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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. 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 midlatitudes. 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 energetic electron precipitation. Plain Language Summary: The study presented here investigates the effects from energetic electron precipitation (EEP) in the southern hemisphere winter of 2003. Electron precipitation is common during periods of enhanced geomagnetic activity and can create reactive nitrogen oxides and hydrogen oxides that can destroy ozone. Most global climate models currently do not include precipitating electrons with energies greater than 30 keV. To test whether this deficiency is important, this investigation compares observations with model simulations that included electrons with energies greater than 30 keV, as observed by the Medium Energy Proton and Electron Detector (MEPED) satellite instruments. In addition, one of the EEP data sets used in the simulations included data from just one of the telescopes on the MEPED instruments, whereas the other included data from both of the telescopes. We found that including both of the telescopes is important for capturing chemistry changes at polar and subpolar latitudes. The model simulation that only included only one of the telescopes showed significant improvement compared to a simulation with only low energy electrons. However, it did not perform as well as the model simulation that included both MEPED telescopes. This work is important because it shows that including energies >30 keV and the full range of precipitating electron pitch angles is necessary to show the impact electrons have on the atmosphere and provides an EEP data set for use in future model simulations. Key Points: Effects of energetic electron precipitation in southern hemisphere 2003 simulated with Whole Atmosphere Community Climate ModelSimulations of middle atmosphere chemistry improve significantly by including >30‐keV precipitating electronsIncluding the full range of pitch angles is necessary to capture precipitating electron impacts at subpolar latitudes [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Dynamics of the ionization troughs determines the variation in a structure of the winter and equinoctial subauroral and auroral ionosphere. Unfortunately, many aspects of this dynamics are still poorly known, which does not allow creating an adequate model of the high‐latitude ionosphere. Such a model is necessary to describe the conditions for propagation of short radio waves. In the international reference model of the ionosphere IRI, the trough as a structure is absent; therefore, IRI poorly describes high latitudes. In this paper, for the first time, a comprehensive pattern of diurnal variations in the different troughs position in winter high‐latitude ionosphere is constructed. They are found to be highly dependent on hemisphere, longitude, and solar activity. The results of this work will be used to build an adequate model of the high‐latitude ionosphere, which is being now created in the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN). Key Points: Comprehensive pattern of diurnal variations in winter ionospheric troughs' position in Northern and Southern hemispheres was obtainedStatistical analysis based on a large dataset of Interkosmos‐19, CHAMP, and Kosmos‐900 satellites was performedDiurnal variations in trough position depend strongly on solar activity, hemisphere, and longitude [ABSTRACT FROM AUTHOR]

This paper describes properties and behavior of magnetic reconnection and flux transfer events (FTEs) on the dayside magnetopause using the global hybrid‐Vlasov code Vlasiator. We investigate two simulation runs with and without a sunward (positive) Bx component of the interplanetary magnetic field (IMF) when the IMF is southward. The runs are two‐dimensional in real space in the noon‐midnight meridional (polar) plane and three‐dimensional in velocity space. Solar wind input parameters are identical in the two simulations with the exception that the IMF is purely southward in one but tilted 45° toward the Sun in the other. In the purely southward case (i.e., without Bx) the magnitude of the magnetosheath magnetic field component tangential to the magnetopause is larger than in the run with a sunward tilt. This is because the shock normal is perpendicular to the IMF at the equatorial plane, whereas in the other run the shock configuration is oblique and a smaller fraction of the total IMF strength is compressed at the shock crossing. Hence, the measured average and maximum reconnection rate are larger in the purely southward run. The run with tilted IMF also exhibits a north‐south asymmetry in the tangential magnetic field caused by the different angle between the IMF and the bow shock normal north and south of the equator. Greater north‐south asymmetries are seen in the FTE occurrence rate, size, and velocity as well; FTEs moving toward the Southern Hemisphere are larger in size and observed less frequently than FTEs in the Northern Hemisphere. Key Points: Sunward IMF tilt results in a smaller tangential field at the magnetopause, slowing dayside reconnectionSmaller tangential field results from a decrease in the IMF component that is shocked up at the bow shockA positive IMF Bx component introduces north‐south asymmetries, where fewer (but larger) FTEs appear on the Southern Hemisphere [ABSTRACT FROM AUTHOR]

We present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF By+ in NH (By− in SH) in most local seasons under both signs of IMF Bz. For By− in NH (By+ in SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During By+ and Bz+ in NH (By− and Bz+ in SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.18 ± 0.09 in winter and 1.17 ± 0.09 in autumn. During By+ and Bz− in NH (By− and Bz− in SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.16 ± 0.07 for FAC. We also find an explicit By effect on auroral currents in a given hemisphere: on average By+ in NH and By− in SH causes larger currents than vice versa. The explicit By effect on divergence‐free current during IMF Bz+ is in very good agreement with the By effect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer. Key Points: Hemispheric asymmetry in auroral currents is larger for interplanetary magnetic field (IMF) By+ in the Northern Hemisphere (NH) (By− in the Southern Hemisphere [SH]) than vice versa during both signs of IMF BzStrongest asymmetry occurs in local winter and autumn for By+ in the NH (By− in the SH) and IMF Bz+ with NH/SH field aligned current ratio of about 1.18IMF By+ in the NH and By− in the SH causes larger auroral currents than vice versa. Effect is stronger for IMF Bz+ than for IMF Bz− [ABSTRACT FROM AUTHOR]

We present a statistical investigation of the seasonal effect on hemispheric asymmetry in the auroral currents during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity. Five years of magnetic data from the Swarm satellites has been analyzed by applying the spherical elementary current system (SECS) method. Bootstrap resampling has been used to remove the seasonal differences between the hemispheres in the data set. In general, the currents are larger in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Asymmetry is larger during low than high Kp and during local winter and local autumn than local summer and local spring. For all Kp conditions together, the NH/SH ratio for FACs in winter, autumn, spring, and summer are 1.17 ± 0.05, 1.14 ± 0.05, 1.07 ± 0.04, and 1.02 ± 0.04, respectively. The largest asymmetry is observed during low Kp in local winter, when the excess in the NH currents is 21 ± 5% in FAC, 14 ± 3% in curl‐free (CF) and 10 ± 3% in divergence‐free (DF) current. We also find that evening sector (13–24 MLT) contributes more to the high NH/SH ratio than the morning (01–12 MLT) sector. The physical mechanisms producing the hemispheric asymmetry are not presently understood. We calculated the solar‐induced ionospheric conductances during low Kp conditions from the IRI model. The model conductance NH/SH ratios are above 1 in autumn and spring, similar to the currents, but below 1 for winter, which is in contradiction with the currents. Therefore, we do not consider solar‐induced conductances as the main explanation for hemispheric asymmetry. Key Points: Hemispheric asymmetry in auroral currents is largest during local winter and local autumn and smallest during local summer and local springLow Kp conditions show the strongest asymmetry in local winter, with maximum NH/SH ratio of 1.21 for FAC and 1.10 for DF currentCurrents in the evening sector contribute more to the high NH/SH ratio than the morning sector [ABSTRACT FROM AUTHOR]

Ionospheric sporadic E layers are irregularities with enhanced plasma density which occur in the ionosphere E region. In the present work, we have focused on an interesting wavenumber 4(WN4) pattern of sporadic E layer based on the Constellation Observing System for Meteorology, Ionosphere and Climate radio occultation S4 indices observation. It is found that amplitude of WN4 pattern has a significant seasonal dependency with the maximum appearing in summer/autumn and minimum in winter. The seasonal variation of the WN4 pattern is basically consistent with the eastward propagating diurnal tide with zonal wavenumber 3 (DE3) tide features. Inconsistencies, mainly an anomalous weak amplitude during the summer in southern hemisphere and propagation direction (speed), are also found. By removing the zonal ion drift factor effect, WN4 pattern of sporadic E layers exhibits a closer relation with the DE3 tide with larger amplitude in the northern hemisphere and ~90° shift in longitude on average during autumn in the southern hemisphere. In general, it is reasonable to conclude that DE3 tide greatly contributes to this WN4 pattern of sporadic E layers. [ABSTRACT FROM AUTHOR]

The local, regional, and global morphology of the ionospheric response of the March 2015 geomagnetic storm has been investigated by different studies. However, the spatio-temporal evolution of the drivers of the global ionospheric response to this storm has not yet been investigated, using multidata sources, in detail. Therefore, in this study drivers of the ionospheric response to the March 17-18, 2015 storm are investigated. Spatial and temporal variations of deposition of solar wind energy are found to be the cause for hemispherical asymmetry of the response of the ionosphere; the American-Canada-Greenland sector, Northern Hemisphere high-latitude ionosphere responded about 12 h earlier than the Southern Hemisphere (SH) high-latitude ionosphere, resulted from hemispherical energy imbalance as detected from Hemispherical Power. The positive ionospheric storm observed in the high-latitude regions is found to be due to solar wind energy deposition at high latitudes. Interestingly, it is found that the Northern hemisphere ionospheric positive storm shifted to the mid-latitude and disappeared there whereas the SH ionospheric positive storm shifted to mid-latitude and then farther to the low-latitude with time; this spatio-temporal evaluation of positive ionospheric storm is found to be due to the spatiotemporal enhancement of the O/N2 ratio. Also, the mid (Europe-African)and low (Brazilian) latitudes positive ionospheric storms prevailed due to prompt penetration electric fields, disturbance dynamo electric fields, and enhancement of the O/N2 ratio. Moreover, the negative ionospheric storm, observed at the NH high and mid-latitudes in the American and Asian sectors, is linked to the reduction of the O/N2 ratio. [ABSTRACT FROM AUTHOR]

An unusual sudden stratospheric warming (SSW) event occurred in the Southern Hemisphere in September 2019. Ground-based and satellite observations show the presence of transient eastward- and westward-propagating quasi-10 day planetary waves (Q10DWs) during the SSW. The planetary wave activity maximizes in the mesosphere and lower thermosphere region approximately 10 days after the SSW onset. Analysis indicates that the westward-propagating Q10DW with zonal wave number s = 1 is mainly symmetric about the equator, which is contrary to theory which predicts the presence of an antisymmetric normal mode for such planetary wave. Observations from microwave limb sounder and sounding of the atmosphere using broadband emission radiometry are combined with meteor radar wind measurements from Antarctica, providing a comprehensive view of Q10DW wave activity in the Southern Hemisphere during this SSW. Analysis suggests that the Q10DWs of various wavenumbers are potentially excited from nonlinear wave-wave interactions that also involve stationary planetary waves with s = 1 and s = 2. The Q10DWs are also found to couple the ionosphere with the neutral atmosphere. The timing of the quasi-10-day oscillations (Q10DOs) in the ionosphere are contemporaneous with the Q10DWs in the neutral atmosphere during a period of relatively low solar and geomagnetic activity, suggesting that the Q10DWs play a key role in driving the ionospheric Q10DOs during the Southern SSW event. This study provides observational evidence for coupling between the neutral atmosphere and ionosphere through the upward propagation of global scale planetary waves. [ABSTRACT FROM AUTHOR]

A sudden stratospheric warming (SSW) is an extremely rare event in the Southern Hemisphere (SH), but occurred in early September 2019. From the Antarctic meteor radar (MR) stations, Davis (68.6°S, 77.9°E) and King Sejong Station (62.2°S, 58.8°W), quasi 10-day oscillations were clearly observed in the zonal mesospheric winds before the central date (DOY 253) of the SSW. From the northern low-latitude Tirupati (13.6°N, 79.4°E) MR, a strong wave activity with a period of ~6 days was detected in the zonal winds right after the central date. This oscillation is also seen in the geopotential height measurements from the Microwave Limb Sounder (MLS) on board the Aura satellite near the Tirupati region. To elucidate the possible source of the quasi 6-day wave (Q6DW), we use a specified dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM) constrained by the reanalysis data from the surface to 50 km. The simulation results show that the amplitude of the westward and equatorward propagating Q6DW was enhanced after the SSW central date in the MLT region, and the Q6DW can be attributed to the baroclinic/barotropic instability in the SH high-latitude mesosphere where the divergence of Eliassen-Palm flux occurred. Thus, we suggest that the Q6DW activity observed by the Tirupati MR and MLS originated from the SH high-latitude mesospheric region. Both the observation and the simulation results clearly demonstrate that the 2019 SH SSW affected not only the high-latitude MLT region but also the low-latitude MLT region. [ABSTRACT FROM AUTHOR]

We report new findings of total electron content (TEC) perturbations in the southern hemisphere at conjugate locations to the northern eclipse on August 21, 2017. We identified a persistent conjugate TEC depletion by 10%--15% during the eclipse time, elongating along magnetic latitudes with at least ~5° latitudinal width. As the Moon's shadow swept southward, this conjugate depletion moved northward and became most pronounced at lower magnetic latitudes (>-20°N). This depletion was coincident with a weakening of the southern crest of the equatorial ionization anomaly (EIA), while the northern EIA crest stayed almost undisturbed or was slightly enhanced. We suggest these conjugate perturbations were associated with dramatic eclipse initiated plasma pressure reductions in the flux tubes, with a large portion of shorter tubes located at low latitudes underneath the Moon's shadow. These short L-shell tubes intersect with the F region ionosphere at low and equatorial latitudes. The plasma pressure gradient was markedly skewed northward in the flux tubes at low and equatorial latitudes, as was the neutral pressure. These effects caused a general northward motion tendency for plasma within the flux tubes, and inhibited normal southward diffusion of equatorial fountain plasma into the southern EIA region. We also identified posteclipse ionospheric disturbances likely associated with the global propagation of eclipse-induced traveling atmospheric disturbances in alignment with the Moon's shadow moving direction. [ABSTRACT FROM AUTHOR]

Energy and momentum can be transferred from the magnetosphere to the ionosphere through field-aligned currents (FACs), which show seasonal variations due to seasonal variation of the ionospheric conductance. Using Magnetospheric Multiscale Mission observations, we investigate the characteristics of kinetic-scale (close to local ion inertial length) FACs in the plasma sheet boundary layer (PSBL), such as occurrence rate, magnitude, and carriers. The results show that: (1) the occurrence rates of FACs exhibit a trident distribution; they are also larger in the northern hemisphere than in the southern hemisphere, especially for the earthward FACs. (2) Distribution patterns vs. plasma β are different for the beam-type and the flow-type FACs. Flow-type FACs are observed closer to the central plasma sheet (higher β), and their occurrence rate decreases linearly toward the tail lobe (lower β), while the beamtype FACs are mainly and equally distributed within 0.1 < β < 1.0. (3) The magnitude of FACs shows little dependence on β, and generally increases closer to the Earth. (4) Occurrence rate and magnitude of FACs both increase during active periods, consistent with ionospheric observations. (5) The main carriers for FACs in the PSBL are thermal electrons, while cold electrons also contribute occasionally, especially during active periods. Hemispheric asymmetry of occurrence rate and dependence on geomagnetic activity of the kinetic-scale FACs in the PSBL are consistent with FACs at ionospheric altitudes. This fact demonstrates that kinetic-scale FACs are significant in magnetosphere-ionosphere coupling and illustrates the possible ionospheric feedback effects to the magnetosphere in the nightside. [ABSTRACT FROM AUTHOR]

We present an event of simultaneous observations of the northern and southern middle‐altitude polar cusps by the Polar spacecraft and Cluster fleet that occurred on 23 September 2004. We examine the possible asymmetries in the fields and plasma parameters, although the proximity of the equinox should limit these asymmetries. Ion sensors reveal two dispersions in both cusps, and data analysis leads to the conclusion that those dispersions are due to pulsed reconnection at a single X‐line, which runs along the subsolar magnetopause. While the electromagnetic and particle energy fluxes injected in both cusp are globally very similar, we report significant differences in ion dispersions, width of the low‐latitude boundary layer, and peak convection velocities. We ascribe these differences to the dipole tilt that introduces an asymmetry in the magnetosheath flow at the exterior cusps. Key Points: Data from Cluster and Polar show a symmetry in ion injections in the magnetospheric cuspsSignatures corresponding to a single reconnection line running along the subsolar magnetopause and pulsed reconnection are highlightedDifferences in convection velocities are found and ascribed to the effect of the tilt angle [ABSTRACT FROM AUTHOR]

A total solar eclipse occurred in the Southern Hemisphere on 2 July 2019 from approximately 17 to 22 UT. Its effect in the thermosphere over South America was imaged from geostationary orbit by NASA's Global-scale Observation of Limb and Disk (GOLD) instrument. GOLD observed a large brightness reduction (>80% around totality) in OI 135.6 nm and N2 LBH band emissions compared to baseline measurements made 2 days prior. In addition, a significant enhancement (with respect to the baseline) in the SO/N2 column density ratio (~80%) was observed within the eclipse's totality. This enhancement suggests that the eclipse induced compositional changes in the thermosphere. After the eclipse passed, a slight enhancement in SO/N2 column density ratio (~7%) was also seen around the totality path when compared to measurements before the eclipse. These observations are the first synoptic imaging measurements of an eclipse's thermospheric effects with the potential to drastically improve and test our understanding of how the thermosphere responds to rapid, localized changes in solar short wavelength radiation. [ABSTRACT FROM AUTHOR]

The strongest Southern Hemisphere minor sudden stratospheric warming (SSW) in the last 40 years occurred in September 2019 and resulted in unprecedented weakening of the stratospheric polar vortex. Ionospheric total electron content (TEC) observations are used to provide an overview of statistically significant anomalies in the low‐latitude ionosphere during this event. Quasi‐semidiurnal perturbations of TEC are observed in response to the SSW, similar to those seen during Northern Hemisphere SSWs. Analysis indicates the existence of quasi‐periodic oscillations in TEC in the crests of the equatorial ionization anomaly, with strong 5‐ to 6‐day and 2‐ to 3‐day periodicities. Ionospheric anomalies from the combined effects of multiple mechanisms exceed a factor of 2, comparable to the strongest anomalies associated with Northern Hemisphere SSWs. These results also indicate a remarkable longitudinal variation in the character and magnitude of variations that could be related to a modulation of the non‐migrating diurnal tide. Plain Language Summary: Sudden stratospheric warming, a large‐scale meteorological disturbance, has been associated with profound anomalies in the Earth atmosphere, from troposphere all the way to the upper thermosphere and ionosphere. During the last decade, numerous studies showed that Arctic sudden stratospheric warmings cause especially large anomalies in the low‐latitude ionosphere. However, it was not clear if similar ionospheric anomalies can be produced by Antarctic sudden stratospheric warming, mostly because Antarctic sudden stratospheric warmings are pretty rare. In this study, we provide an overview of ionospheric anomalies in total electron content observed in September 2019, when very strong sudden stratospheric warming developed over Antarctica. We conclude that Antarctic events produce even more dynamical changes in the ionosphere than Arctic events. We report for the first time large differences in the observed features at locations that are separated by only 30° in longitude. Our results indicate that stratospheric weather can strongly influence the state of the ionosphere not only during December–February period (winter in the Northern Hemisphere), but also during September (equinox conditions). Key Points: Minor Antarctic sudden stratospheric warming of September 2019 produces large ionospheric anomalies at low latitudesChanges of total electron content exceeding a factor of 2 are observedLarge longitudinal differences in total electron content response to Antarctic SSW are reported [ABSTRACT FROM AUTHOR]

The 2015 St. Patrick's Day geomagnetic storm caused numerous disturbances of the ionosphere, particularly, plasma irregularities, large‐scale traveling ionospheric disturbances, and equatorial ionization anomaly enhancement. This study for the first time quantifies the global‐scale impacts of the ionospheric disturbances on Global Positioning System (GPS) precise point positioning (PPP) solutions during this extreme space weather event by taking advantage of 5,500 + GNSS stations installed worldwide. The overall impact was more severe at high latitudes, while PPP degradation at low latitudes was associated with different types of ionospheric disturbances. Specifically, our results show that kinematic PPP solutions degraded following an intensified auroral particle precipitation during the storm's main phase (06–23 UT) when up to ~70% of the high‐latitude stations experienced degraded position solutions in the multimeter range at 16–18 UT. Around magnetic noon and midnight, the storm‐induced plasma irregularities caused notable PPP errors (>10 m) at high latitudes. Interhemispheric differences were observed with a more severe impact seen in the Southern Hemisphere, where PPP outage lasted for ~12 hr during the second main phase (12–23 UT). At low latitudes, post sunset equatorial plasma irregularities were suppressed across most longitudes, but large PPP errors (>2 m) associated with storm‐induced plasma bubbles were registered at the Indian sector at 14–18 UT. The storm‐induced equatorial ionization anomaly enhancement and large‐scale traveling ionospheric disturbances were responsible for the low‐latitude PPP degradation at dayside sectors. This study fills the research gap between physical and practical aspects of severe ionospheric storm effects. Key Points: Ionospheric disturbances caused high‐precision GPS positioning degradation on a global scale during the 2015 St. Patrick's Day stormAt high latitudes, severe position degradation was observed around magnetic noon and midnight coinciding with plasma irregularities zoneAt low latitudes, position errors were due to storm‐induced plasma bubbles, EIA enhancement, and traveling ionospheric disturbances [ABSTRACT FROM AUTHOR]

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 postmidnight 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 FAC region, and collocated with the convection reversal boundary in the postmidnight region. In the longitude sector of 80°–130°E, the high‐latitude trough positions are located at the poleward boundary of the downward FAC region in the midnight region and at the strong westward or antisunward ion flow region in the postmidnight 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. Plain Language Summary: Ionospheric troughs are depletions of the nighttime F region plasma density around the auroral and polar regions. The presence of the trough has a significant impact on the radio transmission. There are two types of troughs: the midlatitude trough and the high‐latitude trough. The midlatitude trough locates on the equatorward side of the auroral oval, and the high‐latitude trough locates within the auroral oval or polar cap. The features and formation mechanisms of the high‐latitude trough in the Southern Hemisphere are not yet clear. This study uses a new multiparameter data set gathered by the Swarm‐A satellite to find out the features and formation mechanisms of the high‐latitude trough in the Southern Hemisphere. We found that the high‐latitude trough in the Southern Hemisphere is a persistent postmidnight feature in winter. It is observed mainly in the eastern longitudes. The features of FACs and ion drift velocity distribution in the high‐latitude trough region depend on longitude. The formation and evolution of the high‐latitude trough is suggested to be associated with downward FACs, flow stagnation in the convection reversal boundary, and westward or antisunward ion flow. Key Points: The high‐latitude trough is a persistent postmidnight feature in winter in the Southern HemisphereThe high‐latitude trough is observed mainly in the eastern longitudesThere is a longitude dependence of features of FACs and ion drift velocity distribution in the high‐latitude trough region [ABSTRACT FROM AUTHOR]

We present statistical investigation of the high‐latitude ionospheric current systems in the Northern Hemisphere (NH) and Southern Hemisphere (SH) during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity levels. Nearly 4 years of vector magnetic field measurements are analyzed from the two parallel flying Swarm A and C satellites using the spherical elementary current system method. The ionospheric horizontal and field‐aligned currents (FACs) for each auroral oval crossing are calculated. The distributions of the mean values of FACs as well as the horizontal curl‐free and divergence‐free currents in magnetic latitude and magnetic local time for each hemisphere and activity level are presented. To estimate the NH/SH current ratios for the two activity levels, we remove seasonal bias in the number of samples and in the Kp distribution by bootstrap resampling. This is done in such a manner that there are equal number of samples from each season in each Kp bin. We find that for the low activity level, the currents in the NH are stronger than in the SH by 12±4% for FAC, 9±2% for the horizontal curl‐free current, and 8±2% for the horizontal divergence‐free current. During the high activity level, the hemispheric differences are not statistically significant. This suggests that the local ionospheric conditions, such as magnetic field strength or daily variations in insolation, may be important and play a larger role during quiet than disturbed periods. This issue must be studied further. Key Points: FACs and horizontal currents from Swarm data are analyzed statistically for both Northern (NH) and Southern Hemispheres (SH) using SECS methodWhen seasonal bias is removed from the data, interhemispheric asymmetry is found for low but not for high magnetic activity caseIn low magnetic activity case both FACs and horizontal currents are about 10% more intense in the NH than in the SH [ABSTRACT FROM AUTHOR]

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. Key Points: We construct an asymmetric three‐dimensional bow shock model with dipole tilt effectsShape and size of bow shock are both affected by the dipole tilt angleThe dipole tilt angle causes asymmetries of bow shock in the meridional plane [ABSTRACT FROM AUTHOR]

The study investigates previously unrecognized wind reversals of the daytime thermosphere in the Southern Hemisphere at latitudes of 50°S–70°S employing observations of O(1D) airglow volume emission rates (VER), temperature, and neutral winds at 170–300‐km height by the Wind Imaging Interferometer. Atomic oxygen densities derived from the Wind Imaging Interferometer O+ (732–733 nm) emission observations were also considered together with contemporaneous observations of total electron content by the TOPEX Poseidon satellite mission. The O(1D) VER for fall equinox (March/April 1994) and summer solstice (January 1995) revealed a peak in the VER over the longitude range of 50°E–150°E, for both seasons and daytime local times. The Doppler temperatures also exhibited a maximum at the same location and times. The co‐located zonal wind field shows a westward trough at 50°E–150°E with wind speeds reaching −450 m/s and an eastward peak at 200°E–300°E. Meridional winds show a "peak and trough" structure over the 50°E–150°E region, with a distinct eastern boundary at ~150°E. During summer solstice the atomic oxygen is depleted at the region of the O(1D) VER enhancement in the presence of westward zonal wind. However, the total electron content observations showed a peak that coincided with the O(1D) VER and temperature enhancements. Plain Language Summary: The study investigates unusual perturbations in upper atmospheric winds that occur at a very specific location in the high‐latitude Southern Hemisphere. These include wind reversals, enhancements in the "red line" atomic oxygen airglow emission at 630.0 nm, temperature, depletion of atomic oxygen, and enhancements of ionospheric electron density. The location is 50°E–150°E longitude, 50°S–70°S latitude. The atmospheric observations were made by the Wind Imaging Interferometer, launched on NASA's Upper Atmosphere Research Satellite in 1991; these continued until 2003. The wind reversals involved westward winds of as strong as 450 m/s. The electron densities were of total electron content integrated along the line of sight from the satellite to the Earth and were obtained from the TOPEX satellite for the same time periods as the Wind Imaging Interferometer observations. Measurements were made at equinox and summer solstice and the atmospheric observations were the same for both. These new observations point the way to future studies of this highly localized phenomenon. Key Points: Previously unrecognized perturbations in thermospheric and ionospheric winds are observed at a specific high latitude location, 50°E–150°E and at 50°S–70°SThe neutral winds show a reversal to westward reaching −450 m/s, with a peak and trough structure of meridional wind, having an eastern boundary at ~150°EEnhancements occur in daytime O(1D) emission, temperature, and TEC; atomic oxygen is depleted [ABSTRACT FROM AUTHOR]

Cross‐track ion drifts measured by the Swarm A satellite are compared with colocated line‐of‐sight Super Dual Auroral Radar Network (SuperDARN) velocities in approximately the same directions. More than 200 Swarm A passes over four polar cap SuperDARN radars in the Northern and Southern Hemispheres are considered. Overall, the Swarm‐based velocities are larger than the SuperDARN velocities; the slope of the best fit line to the data is ~0.67. Somewhat stronger differences are found when Swarm A measurements for the entire year 2016 are compared with SuperDARN vector data from global‐scale convection maps. Swarm ion drift data demonstrate known features of the high‐latitude convection patterns, for example, reverse convection cells at interplanetary magnetic field Βz > 0. The latitudes of the convection reversal boundary inferred from SuperDARN are found to be in reasonable agreement with those determined from Swarm A and Swarm B, with Swarm‐based latitudes occurring roughly 1° more equatorward, typically. Key Points: Swarm cross‐track ion drifts are smaller than SuperDARN radar velocitiesOverall Swarm‐SuperDARN differences are by a factor of 1.5 but smaller in many instancesSwarm shows major features of plasma flow patterns and superior in monitoring flow reversals [ABSTRACT FROM AUTHOR]

All‐sky imagers located in Asiago, Italy (45.87oN, 11.53oE; 40.7o magnetic latitude) and Sutherland, South Africa (32.37oS, 20.81oE; −40.7o magnetic latitude) are used to study magnetically conjugate medium scale traveling ionospheric disturbances (MSTIDs). We present initial results from the first year of joint Asiago‐Sutherland data sets from July 2016 to June 2017. The 630.0‐nm airglow perturbations showing different kinds of waves were frequently observed. Some of these wave events resemble MSTIDs propagating south‐westward in Asiago, typical direction observed at other longitude sectors in the northern hemisphere. They are mostly observed as single bands propagating through the field of view of the all‐sky imagers. We select and analyze five cases of magnetically conjugate bands associated with MSTIDs. The bands observed at Sutherland move mainly westward, noticeably different from the north‐west direction of propagation of MSTIDs observed in the southern hemisphere. We compare the MSTIDs propagation speeds and find that three cases show larger values at Sutherland. When we compare the zonal speeds all the cases show larger values at Sutherland. On average, the propagation speed at Sutherland is 20% larger and the zonal speed is ~35% larger. The westward motion at Sutherland is explained by taking onto account how its magnetic declination (~24oW) affects the orientation of the bands. The larger speed at Sutherland is due to the weaker Earth's magnetic field in the southern hemisphere and the particular configuration of the magnetic field lines in this longitude sector. Key Points: MSTIDs observed at conjugate locations in the Europe‐Africa longitude sector for the first timeDirection of propagation of Sutherland MSTIDs is mostly westward, consequence of its large magnetic declinationPropagation speeds are larger at Sutherland due to weaker Earth's magnetic field and its configuration at this longitude sector [ABSTRACT FROM AUTHOR]