typical conditions in solar wind. A modified magnetic dipole is used as a model of the magnetospheric field. Its modification is found by the method of potentials using cylindrical harmonics. Magnetopause currents are compared with results from a model with confocal paraboloids, which is commonly used in literature.

Description: Abstract Magnetic topology is important for understanding the Martian plasma environment, including particle precipitation, energy transport, cold ion escape, and wave‐particle interaction. In this study, we combine two independent but complementary methods in order to determine magnetic topology based on superthermal electron energy and pitch angle distributions. This approach removes ambiguities that result from using either energy or pitch angle alone, providing a more accurate and comprehensive determination of magnetic topology than previous studies. By applying this combined technique, we are able to identify seven magnetic topologies, including four types of closed field lines, two types of open field lines, and draped. All seven topologies are present in the Mars environment and are mapped in longitude, latitude, solar zenith angle, and altitude with the combined technique near the terminator. We find that closed field lines with double‐sided loss cones are frequently present over stronger crustal field regions at higher altitudes. We also show that the cross‐terminator closed field lines are more spatially confined over strong crustal regions, likely connecting nearby magnetic crustal patches. In contrast, cross‐terminator closed loops over weak crustal regions have more distantly separated footpoints, most likely connecting distant crustal patches.

Description: Abstract Unambiguously estimating the plasma parameters of the ionosphere at altitudes between 130 and 300 km presents a problem for the Incoherent Scatter Radar (ISR). At these ranges, ISR is unable to distinguish between different mixtures of molecular ions (NO+ and O2+) and atomic oxygen ions (O+). Common solutions to this problem are either to employ empirical or theoretical models of the ionosphere, or to add a priori known plasma parameter information obtained from the Plasma Line of the ISR spectrum. Studies have demonstrated that plasma parameters can be unambiguously estimated in almost noiseless scenarios, not commonly feasible during routine monitoring. In this study, we define a theoretical framework to quantify the ambiguity problem and determine the maximum signal fluctuation levels of the ISR signal to unambiguously estimate plasma parameters. We conduct Monte Carlo simulations for different plasma parameters to evaluate the estimation performance of the most commonly used Non‐Linear Least Squares optimization algorithm. Results are shown as probability curves of valid convergence and ‘correct’ estimation. We use simulations to quantify the estimation error when using ionospheric models as initial conditions of the optimization algorithm. We also determine the contribution to the estimation process of different combinations of parameters known from the Plasma Line, the particular contribution of each plasma parameter, and the effect of increasing the level of uncertainty of the parameters known a priori. Results suggest that knowing a priori both electron density and electron temperature parameters allows an unambiguous estimation even at high fluctuation levels.

Description: Abstract Plasmaspheric hiss waves commonly observed in high‐density regions in the Earth's magnetosphere are known to be one of the main contributors to the loss of radiation belt electrons. There has been a lot of effort to investigate the distributions of hiss waves in the plasmasphere while relatively little attention has been given to those in the plasmaspheric plume. In this study, we present for the first time a statistical analysis of the occurrence and the spatial distribution of wave amplitudes and wave normal angles for hiss waves in plumes using Van Allen Probes observations during the period of October 2012 to December 2016. Statistical results show that a wide range of hiss wave amplitudes in plumes from a few pT to 〉 100pT is observed, but a modest (〈 20pT) wave amplitude is more commonly observed regardless of geomagnetic activity in both the midnight‐to‐dawn and dusk sector. By contrast, stronger amplitude hiss occurs preferentially during geomagnetically active times in the dusk sector. The wave normal angles are distributed over a broad range from 0 to 90o with a bimodal distribution: a quasi‐field‐aligned population (〈 20o) with an occurrence rate of 〈 60% and an oblique one (〉 50o) with a relative low occurrence rate of ≲ 20%. Therefore, from a statistical point of view, we confirm that the hiss intensity (a few tens of pT) and field‐aligned hiss wave adopted in previous simulation studies are a reasonable assumption, but stress that the activity‐dependence of the wave amplitude should be considered.

Description: Abstract Dipolarization fronts are typically observed with a density gradient of scale size comparable to an ion gyroradius, which naturally results in an ambipolar electric field in the direction of the gradient. Prevailing models ignore this ambipolar electric field, the separation of ion and electron scale physics, and consequent non‐Maxwellian plasma distributions with strong spatial gradients in velocity, all of which we investigate in this paper. We examine two dipolarization front events observed by the Magnetospheric Multiscale (MMS) mission (one with low plasma beta, one with high plasma beta), develop a rigorous kinetic equilibrium for dipolarization fronts, analyze the linear stability, and explore the nonlinear evolution and observable signatures with kinetic simulations. There are two major drivers of instability in the lower hybrid frequency range: the density gradient (lower hybrid drift instability) and the velocity shear (electron‐ion hybrid instability). We argue the electron‐ion hybrid mode is dominant, and consequently a dipolarization front approaches a steady or saturated state through the emission of waves that relax the velocity shear. A key aspect of these shear‐driven waves is a broadband frequency spectrum that is consistent with satellite observation.

Description: Abstract We evaluate the budgets of available energy (AE) production, transport, and loss under steady‐state forcing of the high‐latitude lower thermosphere during the southern summer time for weak or strong southward interplanetary magnetic field (IMF) (Bz = −2.0 nT or −10.0 nT), using the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model. The AE below 150 km altitude is partitioned into commensurable reservoirs of kinetic energy (KE) and available potential energy (APE). KE density is larger than APE density above 123 km, but they are comparable below 123 km. Whereas Pedersen ion drag generates strong winds and KE, vertical winds and adiabatic cooling above 123 km tend to offset temperature perturbations caused by Joule heating, thereby reducing APE. Below 123 km Hall ion drag and associated vertical motions are important for creating temperature perturbations and APE. With increasingly negative IMF Bz values APE density intensifies more significantly than KE density above about 123 km, while KE density intensifies more significantly than APE density below. KE is generated primarily where the ion‐drag force associated with plasma convection accelerates the neutral gas, and is destroyed primarily where the ion‐drag force opposes the wind. APE is generated primarily where Joule heat is deposited in regions of elevated temperatures, and destroyed where the heat is deposited in regions of reduced temperatures. Ion drag is generally more important than Joule heating for generating AE for steady‐state conditions, but the relative contribution of the ion‐drag forcing compared with heating decreases with descending altitude. AE generation by Joule heating intensifies more significantly than generation by ion drag with increasingly negative IMF Bz values. Transport of AE by horizontal and vertical winds is a significant component of the AE budget. Conversion of APE to KE, and of KE to APE, constitutes an important part of their budgets.

Description: Abstract Electromagnetic ion cyclotron (EMIC) waves can drive precipitation of 10s keV protons and relativistic electrons, and are a potential candidate for causing radiation belt flux dropouts. In this study, we quantitatively analyze three cases of EMIC‐driven precipitation, which occurred near the dusk sector observed by multiple Low‐Earth‐Orbiting (LEO) POES/MetOp satellites. During EMIC wave activity, the proton precipitation occurred from few 10s keV up to 100s keV, while the electron precipitation was mainly at relativistic energies. We compare observations of electron precipitation with calculations using quasi‐linear theory. For all cases, we consider the effects of other magnetospheric waves observed simultaneously with EMIC waves, namely plasmaspheric hiss and magnetosonic waves, and find that the electron precipitation at MeV energies was predominantly caused by EMIC‐driven pitch angle scattering. Interestingly, each precipitation event observed by a LEO satellite extended over a limited L‐shell region (ΔL~0.3 on average), suggesting that the pitch angle scattering caused by EMIC waves occurs only when favorable conditions are met, likely in a localized region. Furthermore, we take advantage of the LEO constellation to explore the occurrence of precipitation at different L shells and MLT sectors, simultaneously with EMIC wave observations near the equator (detected by Van Allen Probes) or at the ground (measured by magnetometers). Our analysis shows that although EMIC waves drove precipitation only in a narrow ΔL, electron precipitation was triggered at various locations as identified by POES/MetOp over a rather broad region (up to ~4.4 hr MLT and ~1.4 L shells) with similar patterns between satellites.

Description: Abstract Daytime periodic wave‐like structures are statistically analyzed for the first time in the low‐latitude ionosphere over the Asian‐Australian sector using total electron content (TEC) from Beidou geostationary satellites during 2016‐2017. These structures have periods of about 18‐28 minutes, which frequently occur during 11:00‐17:00 local time (LT) in the winter at latitudes ranging between 17° N and 25° N (10° ‐ 18° N MLAT) in the Northern Hemisphere, where they have a maximum occurrence rate of 80% at ~21° N (14° N MLAT). In the Southern Hemisphere, daytime periodic wave‐like structures are also observed during 11:00‐15:00 LT in the winter within latitudes ranging between 6.0° S and 11.1° S (15.4° ‐ 21.6° S MLAT), although the peak occurrence rate is only approximately 40%. Compared with stratospheric gravity waves (GWs), the seasonal and latitudinal variations of daytime periodic wave‐like structures are generally consistent with those of stratospheric GWs. This gives a possible argument that daytime periodic wave‐like structures in the low‐latitude ionosphere could be generated in the low‐latitude ionosphere and triggered by GWs from the lower atmosphere.

Description: Abstract We present the ground observation of the modulation of strong electromagnetic ion cyclotron (EMIC) waves by short and long periodicities at Indian Antarctic station, Maitri. The signatures of these waves were evident in the magnetic field variations recorded by an induction coil magnetometer during the interval 4.7‐7.2 UT on 17 September 2011, a moderately disturbed day. These waves were preceded by a gradual increase in the solar wind dynamic pressure, which started at 3.88 UT. The discrete rising tone EMIC waves were observed in the Pc1 frequency band (~0.5‐0.9 Hz). The investigation of the periodicities of the observed wave spectrogram shows the presence of short (≈2.4 minute) and long (≈ 39‐69 minute) periodicities. Our analysis shows that the short periodicities are associated with the Pc5 ULF waves generated by magnetic field line oscillations, while long periodicities might be associated with the ring current drifting ions. A new method, based on the cross‐correlation technique is adopted to determine sweep rates of the discrete rising tones. The average sweep rates estimated in the range of 0.44‐1.9 mHz/s are relatively low as compared to the past reports of sweep rates derived from the satellite observations of EMIC waves. We found that the higher sweep rates are associated with the stronger EMIC waves on the ground, which is in agreement with the theoretical studies. This suggests that the theoretically proposed dependence of sweep rate on strength of EMIC wave in the generation region is retained even during the propagation of these waves on the ground.

Description: Abstract Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations over a ten‐year period from 2008 to 2017, we statistically investigate the thermodynamic properties for magnetosheath ions and their dependence on upstream interplanetary magnetic field (IMF) conditions. The thermodynamic properties for magnetosheath ions are estimated by using the polytropic index averaged over the subinterval that belongs to the same streamline ( ). The THEMIS observations show that the probability distribution of for magnetosheath ions has a major peak at ~1 (quasi‐isothermal conditions) with a longer left‐tail down to ~0 (quasi‐isobaric conditions). The spatial distributions of for two different types according to IMF spiral angle (i.e., Parker spiral and ortho‐Parker spiral IMF orientations) reveal that the ions in the downstream of a quasi‐perpendicular shock (quasi‐perpendicular magnetosheath) exhibit quasi‐isothermal processes, while those in the downstream of a quasi‐parallel shock (quasi‐parallel magnetosheath) show lower than unity (down to ~0.8) implying the anticorrelation between the ion temperature and the ion number density variations. Moreover, in the quasi‐parallel magnetosheath tends to decrease with increasing magnetic local time (MLT) distance from the magnetic local noon. These results indicate that the thermodynamic properties for magnetosheath ions depend on the bow shock geometry (quasi‐perpendicular bow shocks versus quasi‐parallel bow shocks) and are presumably controlled by a variety of instabilities, waves, and turbulence in the magnetosheath.

Description: Abstract Using data from the Relativistic Electron Proton Telescope (REPT) on the Van Allen Probes, the effects of geomagnetic storms and solar wind conditions on the ultrarelativistic electron (E〉~3 MeV) flux enhancements in the outer radiation belt, especially regarding their energy dependence, are investigated. It is showed that, statistically, more intense geomagnetic storms are indeed more likely to cause flux enhancements of ~1.8 – 7.7 MeV electrons, though large variations exist. As the electron energy gets higher, the probability of flux enhancement gets lower. To shed light on which conditions of the storms are preferred to cause ultrarelativistic electron flux enhancement, detailed superposed epoch analyses of solar wind parameters and geomagnetic indices during moderate and intense storms with/without flux enhancements of different energy electrons are conducted. The results suggest that, the storms with higher solar wind speed, sustained southward IMF Bz, lower solar wind number density, higher solar wind Ey, as well as elevated and sustained substorm activity are more likely to cause ultrarelativistic electron flux enhancements in the outer belt. Comparing results of different energy electrons, the solar wind speed and AE index are the two parameters mostly correlated with the energy‐dependent acceleration of ultrarelativistic electrons: storms with higher solar wind speed and elevated and sustained substorm activity are more likely to cause flux enhancement of ultrarelativistic electrons with higher energies. This suggests the important roles of inward radial diffusion as well as the source and seed populations provided by substorms on the energy‐dependent acceleration of ultrarelativistic electrons.

Description: Abstract We analyze three substorms that occur on 1) 9 March 2008 05:14 UT, 2) 26 February 2008 04:05 UT, and 3) 26 Feb 2008 04:55 UT. Using ACE solar wind velocity, IMF Bz values, we calculate the rectified (southward Bz) solar wind voltage propagated to the magnetosphere. The solar wind conditions for the two events were vastly different, 300 kV for 9 March 2008 substorm, compared to 50 kV for 26 February 2008. The voltage is input to a nonlinear physics based model of the magnetosphere called WINDMI. The output is the westward auroral electrojet current which is proportional to the auroral electrojet (AL) index from WDC Kyoto and the SuperMAG auroral electrojet index (SML). Substorm onset times are obtained from the superMAG substorm database, Pu et al. [2010], Lui [2011] and synchronized to THEMIS satellite data. The timing of onset, model parameters and intermediate state space variables are analyzed. The model onsets occurred about 5 to 10 mins earlier than the reported onsets. Onsets occurred when the geotail current in the WINDMI model reached a critical threshold of 6.2 MA for the 9 March 2008 event, while, in contrast, a critical threshold of 2.1 MA was obtained for the two 26 February 2008 events. The model estimates 1.99 PJ of total energy transfer during the 9 March 2008 event, with 0.95 PJ deposited in the ionosphere. The smaller events on 26 February 2008 resulted in a total energy transfer of 0.37 PJ according to the model, with 0.095 PJ deposited in the ionosphere.