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During the main phase of geomagnetic storms, the horizontal ground magnetic component is more depressed at dusk than at dawn, which was originally explained by the partial ring current model. Later studies questioned the idea that the ring current intensifies primarily at dusk, and the cause of this dawn‐dusk asymmetry still remains to be understood. The present study targets this old issue by statistically examining SuperMAG geomagnetic (sub‐)indices. The results are summarized as follows: (1) the dawn‐dusk asymmetry is correlated with the enhancement of the dawnside westward auroral electrojet (AEJ); (2) its correlation with the duskside eastward AEJ, which was considered by the classical partial ring current model, is insignificant; (3) the magnetic depression tends to be larger on the dayside than on the nightside, and the asymmetry is correlated with the westward AEJ in the midnight sector; (4) the dawn‐dusk asymmetry enhances typically for 1–2 h, and the day‐night asymmetry for <1 h; (5) the dawn‐dusk and day‐night asymmetries are not correlated with each other. It is suggested that the dawn‐dusk asymmetry is associated with a dawnside wedge current system with an enhanced westward AEJ (1 & 2), and it is suggested to be driven by a substorm‐like process, and possibly also by enhanced global convection (4). The day‐night asymmetry is probably associated with the formation of the substorm wedge current system (3 & 4). Although the associated dawnside and midnight westward AEJs may often enhance simultaneously, they are possibly driven by different processes (4 & 5). Key Points: Dawn‐dusk asymmetry of storm‐time ground magnetic depression is correlated with dawnside westward auroral electrojet (AEJ) but not with duskside eastward AEJDuring storms ground magnetic field is more depressed on the dayside than on the nightside probably by the substorm wedge current systemDawn‐dusk and day‐night asymmetries are not correlated with each other, and develop at different timescales, 1–2 h for D‐D and 1 h for D‐N [ABSTRACT FROM AUTHOR]

On a geomagnetic quiet night of October 29, 2018, we captured an observational evidence of the onset of dark band structures within the field‐of‐view of an all‐sky airglow imager operating at 630.0 nm over a geomagnetic low‐mid latitude transition region, Hanle, Leh Ladakh. Simultaneous ionosonde observations over New Delhi shows the occurrence of spread‐F in the ionograms. Additionally, virtual and peak height indicate vertical upliftment in the F layer altitude and reduction in the ionospheric peak frequency were also observed when the dark band pass through the ionosonde location. All these results confirmed that the observed depletions are indeed associated with ionospheric F region plasma irregularities. The rate of total electron content index (ROTI) indicates the absence of plasma bubble activities over the equatorial/low latitude region which confirms that the observed event is a mid‐latitude plasma depletion. Our calculations reveal that the growth time of the plasma depletion is ∼2 h if one considers only the Perkins instability mechanism. This is not consistent with the present observations as the plasma depletion developed within ∼25 min. By invoking possible Es layer instabilities and associated E‐F region coupling, we show that the growth rate increases roughly by an order of magnitude. This strongly suggests that the Cosgrove and Tsunoda mechanism may be simultaneously operational in this case. Furthermore, it is also suggested that reduced F region flux‐tube integrated conductivity in the southern part of onset region created conducive background conditions for the growth of the plasma depletion on this night. Plain Language Summary: It is well known that the plasma irregularities/depletions in the ionosphere degrade the satellite‐based communication navigation signals, significantly. Thus, understanding the plausible onset condition and characteristics of these depletions are vital for the better space weather forecasting. Occurrence of plasma depletion in the equatorial and high latitude regions are associated with Generalized Rayleigh Taylor instability mechanism and their characteristics are mostly well reported. On the other hand, though the characteristics of the mid‐latitude field aligned irregularities are reported by a few investigators, there is no direct observational evidence for the onset of the mid‐latitude plasma depletion till now. Thus, the present investigation provides first optical observational evidence of in‐situ generation of plasma depletion and provides some insight on the possible background conditions which supported the onset over the geomagnetic low‐mid latitude transition region. Key Points: A remarkable observational evidence of onset of plasma depletions is captured over geomagnetic transition region in a span of 25 minIt is argued that both mid‐latitude sporadic‐E layer and Perkins instabilities are simultaneously needed to explain the growth timeIt is also proposed that reduced flux tube integrated Pedersen conductivity over south of onset location favored the growth of the bubble [ABSTRACT FROM AUTHOR]

In the publication Troshichev et al. (2006) (https://doi.org/10.1029/2005JA011402) on the polar cap (PC) indices, PCN and PCS, an error was made by using components of the interplanetary magnetic field (IMF) in their geocentric solar ecliptic (GSE) representation instead of the prescribed geocentric solar magnetospheric (GSM) representation for calculations of index scaling parameters. The mistake has caused a trail of incorrect relations and wrong conclusions extending since 2006 up to now (2020) which should be discontinued, for instance, by issuing a corrigendum note from the authors. The present contribution explains the error and discusses in an extended example its consequences for one of the publications that have referred to the invalid scaling parameter set. Further investigations reported here of the PC index versions recommended by the International Association for Geomagnetism and Aeronomy (IAGA) indicate occurrences of similar problems in the present derivation of index scaling parameters. Plain Language Summary: The publication Troshichev et al. (2006) (https://doi.org/10.1029/2005JA011402) on the polar cap (PC) indices, PCN and PCS, has by mistake used components of the interplanetary magnetic field (IMF) in their geocentric solar ecliptic (GSE) representation instead of the prescribed geocentric solar magnetospheric (GSM) representation for calculations of index scaling parameters. The mistake has caused a trail of incorrect relations and wrong conclusions extending since 2006 up to now (2020) which should be discontinued. The examinations have been extended to the PCN version recently endorsed by the International Association for Geomagnetism and Aeronomy (IAGA). The analyses indicate that the scaling parameters for this PC index version are also questionable. Key Points: The use of invalid scaling parameters for Polar Cap (PC) index calculations is disclosedRelated adverse consequences for derived publications on PC indices are analyzedA summary of publications devaluated by their use of inconsistent PC index versions is provided [ABSTRACT FROM AUTHOR]

Rapid changes of magnetic fields associated with nighttime magnetic perturbation events (MPEs) with amplitudes |ΔB| of hundreds of nT and 5–10 min duration can induce geomagnetically induced currents (GICs) that can harm technological systems. Here we present superposed epoch analyses of large nighttime MPEs (|dB/dt| ≥ 6 nT/s) observed during 2015 and 2017 at five stations in Arctic Canada ranging from 64.7° to 75.2° in corrected geomagnetic latitude (MLAT) as functions of the interplanetary magnetic field (IMF), solar wind dynamic pressure, density, and velocity, and the SML, SMU, and SYM/H geomagnetic activity indices. Analyses were produced for premidnight and postmidnight events and for three ranges of time after the most recent substorm onset: (a) 0–30 min, (b) 30–60 min, and (c) >60 min. Of the solar wind and IMF parameters studied, only the IMF Bz component showed any consistent temporal variations prior to MPEs: a 1–2 h wide 1–3 nT negative minimum at all stations beginning ∼30–80 min before premidnight MPEs, and minima that were less consistent but often deeper before postmidnight MPEs. Median, 25th, and 75th percentile SuperMAG auroral indices SML (SMU) showed drops (rises) before pre‐ and post‐midnight type A MPEs, but most of the MPEs in categories B and C did not coincide with large‐scale peaks in ionospheric electrojets. Median SYM/H indices were flat near −30 nT for premidnight events and showed no consistent temporal association with any MPE events. More disturbed values of IMF Bz, Psw, Nsw, SML, SMU, and SYM/H appeared postmidnight than premidnight. Key Points: Superposed epoch analyses of 2 years of >6 nT/s magnetic perturbation events (MPEs) from 5 high latitude Arctic stations are presentedOf the solar wind and interplanetary magnetic field (IMF) parameters studied, only IMF Bz showed any consistent pattern: a drop and rise prior to MPE occurrenceMost of the MPEs that occurred more than 30 min after a substorm onset did not coincide with peaks in the westward electrojet [ABSTRACT FROM AUTHOR]

Geomagnetic substorm is one of the essential solar wind‐magnetosphere energy coupling processes. In this study, we present a statistical analysis of substorms with different intensities that occurred from 1982 to 2012. We investigated the substorm occurrence rate, the time evolution of solar wind and geomagnetic parameters, as well as the SME, SMU and, SML indices during these substorms by using OMNI and SuperMag networks data sets. We find that substorms more frequently occur during solar declining phases and in spring and autumn due to the Russel‐McPherron effect. The superposed SME index shows a double‐peak near midnight and dusk during intense substorms, standing for comparable eastward and westward electrojets. We suggest that there is an additional current wedge during intense substorms located near the dusk. During the substorm expansion phase, eastward and westward electrojets are expanding against their current direction. The results presented in this study provide a comprehensive view of substorm characteristics with different intensities and may facilitate the understanding of the development of auroral electrojets in the ionosphere, even the current system in the magnetosphere. Key Points: Occurrence of modest substorms and intense substorms show different annual, monthly, and diurnal variationsSuperposed SME index during intense substorms shows a double‐peak distribution dominated by westward and eastward electrojets, respectivelyWe found electrojets expansions during the substorm expansion phase and the recovery phase [ABSTRACT FROM AUTHOR]

Long‐term thermospheric mass density disturbances due to magnetospheric forcing are not clear due to a lack of measurement data and imprecise models. In this study, the Global Navigation Satellite System (GNSS) precise orbits of CAScade SmallSat and IOnospheric Polar Explorer (CASSIOPE) are used to infer high‐resolution thermospheric mass densities between the years 2014 and 2020. The CASSIOPE densities are comprehensively validated at altitudes from 325 to 425 km at intervals of 25 km with the High Accuracy Satellite Drag Model (HASDM) density database, and further compared with the Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Exosphere/2000 (NRLMSISE‐00) and the Jacchia‐Bowman/2008 (JB2008) empirical models. The CASSIOPE densities are very similar to the HASDM and JB2008 densities, while the NRLMSISE‐00 largely overestimates (∼150%) during low solar‐flux conditions. For density values above ∼10−12 kg/m3, the correlation of CASSIOPE with HASDM is ∼5% better than the models, and the standard deviation is within 10% of the background density. For density values below ∼10−12 kg/m3, systematic errors have shown to reduce the precision of the CASSIOPE densities. By setting geomagnetic contributions to zero in the models, the density disturbances due to magnetospheric forcing are isolated from the CASSIOPE time‐series, allowing investigation into the correlations and time‐delay responses to the models and to the merging electric field (Em). A new linear dependence of the time delay to the Em was found and then parameterized in this study. Time delays occurred at the 4–7 h range during geomagnetic storms, and at 9–11 h during quiet conditions; neither had significant dependence on altitude. The results represent the validation of the first high‐resolution thermospheric mass density estimates inferred from commercial‐off‐the‐shelf GNSS receivers. Key Points: Thermospheric mass densities from 2014 to 2020 are estimated from CAScade SmallSat and IOnospheric Polar Explorer Global Navigation Satellite System (GNSS) precise orbitsThe high‐resolution thermospheric mass densities inferred from commercial‐off‐the‐shelf GNSS receivers are validatedThe density disturbances due to magnetospheric forcing are investigated for correlations and time‐delay responses to models and indices [ABSTRACT FROM AUTHOR]

The Magnetospheric Multiscale spacecraft observations in the burst mode allow the determination of the characteristics of resonant electrons interacting with quasi‐parallel whistler waves during prolonged dipolarizations in the near Earth magnetotail at −22 RE < X ≤ −8 RE. We have detected 163 whistler bursts observed during 48 dipolarization events. The bursts were registered within ∼13 min following the dipolarization onset when the burst mode observations were available. In the majority of events, electrons with energies Wres ≥ 10 keV and pitch angles αres ∼ 100°–130° and αres ∼ 50°–80°made the maximum positive contribution to the growth rate of whistler waves propagating quasi‐parallel and antiparallel to the ambient magnetic field, respectively. Our analysis shows that electrons with Wres ∼ 10–20 keV could potentially be scattered into the loss cone by the low frequency whistler waves with fw ∼ (0.05–0.2)fce (fw is the wave frequency and fce is the electron gyrofrequency). The electrons that are scattered into the loss cone can contribute to electron precipitation within ∼13 min following the dipolarization onset. We suggest that whistler waves that are excited due to cyclotron instability driven by the temperature anisotropy of suprathermal electrons (≥2 keV) may, in turn, affect electron distribution in this energy range. Specifically, lower energy resonant electrons transfer a part of their kinetic energy to waves, while more energetic electrons absorb wave energy increasing their kinetic energy. This may lead to the transformation of higher‐energy part of electron distribution from Maxwellian to the power law shape. Plain Language Summary: Magnetic dipolarization is an important process in the magnetotail dynamics which manifests in transformation of initially stretched tail‐like configuration into the dipole‐like configuration. The dipolarization is followed by various processes of energy dissipation and transformation including plasma heating and acceleration as well as generation of different wave modes. In collisionless plasma, wave‐particle interactions play a key role in energy exchange between different populations of plasma particles. Also particle interactions with waves may cause particle scattering into the loss cone and their precipitation in the auroral region. The processes of wave‐particle interactions usually occur at very short time scales, and their investigation "in situ" requires observations with high time resolution. In this study, we use the Magnetospheric Multiscale spacecraft observations in burst mode and determine the characteristics of resonant electrons interacting with quasi‐parallel whistler waves at time scales of the order of a few seconds. We have found that quasi‐parallel whistler waves can be generated in the course of dipolarization due to cyclotron instability driven by the temperature anisotropy of suprathermal electrons (≥2 keV). Once have been generated, these waves, in turn, affect electron distribution in this energy range. Specifically, lower energy resonant electrons transfer a part of their kinetic energy to waves, while more energetic electrons absorb wave energy increasing their kinetic energy. We also found that some fraction of suprathermal electrons can be potentially scattered into the loss cone by the low‐frequency whistler waves. Such electrons can contribute to electron precipitation within ∼13 min following the dipolarization onset. Key Points: Perpendicular anisotropy of electrons with energies more than 10 keV is responsible for whistler waves generation during dipolarizationsWhistler waves play an important role in energy exchange between different parts of electron spectrum in energy range more than 2 keVElectrons with energies 10–20 keV can be scattered into the loss cone due to their interaction with the low‐frequency whistler waves [ABSTRACT FROM AUTHOR]

Using the list of the omega structures based on the Magnetometers‐Ionospheric Radars‐All‐sky Cameras Large Experiment network (Partamies et al., 2017, https://doi.org/10.5194/angeo-35-1069-2017), we obtained a number of important statistical characteristics describing the surface magnetic field. Based on 438 events, typical magnetic variations associated with the passage of the single omega were obtained. The typical variation, obtained using superposed epoch analysis, is associated with a local bending of the westward electrojet and statistically confirms the distribution of equivalent ionospheric currents obtained in earlier observations of single omegas. It was found that during low and moderate geomagnetic activity, the appearance of the omega structures in the dark morning magnetic local time (MLT) sector results in two times higher than average dB/dt on the ground surface. Also, the velocity, direction of movement, and area of omega structures were calculated. It is shown that faster and bigger omegas produce larger time derivatives of the ground magnetic field. Furthermore, we demonstrate that in the 03–08 MLT sector, superposed magnetic variations for the arbitrary events of very high time derivatives |dB/dt| > 10 nT/s, reveal magnetic signatures similar to omegas. Our findings, together with the results described by Apatenkov et al. (2020, https://doi.org/10.1029/2019gl086677), emphasize the important role of omega structures in the formation of large geomagnetically induced currents. Key Points: Superposed‐epoch analysis confirmed the typical ground magnetic variations for omega structure—bipolar pulse in BY and short BZ decreaseOmega structures produce 50%–100% higher than averaged dB/dt during low and moderate geomagnetic activityStatistically confirmed that ground dB/dt value depends on omegas velocity and size [ABSTRACT FROM AUTHOR]

Recent observations in the quasi‐parallel bow shock by the Magnetospheric Multiscale spacecraft show rapid heating and acceleration of ions up to an energy of about 100 keV. It is demonstrated that a prominent acceleration mechanism is the nonlinear interaction with a spectrum of waves produced by gradient‐driven instabilities, including the lower hybrid drift (LHD) instability, modified two‐stream (MTS) instability, and electron cyclotron drift (ECD) instability. Test‐particle simulations show that the observed spectrum of waves can rapidly accelerate protons up to a few hundreds of keV by the ExB mechanism. The ExB wave mechanism is related to the surfatron mechanism at shocks, but through the coupling with the stochastic heating condition, it produces significant acceleration on much shorter temporal and spatial scales by the interaction with bursts of waves within a cyclotron period. The results of this study are built on the heritage of four‐point measurement techniques developed for the Cluster mission and imply that the concepts of Fermi acceleration, diffusive shock acceleration, and shock drift acceleration are not needed to explain proton acceleration to hundreds of keV at the Earth's bow shock. Plain Language Summary: The acceleration, or energization, of particles is a common and fundamental process throughout the universe. In particular, particle acceleration is a common occurrence at collisionless shocks that occur in plasmas in our solar system and beyond. This study presents new observations of the acceleration of protons to high energies (hundreds of keV) by waves at the bow shock upstream of the Earth, where the solar wind first encounters Earth's magnetic field. The observations by NASA's Magnetospheric Multiscale (MMS) mission show a specific event where energized protons are measured, along with wave activity that is consistent with theoretical expectations for a certain type of interaction between the waves and particles that allows the waves to transfer energy to the protons. Numerical simulations are also performed and the results agree with what was observed in real life by the spacecraft. The results are important because they provide insight into potential processes that can create high‐energy particles both near the Earth and at other astrophysical systems. Key Points: At quasi‐parallel shocks, ions are rapidly accelerated from 10 eV to 100 keV by the ExB mechanism of electrostatic wavesThe waves are produced by current‐driven instabilities with frequencies between the proton and the electron cyclotron frequenciesThe acceleration occurs locally within a few gyroperiods [ABSTRACT FROM AUTHOR]

We study the properties of the complex network for solar proton flux (SPF) at six integral energy channels (>1, >5, >10, >30, >50, and >100 MeV). We construct a growing visibility network based on the SPF time series for proton for each channel, which observed by Geostationary Operational Environmental Satellites from April 14, 2010 to May 9, 2018. The clustering coefficient values for SPF networks are much larger than those of the equivalent random network. The logarithmic behavior of the characteristic path length refers to the characteristics of the small‐world network. The degree distribution of nodes obeys the power‐law behavior with certain exponents that reflects the scale‐free nature of the networks. For the constructed SPF networks, the average clustering coefficient shows a jump when the large flares occurred. We show that both western solar longitude large flares (X‐class, M‐class) and flares associated with coronal mass ejections (with speed greater than 500 km s−1) are relatively more related to the peaks (more than the median) of both SPF time series and PageRank of the networks. We conclude that both the PageRank and clustering coefficient of the SPF network are useful indicators to identify the source of SPFs related to flares and flares associated with coronal mass ejections. Key Points: The network approach confirms that flares are one of the most important sources of the solar proton fluxProton flux forms a scale‐free network with small‐world behavior, which shows large peaks are linked with several small neighboring peaksDue to filtering out noise and interferences by mapping a proton flux to a network, the network measures show high correlations with flares [ABSTRACT FROM AUTHOR]

Fast earthward plasma flows are commonly observed in the magnetotail plasma sheet. These flows are often termed as bursty bulk flows because of their bursty nature, and they are considered to be generated by magnetic reconnection. Close to the neutral sheet (Bx ∼ 0), the fast flows are considered to be associated with an enhanced dawn‐to‐dusk electric field (Ey > 0), which together with the northward magnetic field component (Bz > 0) protrude the plasma earthward via enhanced E × B‐drift. Sometimes, reversals in the dawn‐dusk velocity component perpendicular to the magnetic field (V⊥y) are measured in association with Bx sign changes in the flows. This suggests that the electric field component in the north‐south direction (Ez) can play a role in determining the dawn‐dusk direction of the enhanced drift. We present data measured by the Magnetospheric Multiscale, which demonstrate that Ez can have a dictating role for V⊥y of fast flows. Furthermore, it is shown that the critical contribution of Ez is not limited only to V⊥y, but it can also dominantly determine the enhanced drift of the fast flows in the X direction (V⊥x). The latter can occur also near and at the neutral sheet, which adds an alternative configuration to the conventional picture of Ey and Bz being the main players in driving the earthward fast flows. The domination of Ez in the studied events appears with potential signatures of an influence of a nonzero dawn‐dusk component of the interplanetary magnetic field (IMF By) on the magnetotail. Key Points: Magnetospheric Multiscale data for earthward fast flow events exhibiting reversals in V⊥y are investigatedEz can have a dictating role in determining V⊥y in fast flowsThe critical contribution of Ez is not only limited to V⊥y, but also earthward V⊥x can dominantly be determined by Ez [ABSTRACT FROM AUTHOR]

An energy spectrum of electrons from 180 to 550 keV precipitating into the dayside polar ionosphere was observed under a geomagnetically quiet condition (AE ≤ 100 nT, Kp = 1‐). The observation was carried out at 73–184 km altitudes by the HEP instrument onboard the RockSat‐XN sounding rocket that has been launched from Andøya, Norway. The observed energy spectrum of precipitating electrons follows a power law of −4.9 ± 0.4 and the electron flux does not vary much over the observation period (∼274.4 s). A nearby ground‐based VLF receiver observation at Lovozero, Russia shows the presence of whistler‐mode wave activities during the rocket observation. A few minutes before the RockSat‐XN observation, POES18/MEPED observed precipitating electrons, which also suggest whistler‐mode chorus wave activities at the location close to the rocket trajectory. A test‐particle simulation for wave‐particle interactions was carried out using the data of the Arase satellite as the initial condition which was located on the duskside. The result of the simulation shows that whistler‐mode waves can resonate with sub‐relativistic electrons at high latitudes. These results suggest that the precipitation observed by RockSat‐XN is likely to be caused by the wave‐particle interactions between whistler‐mode waves and sub‐relativistic electrons. Plain Language Summary: Sub‐relativistic electrons precipitating into the Earth's dayside polar ionosphere are observed by a sounding rocket under geomagnetically quiet conditions. An energy spectrum of these electrons in an energy range from 180 to 550 keV is reported at the rocket altitude. A possible mechanism for generating this precipitation is the resonance scattering of electrons by whistler‐mode waves, which we conducted a test‐particle simulation based on the ground and satellite observations. Key Points: A sounding rocket observed an energy spectrum of sub‐relativistic electron precipitation in the dayside polar ionosphere during quiet timeGround and satellite observations suggest that the precipitation observed by RockSat‐XN was caused by the whistler‐mode wavesA test‐particle simulation for wave‐particle interactions based on the data of the Arase satellite supports the RockSat‐XN observation [ABSTRACT FROM AUTHOR]

Enhanced ion line spectra as a response to magnetic field‐aligned high frequency (HF) pumping of the overdense polar ionosphere with left‐handed circular polarization, can be observed at the top and bottomside F‐region ionosphere under certain conditions. The European Incoherent Scatter (EISCAT) UHF radar was directed in magnetic zenith on October 18th and 19th, 2017 while stepping the pump frequency of the EISCAT Heating facility both upward and downward across the double resonance frequency of the fourth harmonic of the electron gyrofrequency and the local upper hybrid frequency, in a 2‐min‐on, 5‐min‐30‐s‐off pump cycle. We present observations of two separate cases of topside HF‐enhanced ion lines (THFIL). THFIL simultaneous to bottomside HFIL (BHFIL), and conditioned by the relative proximity of the HF pump frequency to the bottomside double resonance frequency, consistent with previous observations (Rexer et al., 2018, https://doi.org/10.1029/2018JA025822) were observed for HF pulses on 19th October. Recurring THFIL appearing after BHFIL have faded and conditioned by the relative proximity to the double resonance frequency on the topside ionosphere, were observed on 18th October. The THFIL are consistent with transionospheric propagation through artificial radio windows by L mode guiding of the pump wave, facilitated by large scale density ducts present in the plasma. Key Points: Observations of high frequency enhanced ion line spectra at the topside ionosphereThe appearance of the topside enhancements are affected by the proximity to the topside double resonance frequencyWe interpret the observations as a result of L mode wave propagation through artificial radio windows [ABSTRACT FROM AUTHOR]

Terrestrial Very‐Low‐Frequency (VLF) energy from both lightning discharges and radio transmitters has a role in affecting the energetic electrons in the Van Allen radiation belts, but quantification of these effects is particularly difficult, largely due to the collisional damping experienced in the highly variable electron density in the D‐ and E‐region ionosphere. The Faraday International Reference Ionosphere (FIRI) model was specifically developed by combining lower‐ionosphere chemistry modeling with in situ rocket measurements, and represents to date the most reliable source of electron density profiles for the lower ionosphere. As a full‐resolution empirical model, FIRI is not well suited to D‐ and E‐region ionosphere inversion, and its applicability in transionospheric VLF simulation and in remote sensing of the lower ionosphere is limited. Motivated by how subionospheric VLF remote sensing has been aided by the Wait and Spies (WS) profile (Wait & Spies, 1964), in this study, we parameterize the FIRI profiles and extend the WS profile to the E‐region ionosphere by introducing two new parameters: the knee altitude hk and the sharpness parameter for the E‐region ionosphere βE. Using this modified WS profile, we calculate the expected signals at different receiver locations from the NAA, NPM, and NWC transmitters under the full range of possible ionospheric conditions. We also describe and validate a method about how these results can be readily used to translate VLF measurements into estimates of the lower ionosphere electron density. Moreover, we use this method to evaluate the sensitivity of different ground receiver locations in lower‐ionosphere remote sensing. Key Points: We describe an electron density model of the D‐ and E‐region ionosphere for transionospheric VLF propagationWe parameterize the Faraday International Reference Ionosphere model and extend the Wait and Spies profile to E‐region altitudesWe calculate the expected transmitter signal under the full range of possible ionosphere conditions for lower ionosphere remote sensing [ABSTRACT FROM AUTHOR]

We present the results of a multi‐point and multi‐instrument study of electromagnetic ion cyclotron (EMIC) waves and related energetic proton precipitation during a substorm. We analyze the data from Arase (ERG) and Van Allen Probes (VAPs) A and B spacecraft for an event of 16 and 17 UT on December 1, 2018. VAP‐A detected an almost dispersionless injection of energetic protons related to the substorm onset in the night sector. Then the proton injection was detected by VAP‐B and further by Arase, as a dispersive enhancement of energetic proton flux. The proton flux enhancement at every spacecraft coincided with the EMIC wave enhancement or appearance. This data show the excitation of EMIC waves first inside an expanding substorm wedge and then by a drifting cloud of injected protons. Low‐orbiting NOAA/POES and MetOp satellites observed precipitation of energetic protons nearly conjugate with the EMIC wave observations in the magnetosphere. The proton pitch‐angle diffusion coefficient and the strong diffusion regime index were calculated based on the observed wave, plasma, and magnetic field parameters. The diffusion coefficient reaches a maximum at energies corresponding well to the energy range of the observed proton precipitation. The diffusion coefficient values indicated the strong diffusion regime, in agreement with the equality of the trapped and precipitating proton flux at the low‐Earth orbit. The growth rate calculations based on the plasma and magnetic field data from both VAP and Arase spacecraft indicated that the detected EMIC waves could be generated in the region of their observation or in its close vicinity. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are believed to play a significant role in the dynamics of energetic protons and relativistic electrons in the Earth's magnetosphere. The properties of these waves are being intensively studied. We consider the conditions of the EMIC wave generation and the dynamics of the wave source during a substorm event using a unique configuration of three spacecraft (Arase and two Van Allen Probes). All spacecraft were at approximately the same distance from the Earth, forming a chain across the evening local time sector. Analyzing parameters of the wave generation obtained from in situ measured proton distribution function, we came to the conclusion that the waves could be generated within the substorm area, sometimes close to, but not necessary at the spacecraft location. As the substorm expands in longitude, the EMIC wave source exhibits a longitudinal drift. When substorm expansion stops, the wave generation region expands due to the magnetic drift of protons injected during the substorm. The observed wave properties show that the waves are able to precipitate energetic protons into the atmosphere. This is confirmed by observations of low orbiting satellites measuring proton precipitating fluxes. Key Points: Westward propagation of the EMIC wave generation region is due to both the substorm expansion and azimuthal drift of injected protonsStrong pitch‐angle diffusion regime is confirmed by observations of proton fluxes at low altitude and the diffusion coefficient calculationThe diffusion coefficient maximum corresponds well to the energy range of the observed proton precipitation [ABSTRACT FROM AUTHOR]

The process of magnetic reconnection in a flux tube can occur in a time‐stationary fashion as slippage reconnection or in a time‐dependent manner based on, for example, the tearing instability. However, it is not well known under which conditions a system can sustain slippage reconnection. Likewise, it is unclear whether systems can exhibit slippage reconnection and time‐dependent reconnection simultaneously. In order to investigate these questions, we employ a set of 3D magnetohydrodynamic simulation. Using these simulations, we model a twisted flux tube with a spatially localized resistive region in the center of the simulation box, and an applied driver on one end plane of the simulation box. As a result, the Poynting flux injected at the boundary propagates to the resistive region and dissipates there. If the driver is nearly circular, we find a predominance of slippage reconnection, which decouples the applied driver entirely from one half of the flux tube. Second, a tearing‐like instability occurs with slippage reconnection in the resistive region when the resistivity is strong enough and the cross section of the flux tube, as defined by the shape of the applied velocity perturbation, is elliptical enough. In this case we find the surprising feature that magnetic field perturbation generated by the tearing‐like instability radiates away from the resistive region to influence the entire flux tube shape. Key Points: Magnetic reconnection in a twisted magnetic flux tube can be time‐stationary, leading to slippage reconnectionIn case the flux tube profile is elliptical enough, tearing‐like reconnection can occur simultaneously with slippageThese results pertain to a variety of plasma systems involving dissipation in twisted flux tubes [ABSTRACT FROM AUTHOR]

We present results and analysis of finite‐difference time‐domain (FDTD) simulations of electromagnetic waves scattering off meteor head plasma using an analytical model and a simulation‐derived model of the head plasma distribution. The analytical model was developed by (Dimant & Oppenheim, 2017b, https://doi.org/10.1002/2017JA023963) and the simulation‐derived model is based on particle‐in‐cell (PIC) simulations presented in (Sugar et al., 2019, https://doi.org/10.1029/2018JA026434). Both of these head plasma distribution models show the meteor head plasma is significantly different than the spherically symmetric distributions used in previous studies of meteor head plasma. We use the FDTD simulation results to fit a power law model that relates the meteoroid ablation rate to the head echo radar cross section (RCS), and show that the RCS of plasma distributions derived from the Dimant‐Oppenheim analytical model and the PIC simulations agree to within 4 dBsm. The power law model yields more accurate meteoroid mass estimates than previous methods based on spherically symmetric plasma distributions. Key Points: We use finite‐difference time‐domain simulations to simulate the radar cross section of different meteor head plasma models derived from an analytical model and particle‐in‐cell simulationsThe radar cross sections of the analytical model and the simulation‐derived plasma distributions agree to within 4 dBsmThe magnetic field orientation with respect to the meteor path has a small effect on the head echo's radar cross section [ABSTRACT FROM AUTHOR]

Regions 1 and 2 (R1 and R2) field-aligned currents (FACs), manifestations of large-scale convection in Earth's magnetosphere-ionosphere (M-I) system, often contain intense FAC layers of mesoscale latitudinal width near the R1/R2 interface. We refer to such layers as "embedded" R1 and R2 FACs. Likely resulting from enhanced magnetosphere-ionosphere (M-I) convection, these FACs may indicate M-I configuration change and contribute significantly to substorm current wedges. We present several events in which embedded FACs were observed by low-altitude spacecraft in the ionosphere. All the events occurred during active geomagnetic conditions or a substorm growth phase, and most map to an equatorial location on the nightside. When an embedded FAC is upward, it coincides with inverted-V electron precipitation and a discrete auroral arc. If an upward embedded FAC is in the postmidnight-to-dawn sector, a dawnside auroral polarization stream appears immediately poleward of it, so it may be important for ionospheric heating, M-I convection, and instabilities. Our results establish embedded FACs as a frequently appearing, fundamental phenomenon for understanding and modeling the magnetosphere-ionosphere system. [ABSTRACT FROM AUTHOR]

A novel algorithm for estimating both the linear and quadratic gradients of physical quantities based on multiple spacecraft observations using the least squares method is put forward. Using 10 or more spacecraft constellation measurements as input, this new algorithm can yield both the linear and quadratic gradients at the barycenter of the constellation. Iterations were used in the algorithm. Tests on cylindrical flux ropes, dipole magnetic field, and modeled geomagnetospheric field were carried out. The results of these tests indicate that the linear gradient obtained is of second-order accuracy, while the quadratic gradient is of first-order accuracy. The test on the modeled geomagnetospheric field showed that, the greater the number of spacecraft in the constellation, the greater the accuracy of the quadratic gradient calculated. However, the accuracy of the linear gradient obtained was independent of the number of spacecraft. The feasibility, reliability, and accuracy of this algorithm have been successfully verified. This algorithm could find wide applications in the design of future multiple spacecraft missions as well as in the analysis of multiple-point measurement data. [ABSTRACT FROM AUTHOR]

Neutral temperature responses in the mesosphere and lower thermosphere (MLT) to severe geomagnetic storms induced by coronal mass ejections (CMEs) are of growing interest to the space science research community. Recently, it was found that geomagnetic activities produced by the corotating interaction regions (CIRs) caused comparable effects on the Earth's upper atmosphere. In this work, we carried out a comparative study of the temperature responses in the MLT region to these two types of geomagnetic activities, using the temperature measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments onboard the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite. Our results demonstrate that CIR-induced geomagnetic activity produced temperature variations in the MLT region and that this effect can penetrate downward to ~100 km at high latitudes in both hemispheres. Temperature enhancements penetrated deeper during CME-induced geomagnetic activities, but the heating effects lasted longer during CIRinduced geomagnetic activities. There is a hemispherical asymmetry in the geomagnetical activity induced temperature changes in the MLT region. The temperature enhancements are stronger in the southern hemisphere than in the northern hemisphere during CME events. [ABSTRACT FROM AUTHOR]

The Wharton Basin earthquake sequence on April 11, 2012, offshore Sumatra, represents the two largest (Mw > 8.0) strike-slip earthquakes ever recorded. Ground fault displacements generated a spectrum of acoustic-gravity waves due to solid Earth-atmosphere coupling. Wave-like perturbations in Total Electron Content (TEC) were therefore observed in ground-based Global Positioning System data. The waves arrive about 10 min after each earthquake and their spectral analysis reveals the presence of acoustic resonance frequencies of 3.8 and 4.4 mHz. The acoustic wave speeds of 0.9-1.2 km/s suggest coseismic ground movement as the primary wave generating mechanism instead of seismic Rayleigh waves. Gravity waves with frequencies below 2 mHz traveling with lower speeds of 0.21 km/s are also detected. Ray tracing using a simple numerical model traced the source of observed ionospheric perturbations to within 150 km distance of the epicenters. Large amplitude ionospheric disturbances were found to travel mostly in a north-south direction, an observation explained by the orientation of Earth's geomagnetic field. [ABSTRACT FROM AUTHOR]

Equatorial magnetosonic waves (EMS), together with chorus and plasmaspheric hiss, play key roles in the dynamics of energetic electron fluxes in the magnetosphere. Numerical models, developed following a first principles approach, that are used to study the evolution of high energy electron fluxes are mainly based on quasilinear diffusion. The application of such numerical codes requires statistical models for the distribution of key magnetospheric wave modes to estimate the appropriate diffusion coefficients. These waves are generally statistically modeled as a function of spatial location and geomagnetic indices (e.g., AE, Kp, or Dst). This study presents a novel dynamic spatiotemporal model for EMS wave amplitude, developed using the Nonlinear AutoRegressive Moving Average eXogenous machine learning approach. The EMS wave amplitude, measured by the Van Allen Probes, are modeled using the time lags of the solar wind and geomagnetic indices as inputs as well as the location at which the measurement is made. The resulting model performance is assessed on a separate Van Allen Probes data set, where the prediction efficiency was found to be 34.0% and the correlation coefficient was 56.9%. With more training and validation data the performance metrics could potentially be improved, however, it is also possible that the EMS wave distribution is affected by stochastic factors and the performance metrics obtained for this model are close to the potential maximum. [ABSTRACT FROM AUTHOR]

We analyze the geomagnetic data recorded at 78 stations from 13 to 31 March 2015. Using the empirical mode decomposition (EMD) method, we focus our attention on geomagnetic signal due to sources which are external to the Earth, that is, due to current systems flowing in the ionosphere and magnetosphere. We analyze the short timescale fluctuations (τ < 200 min) of this magnetic signal, their dependence on magnetic latitude, magnetic local time, and geomagnetic activity. At high geomagnetic latitudes (>|60°|), these short timescale fluctuations constitute more than 30% of the external magnetic field. Their maximum contribution occurs along the auroral oval suggesting that they are mainly triggered by the ionospheric electric current systems active in these regions. A discussion of the relevance of these short timescale magnetic fluctuations to result in a more significant modeling and prediction of geomagnetically induced currents in the auroral zones is also provided. Key Points: The multiscale nature of geomagnetic field fluctuations recorded on the ground is analyzedShort‐timescale magnetic fluctuations constitute a significant contribution to the magnetic field of external origin at high latitudesThe main source of short timescale magnetic fluctuations seems to be the occurrence of magnetospheric substorms [ABSTRACT FROM AUTHOR]

A study of 37 large solar storms occurring in the equinox periods from 2000 to 2018 is presented to quantify their effects on total electron content (TEC) variations in the ionosphere over the North American sector. The dependence of storm‐related TEC enhancements and depletions is studied as a function of various storm parameters using machine‐learning and statistical algorithms to quantify their relative importance. The variables under consideration include the onset time and duration of each storm, as well as characteristic scalar values derived from ring current variations detected by a network of ground‐based magnetometers. The TEC data used in the study are taken from the Continuously Operating Reference Station network of ground‐based GPS measurements, archived in the Madrigal database. The agreement and disagreement of the results across the different methods studied, along with the relative uncertainties in model prediction capability of the machine‐learning study, show that the geo‐effectiveness of large storms in the equinox periods is more closely related to the storm onset time and to ionospheric preconditioning than to any other parameters used in characterizing the storm. The effects in the higher latitude regions are shown to be very different from the lower latitude regions over the North American sector. This behavior can possibly be explained through storm‐induced changes in the atmospheric chemistry and the related temperature‐dependent production and loss processes at the higher latitudes. Key Points: Large geomagnetic storms reveal which characteristics of the measured Symmetric H‐component response are closely related to total electron content (TEC) perturbations over the USMachine learning and other statistical methods are independently applied to regions with distinct geomagnetic geometriesThe onset time of the storms, especially with respect to local sunrise, is the parameter most closely linked to the TEC response [ABSTRACT FROM AUTHOR]

Geomagnetic substorms are major energy transfer events where energy stored in the Earth's magnetotail is released into the ionosphere. Substorm phenomena, including auroral activities, earthward Poynting flux, magnetic field dipolarization, etc, have been extensively studied. However, the complex interplay among them is not fully understood. In a fortuitous event on June 7, 2013, the twin Van Allen Probes (separated by 0.4 h in local time) observed bursts of earthward Alfvenic Poynting flux in the vicinity of the plasma sheet boundary layer (PSBL). The Poynting flux bursts correlate with enhancements of auroral brightness around the footpoints of both spacecraft. This indicates a temporal and spatial correlation between the auroral brightening and Poynting flux bursts, and that the auroral motion is directly linked to the perpendicular expansion of the Alfven wave. These observations suggest that the Alfvenic Poynting flux is a primary driver for the auroral electron acceleration. Around the time of auroral brightening, a dipolarization was seen to propagate more than 4 h in local time during a 20 min period. The azimuthal phase speed of this dipolarization (2 deg/min) is too small to explain the azimuthal motion of the aurora (13.6 deg/min), but the dipolarization could be related to the generation of the Alfvenic Poynting flux through phase mixing at strong density gradients like those in the PSBL. Key Points: We report the one‐to‐one correlation between a discrete arc and enhanced Poynting fluxAlfvenic Poynting flux directly powers auroral accelerationDipolarization may be related to Alfven wave generation [ABSTRACT FROM AUTHOR]

As an application of the completely general, ideal two‐fluid analysis of waves in a warm ion‐electron plasma, where six unique wave pair labels (S, A, F, M, O, and X) were identified, we here connect to the vast body of literature on whistler waves. We show that all six mode pairs can demonstrate whistling behavior, when we allow for whistling of both descending and ascending frequency types, and when we study the more general case of oblique propagation to the background magnetic field. We show how the general theory recovers all known approximate group speed expressions for both classical whistlers and ion cyclotron whistlers, which we here extend to include ion contributions and deviations from parallel propagation. At oblique angles and at perpendicular propagation, whistlers are investigated using exact numerical evaluations of the two‐fluid dispersion relation and their group speeds under Earth's magnetosphere conditions. This approach allows for a complete overview of all whistling behavior and we quantify the typical frequency ranges where they must be observable. We use the generality of the theory to show that pair plasmas in pulsar magnetospheres also feature whistling behavior, although not of the classical type at parallel propagation. Whistling of the high‐frequency modes is discussed as well, and we give the extension of the Appleton‐Hartree relation for cold plasmas, to include the effect of a nonzero thermal electron velocity. We use it to quantify the Faraday rotation effect at all angles, and compare its predictions between the cold and warm Appleton‐Hartree equation. Key Points: Group speeds can be computed numerically, highlighting whistling behaviorWhistler group speed approximations are extended to nonparallel propagation with respect to the background magnetic fieldThe widely used Appleton‐Hartree equation is extended to include a nonzero thermal electron velocity [ABSTRACT FROM AUTHOR]

For the investigation of time variations of critical frequency (foF2), solar radiation flux at 10.7 cm wavelength (F10.7 index) and geomagnetic activity index Kp, we use conditional independence graphical models which describe and transparently represent the structure of relationships in the time series. We employ multivariate statistic methods applied to daily observational data obtained from midlatitude ionosondes within the period 1994–2009 (23rd Solar Cycle). It is demonstrated that conditional independence graphical models represent a robust method of multivariate statistical analysis, useful for finding a relation between one of the main ionospheric parameters and space weather conditions. This method appears to be more appropriate than correlation analysis between foF2 and the main geomagnetic and solar indices, especially for long‐term data for which the model characteristics may change or time series can be interrupted. We compare the results obtained by the graphical models method with cross‐correlation analysis. We found that the method is suitable for the analysis of the dependence between foF2 values and time‐shifted F10.7 time series. In particular, we clearly identified +0 day shift in all cases, and +1‐, +2–3‐, and +4–5‐day shifts in European, American, and East Asia sector, respectively. The conditional independence graphs method can be applied even in the case when classical parametric methods are not convenient. Key Points: Conditional independence graphical models are useful for finding a relations between the ionosphere and space weather conditionsWe clearly identified dependence between foF2 and time‐shifted F10.7 time series different in European, American, and East Asia sectorRegional character of the atmosphere at tropospheric heights may be crucial to explain the three different dependencies of foF2 and F10.7 [ABSTRACT FROM AUTHOR]

Coronal Mass Ejections (CMEs) are key drivers of space weather activity but most predictions have been limited to the expected arrival time of a CME, rather than the internal properties that affect the severity of an impact. Many properties, such as the magnetic field density and mass density, follow conservation laws and vary systematically with changes in the size of a CME. We present ANTEATR‐PARADE, the newest version of the ANTEATR arrival time model, which now includes physics‐driven changes in the size and shape of both the CME's central axis and its cross section. Internal magnetic and thermal and external drag forces affect the acceleration of the CME in different directions, inducing asymmetries between the radial and perpendicular directions. These improvements should lead to more realistic CME velocities, both bulk and expansion, sizes and shapes, and internal properties. We present the model details, an initial illustration of the general behavior, and a study of the relative importance of the different forces. The model shows a pancaking of both the cross section and central axis of the CME so that their radial extent becomes smaller than their extent in the perpendicular direction. We find that the initial velocities, drag, any form of cross section expansion, and the precise form of thermal expansion have strong effects. The results are less sensitive to axial forces and the specific form of the cross section expansion. Plain Language Summary: Coronal Mass Ejections (CMEs) are large explosions of matter and magnetic field that violently erupt from the Sun. When they hit the Earth they cause negative effects in human technology so it is important to forecast them. Most models of CMEs only predict if and when a CME could impact the Earth but not the properties that affect the severity of the impact. Many of these properties scale with the CME size and shape so we need to understand how the CME expands between the Sun and the Earth. We have taken a model for the arrival time and added new forces to better understand the evolution of a CME's size and shape. This should help us better predict factors like magnetic field strength, number density, and velocity. Our model reproduces a previously known trend of CMEs becoming squished in the radial direction or "pancaking." We explore which forces are the most important for causing this effect. We find that drag forces and the expansion of the CME cross section are more important than axial forces. We also see that the model results depend on how we initially convert the total CME speed into initial propagation and expansion speeds. Key Points: ANTEATR‐PARADE adds CME expansion and deformation to the interplanetary drag modelDrag and any form of cross sectional pressure gradient are the most important forces for deformationThe initial expansion velocity has a very strong effect on the total amount of deformation [ABSTRACT FROM AUTHOR]

Above lunar crustal magnetic anomalies, large fractions of solar wind electrons and ions can be scattered and stream back toward the solar wind flow, leading to a number of interesting effects such as electrostatic instabilities and waves. These electrostatic structures can also interact with the background plasma, resulting in electron heating and scattering. We study the electrostatic waves and electron heating observed over the lunar magnetic anomalies by analyzing data from the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft. Based on the analysis of two lunar flybys in 2011 and 2013, we find that the electron twostream instability (ETSI) and electron cyclotron drift instability (ECDI) may play an important role in driving the electrostatic waves. We also find that ECDI, along with the modified two-stream instability (MTSI), may provide the mechanisms responsible for substantial isotropic electron heating over the lunar magnetic anomalies. [ABSTRACT FROM AUTHOR]

The occurrence of tailward flows in the magnetotail plasma sheet is closely linked to the dynamics of earthward bursty bulk flows (BBFs). Tailward flows that are observed in the vicinity of these BBFs (or TWABs -- Tailward flows around BBFs) may hold unique information on its origin. In this study, we conduct a statistical survey on TWABs by using data from the Cluster mission. We find that TWABs are observed in the vicinity of ~75% of the BBFs and their occurrence does not depend on BBF velocity magnitude. TWABs have a flow convection pattern consistent with the general tailward flows (GTWs) in the plasma sheet and they do not resemble vortical-like flows. However, TWABs have a flow velocity magnitude twice larger than the GTWs. The plasma density and temperature of TWABs are comparable with BBFs. It is more common to observe a TWAB succeeding than preceding a BBF. However, there is no distinctive difference (in flow pattern, plasma density and temperature) between preceding and succeeding TWABs. We suggest that TWABs are likely the "freshly" rebounded BBFs from the near-Earth region where the magnetic field is stronger. TWABs may represent the early stage of the evolution of tailward flows in the plasma sheet. We also discuss and argue that other mechanisms such as shear-induced vortical flows and tailward slipping of depleted flux tubes cannot be the principal causes of TWABs. [ABSTRACT FROM AUTHOR]

The nitric oxide (NO) 5.3 μm radiative emission is the dominating and most efficient cooling agent in the thermosphere above 100 km. The NO 5.3 μm radiative cooling is an important parameter, particularly during geomagnetic storm events, to quantify the energy budget within the magnetosphere-ionosphere-thermosphere system. We utilize the TIMED (Thermosphere-Ionosphere- Mesosphere Energetics and Dynamics)/SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) observations of NO radiative cooling to investigate the storm-time hemispheric asymmetry during 2002-2018. The auxiliary parameters such as NO density, atomic oxygen density, thermospheric temperature, and the meridional wind are obtained from the TIEGCM (Thermosphere-Ionosphere-Electrodynamics General Circulation Model) simulations. We analyze the events that occurred close to (or during) the equinox periods to get rid of the effect of seasonal asymmetry. The TIEGCM-simulated NO density and meridional wind show hemispheric asymmetry during geomagnetic storm event. Almost no hemispheric variation in TIEGCM-atomic oxygen and -thermospheric temperature is noticeable. The SABER-observed and the TIEGCM-simulated radiative flux exhibits higher values in the northern and southern hemispheres, respectively, during the events close to (or during) March and September equinox. Our analysis shows that the storm-time meridional wind could play an important role resulting in the hemispheric asymmetry by changing the density and temperature. [ABSTRACT FROM AUTHOR]

Hemispheric asymmetries of the Vertical Total Electron Content (VTEC) were observed during the first recovery phase of the geomagnetic storm on September 7-8, 2017. These asymmetries occurred at the mid latitudes at two different local times simultaneously: In the European-African sector (early morning), the storm time VTEC in the southern/northern hemisphere was higher/lower than the quiet time value, suggesting the southern/northern hemisphere entered the positive/negative phase (N-S+). In the East Asian-Australian sector (afternoon), the storm time VTEC change was positive in the northern hemisphere, but negative in the southern hemisphere (N+S-). The electron density profiles from digisondes demonstrated that the asymmetries appeared in the F region density as well. The plasma drifts data from digisondes, the column-integrated [O]/[N2] ratio from GUVI onboard the TIMED satellite, and the detrended VTEC were utilized to study the drivers of the asymmetries. Traveling Ionospheric Disturbance (TID) signatures were identified in the digisonde drift and detrended VTEC data before the appearance of the asymmetry. The magnitude of TIDs was larger in the hemisphere where the negative phase occurred later. The storm time [O]/[N2] ratio change was positive in Africa (S+) and negative in Europe (N-). However, the [O]/[N2] measurements were not available in the East Asian-Australian sector during the focused period. The hemispheric differences in the vertical drifts were also observed in both sectors. Therefore, the observed hemispheric asymmetries in both sectors are suggested to be due to the hemispheric asymmetries in the thermospheric composition change, vertical drift, and TID activity. [ABSTRACT FROM AUTHOR]

We observationally address substorm-related ground magnetic disturbances in terms of the substorm current wedge (SCW) system. The study consists of three data analyses, which approach different aspects of this subject. First, by comparing nightside magnetic disturbances below and above the ionosphere, we confirm that substorm-related midlatitude magnetic disturbances around the edges of the SCW can be attributed to a remote current system (i.e., SCW) rather than to a local ionospheric current. Second, we statistically examine the magnetic local time (MLT) distributions of the correlation between midlatitude magnetic variations and the nightside westward auroral electrojet (AEJ). We confirm that nightside magnetic disturbances are consistent with the SCW, and find that correlated magnetic variations extend to the midday sector. Finally, we introduce midlatitude and high-latitude geomagnetic indices for each MLT quadrant, and find that their variations are often correlated with the nightside AEJ intensity. From the magnitude of the correlated variations it is inferred that midday magnetic disturbances at both midlatitudes and high latitudes are remote effects of the SCW. In the dawn-sector and dusk-sector auroral zone, in contrast, correlated magnetic disturbances can be attributed mostly to the global enhancement of AEJs, which suggests that the global region-1 system also changes in correlation with the nightside westward AEJ during substorms. These results consistently indicate that ground magnetic disturbances are correlated globally with the substorm westward AEJ. In contrast, the geomagnetic effect of midlatitude ionospheric currents must be relatively small not only on the night side but also on the day side. [ABSTRACT FROM AUTHOR]

The dynamical complexities of geospace has been examined by investigating the characteristic chaotic properties of hourly Dst values obtained from the World Data Center for Geomagnetism, Kyoto, over four solar cycles (1964-2008) using nonlinear methods: sample entropy, Lyapunov exponents, correlation dimension, recurrence quantification analysis, and multifractal detrended fluctuation. Throughout the solar cycles considered, the space environment was observed to be chaotic based on Lyapunov exponents obtained. The order of increasing complexity in the solar cycles based on recurrence rate (RR) values is Cycle 21 < 22 < 23 < 20. Similar patterns and trend were observed in the correlation dimension values for the different solar cycles. The peak of each solar cycle is associated with high chaoticity. In all the analysis considered, high values of sample entropy, Lyapunov exponent, correlation dimension, correspond to increasing chaoticity. However, lower value of RR signifies more chaotic activity. The solar cycle 20 with the greatest values of mean sunspot number (651.62) and standard deviation of sunspot number (1,656.90) has the greatest chaoticity. The chaoticity in the magnetosphere increases with increasing geomagnetic activity as captured by Dst index. Similar trends were not observed in BZ and solar wind dynamic pressure which showed that the observed chaoticity is due to internal variations. The multifractal spectrum of Dst at all solar cycles considered revealed that the chaotic activities in the magnetosphere are not influenced by the local fluctuations in the region. The degree of multifractality were found to be in the order Cycle 20 > Cycle 23 > Cycle 21 > Cycle 22. [ABSTRACT FROM AUTHOR]

t In recent years, there has been a growing body of direct experimental evidence demonstrating electromagnetic ion cyclotron (EMIC) waves driving energetic electron precipitation (EEP) at unexpectedly low, sub-MeV energies—as low as only a few hundred keV. EMIC-wave driven scattering at these energies has important ramifications for our understanding of not only radiation belt electron dynamics, but also the importance of EMIC-driven EEP to the chemical balance of the Earth's atmosphere. In this study, we use three experimentally derived EMIC-driven EEP flux spectra to investigate the impact of this precipitation on trapped radiation belt fluxes. In doing so, we resolve an apparent contradiction with earlier results derived from trapped electron flux populations that suggested EMIC waves only caused significant scattering at ultrarelativistic energies. We show that strong sub-MeV EEP measurements are not necessarily mutually exclusive with a strongly relativistic-only trapped flux response, as the sub-MEV peak precipitation is comparatively much smaller than the trapped population at those energies. Using a further six EEP spectra, we also demonstrate that EMIC-driven EEP can generate significant ionization of the Earth's atmosphere above 40 km, leading to the loss of mesospheric ozone. We find poor correlation between EMIC-driven EEP fluxes and geomagnetic activity proxies, such that EMIC-driven EEP is likely to be poorly specified in the forcing factors of modern coupled-climate models. [ABSTRACT FROM AUTHOR]

This study examined the variability of the night-time equatorial thermospheric meridional and zonal wind speeds using an optical Fabry-Perot interferometer (FPI) located in Abuja, Nigeria (Geographic: 8.99°N, 7.39°E; Geomagnetic latitude: −1.60). The study period covered 9 months with useable data of 139 nights between March 2016 and January 2018. The hourly zonal wind speed is between −124 and 163 ms−1 and that of the meridional wind ranged between −70 and 95 ms−1. Comparison of FPI ground-based measurements with estimates from the Horizontal Wind Model (HWM-14) accurately reproduced the post-midnight meridional component, but for some departure of ∼50 ms−1 during pre-midnight. A very good agreement is observed between the predicted and measured zonal winds speed in the month of January 2018. However, the HWM-14 more often underestimated the zonal wind speeds in the months of 2016. Hence, this necessitates a call for improvement of the HWM-14 by using newly observed data in order to better characterize the West African sector. The varying zonal winds showed modal periods of 25.9 and 133.5 days, which are quasi 27 days and quasiterannual periodic variations, respectively. On the meridional wind, oscillatory periods of 133.5 and 23.1 days are seen in year 2016 and 2017, respectively. [ABSTRACT FROM AUTHOR]

In situ electron density profiles obtained from Arase in the night magnetic local time (MLT) sector and from RBSP-B covering all MLTs are used to study the small-scale density irregularities present in the plasmasphere and near the plasmapause. Electron density perturbations with amplitudes >10% from background density and with time-scales less than 30-min are investigated here as the small-scale density irregularities. The statistical survey of the density irregularities is carried out using nearly 2 years of density data obtained from RBSP-B and 4 months of data from Arase satellites. The results show that density irregularities are present globally at all MLT sectors and L-shells both inside and outside the plasmapause, with a higher occurrence at L > 4. The occurrence of density irregularities is found to be higher during disturbed geomagnetic and interplanetary conditions. The case studies presented here revealed: (1) The plasmaspheric density irregularities observed during both quiet and disturbed conditions are found to coexist with the hot plasma sheet population. (2) During quiet periods, the plasma waves in the whistler-mode frequency range are found to be modulated by the small-scale density irregularities, with density depletions coinciding well with the decrease in whistler intensity. Our observations suggest that different source mechanisms are responsible for the generation of density structures at different MLTs and geomagnetic conditions. [ABSTRACT FROM AUTHOR]

Under nominal solar wind conditions, a tenuous wake forms downstream of the lunar nightside. However, the lunar plasma environment undergoes a transformation as the Moon passes through the Earth's magnetotail, with hot subsonic plasma causing the wake structure to disappear. We investigate the lunar wake response during a passing coronal mass ejection (CME) on March 8, 2012 while crossing the Earth's magnetotail using both a magnetohydrodynamic (MHD) model of the terrestrial magnetosphere and a three-dimensional hybrid plasma model of the lunar wake. The CME arrives at 1 AU around 10:30 UT and its impact is first detected inside the geomagnetic tail after 11:10 UT by the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (THEMIS-ARTEMIS) satellites in lunar orbit. A global magnetospheric MHD simulation using Wind data for upstream conditions with the OpenGGCM model reveals the magnetosheath compression to the lunar position from 11:20--12:00 UT, accompanied by multiple flux rope or plasmoid-like features developing and propagating tailward. MHD results support plasma changes observed by the THEMIS-ARTEMIS satellites. Lunar-scale simulations using the Amitis hybrid code show a short and misaligned plasma wake during the Moon's brief entry into the magnetosheath at 11:20 UT, with plasma expansion into the void being aided by the higher plasma temperatures. Sharply accelerated flow speed and a compressed magnetic field lead to an enhanced electric field in the lunar wake capable of generating sudden changes to the nightside near-surface electric potential. [ABSTRACT FROM AUTHOR]

We present the first multi-wavelength imaging observations of strong thermal emission velocity enhancement (STEVE) using an all-sky imager at Athabasca (magnetic latitude = 61.5°N), Canada. This study is based on three STEVE events which were accompanied by picket fence structures in the green-line. Although the STEVE arc was dominant in 630 and 557.7-nm, weak emissions were also found in other wavelengths including OI at 844.6, Hβ, Na, and the nominal background filter at 572.5-nm. As observed at 630 and 557.7-nm, the STEVE arc started as a faint arc close to the auroral oval and moved equatorward. The 557.7-nm arc exhibited picket fence structure at later times after it moved equatorward. The picket fence was sometimes found to persist even after the 630-nm arc had disappeared. During a particular event, the STEVE arcs in both the 630 and 557.7-nm were found to carry a ribbon-like motion moving along the arc. We have found that STEVE arcs are embedded in a region of weak diffuse auroral emissions. The STEVE arcs have sharp boundaries and these boundaries are different in red- and greenline. The sharp decrease in the intensity at the immediate poleward edge of the STEVE arc appears as a "dark-band" in the green-line images. Based on the horizontal component of the geomagnetic field at Fort Smith (magnetic latitude 67.28°N), we find that the STEVE arc detachment, its equatorward motion, and its brightness coincided with changes in the magnetic activity during the recovery phase of a substorm. [ABSTRACT FROM AUTHOR]

Magnetosheath reconnection due to the interaction of an interplanetary directional discontinuity with the bow shock and Earth's magnetosphere under an initially northward interplanetary magnetic field (IMF) has been investigated in previous simulations (e.g., Guo et al., 2018, https://doi. org/10.1029/2018ja025679). Under an initially southward IMF, the magnetosheath reconnection could interact with reconnection at the magnetopause. In this study, using three-dimensional (3D) global-scale hybrid simulations, we present cases with incoming tangential discontinuities (TDs) in an initially southward IMF, which possess various magnetic field rotation angles (ΔΦ) and half-width (w), to study the effects of pre-existing magnetopause reconnection on the formation of magnetosheath flux ropes, as well as the subsequent interaction between the magnetopause reconnection and magnetosheath reconnection, with downstreams of both Quasi-perpendicular (Q-...) and Quasi-parallel (Q-II) shock examined. The initial IMF is assumed to be oblique, with a finite Bx and Bz, similar to that in Guo et al. (2018), https:// doi.org/10.1029/2018ja025679 but with a southward Bz < 0. Compared with the cases with an initially northward IMF, magnetopause reconnection weakens the compression processes of the TD and leads to less frequent reconnection in the magnetosheath. The existence and the structure of magnetosheath reconnection are found to strongly depend on the parameters w and ΔΦ of the TD. When interacting with the magnetopause reconnection, the magnetosheath flux ropes can re-reconnect with the geomagnetic dipole field lines, forming new structures of magnetopause flux ropes. The resulting evolution of flux rope configuration is illustrated. [ABSTRACT FROM AUTHOR]

The Global-scale Observations of the Limb and Disk (GOLD) mission, launched in 2018, aims to investigate the low latitude ionosphere from a geostationary orbit at 47.5°W. It uses two identical spectrometers measuring the wavelength range from 134.0 to 163.0 nm. The configuration of the Earth's magnetic field shows that the largest offset between geographic and geomagnetic equators occurs in the longitude sectors sampled by GOLD. In an attempt to investigate the longitude dependence of the occurrence rate and time of onset of plasma bubbles, or plasma depletions, GOLD data were separated in three sectors: 65°-55°W, 50°-40°W, and 10°W--0°. Observations of the nighttime emissions in 135.6 nm on November 2018 and March 2019 show plasma depletions occurring very frequently at these longitudes. The growth rate of the Rayleigh-Taylor instability was computed at these longitudes under similar low solar activity conditions, assuming an empirical model of upward plasma drifts. The time and value of the maximum growth rates obtained cannot always explain the observations. On average, the observed occurrence rate of plasma depletions is high, with a maximum of 73% (observed during November 2018 at ~45°W). Most of the depletions observed in November at 45°W and 60°W occur within 1 h after sunset. When compared with the November 2018 observations, depletions in March 2019 occur at later times. [ABSTRACT FROM AUTHOR]

We present a new high-resolution empirical model for the ionospheric total electron content (TEC). TEC data are obtained from the global navigation satellite system (GNSS) receivers with a 1° x 1° spatial resolution and 5-min temporal resolution. The linear regression model is developed at 45°N, 0°E for the years 2000--2019 with 30-min temporal resolution, unprecedented for typical empirical ionospheric models. The model describes dependency of TEC on solar flux, season, geomagnetic activity, and local time. Parameters describing solar and geomagnetic activity are evaluated. In particular, several options for solar flux input to the model are compared, including the 10.7 cm solar radio flux (F10.7), the Mg II core-to-wing ratio, and formulations of the solar extreme ultraviolet flux (EUV). Ultimately, the extreme ultraviolet flux presented by the Flare Irradiance Spectral Model, integrated from 0.05 to 105.05 nm, best represents the solar flux input to the model. TEC time delays to this solar parameter on the order of several days as well as seasonal modulation of the solar flux terms are included. The Ap3 index and its history are used to reflect the influence of geomagnetic activity. The root mean squared error of the model (relative to the mean TEC observed in the 30-min window) is 1.9539 TECu. A validation of this model for the first 3 months of 2020 shows excellent agreement with data. The new model shows significant improvement over the International Reference Ionosphere 2016 (IRI-2016) when the two are compared during 2008 and 2012. [ABSTRACT FROM AUTHOR]

On a geomagnetically quiet night of July 07, 2018, the allsky imager at Hanle, Leh Ladakh (32.7°N, 78.9°E; Mlat. ~24.1°N), India captured a number of plasma structures in O (1D) 630.0 nm airglow emission. In a short observational period of around 3 h, two different medium scale traveling ionospheric disturbances and two different mid-latitude plasma depletions were observed. The most interesting aspect of the night was the drastic difference in the behavior of the two plasma depletions, which simultaneously existed within the imager's field of view for a long time. One of the plasma depletions, which we have termed as mid-latitude field-aligned plasma depletion, was nearly field aligned, drifted westward very slowly and became more prominent later in the night. The other depletion showed a much more ambiguous behavior with large reduction in its drift as well as significant change in its orientation relative to the geomagnetic meridian. In the present study, we have tried to explain this contrasting behavior in two different scenarios and have discussed arguments supporting each scenario along with their limitations. These observations are particularly useful considering the location of Hanle, which lies at a transition of geomagnetically low-mid latitudes, which are governed by different electrodynamics. [ABSTRACT FROM AUTHOR]

Aurora provides an essential diagnostic to spatial and temporal variations of terrestrial space environment and is also an important proxy of solar and geomagnetic activities. Contemporary auroral observations have just continued for more than half a century. Visual auroral phenomena recorded in historical books provide key clues to understand the solar and geomagnetic activities in the long history prior to modern era. In this study, we compiled a new auroral catalog from ancient Korean historical books, including 2013 auroral records with day-level resolution from 1012 to 1811 CE, especially for the records searched from the Seungjeongweon Ilgi. The number of auroral records in this new catalog is greatly enlarged compared with previous lists. The occurrence of the aurora in the new catalog is generally consistent with previous data sets. This extended data set provides valuable support for various studies related to solar-terrestrial space weather and ancient climates in the past millennium. [ABSTRACT FROM AUTHOR]

Observations from the Magnetospheric Multiscale (MMS) mission are used to quantify the maximum effect of magnetospheric H+ and He+ on dayside magnetopause reconnection. A data base of current-sheet crossings from the first 2 years of the MMS mission is used to identify magnetopause crossings with the highest He+ concentrations. While all of these magnetopause crossings exhibit evidence of plasmaspheric plume material, only half of the crossings are directly associated with plasmaspheric plumes. The He+ density varies dramatically within the magnetosphere adjacent to the magnetopause, with density variations of an order of magnitude on timescales as short as 10 s, the time resolution of the composition instrument on MMS. Plasma wave observations are used to determine the total electron density, and composition measurements are used to determine the mass density in the magnetosheath and magnetosphere. These mass densities are then used with the magnetic field observations to determine the theoretical reduction in the reconnection rate at the magnetopause. The presence of high-density plasmaspheric plume material at the magnetopause causes transient reductions in the reconnection rate of up to -40%. Plain Language Summary As the solar wind propagates from the Sun to the Earth, it encounters two boundaries that limit its access to the near-Earth environment. The first is a bow shock that heats, slows, and deflects the solar wind. The second is the Earth's magnetopause, where the shocked solar wind is deflected around the Earth's magnetic field region called the magnetosphere. Magnetic reconnection at the magnetopause creates an interconnection between the magnetic field of the shocked solar wind and the Earth's magnetic field. The rate at which these magnetic fields interconnect, the reconnection rate, depends on the amount of plasma (ions and electrons) on either side of the magnetopause. This research uses observations from the Magnetospheric Multiscale mission to look at this rate for times when there is substantial plasma on the magnetospheric side of the magnetopause. These instances are rare; however, when they do occur, they reduce the reconnection rate by a substantial amount (up to 40%). [ABSTRACT FROM AUTHOR]

In this study, the thermospheric mass densities based on satellite Precise Orbit Determination (POD) data from Gravity Recovery and Climate Experiment (GRACE), Swarm-A, and Swarm-B were used to explore the thermospheric variations in response to the October 2016 and September 2017 geomagnetic storms which occurred around the equinox. Two of the three satellites were nearly coplanar and the third one flew at a separated orbital plane. Based on the nearly coplanar observations of GRACE and Swarm, the thermospheric mass densities showed different hemispheric asymmetries depending on altitudes. The observations from different local times (LTs) revealed that the density enhancements were similar between the two hemispheres when the LTs were 8-10/20-22 LTs. However, the enhancements were stronger in the southern hemisphere than those in the northern hemisphere at around 16 LT. Moreover, the density enhancements at low latitudes underwent significant day-night variation as the daytime densities at low latitudes increased more rapidly and strongly than the nightside. Our results demonstrated that the satellite POD data could be used to retrieve high-cadence thermospheric density to study the variations of the upper atmosphere. [ABSTRACT FROM AUTHOR]

This study explores the dynamics of the main ionospheric trough (MIT) and ring ionospheric trough (RIT) during geomagnetic disturbances. The RIT is formed by a decay process of the magnetospheric ring current during the recovery phase of a geomagnetic storm/substorm. As a result, it is usually observed within the latitude belt 48°-60° GMLat, that is, the RIT is a midlatitude trough in contrast to the subauroral MIT. To explore the mutual dynamics of the MIT and RIT, 120 geomagnetic disturbances with Kp > 4, according to Kosmos-1809, Kosmos-900, Interkosmos-19, and CHAMP satellites, are considered. At night, the RIT is almost always formed at the recovery phase of geomagnetic disturbances. The RIT occurs most frequently in the morning and less often before midnight. The dynamics of the MIT and RIT during geomagnetic disturbances is quite complex: both troughs can be observed either separately or simultaneously, the MIT can transform abruptly into RIT and vice versa. The position of the RIT depends on Kp and longitude, but not always as distinctly as the position of the MIT. The RIT can be observed for 1.5 days during the recovery phase of a geomagnetic disturbance. The RIT can be observed simultaneously in both hemispheres at close conjugate latitudes. Plain Language Summary The dynamics of the main ionospheric trough (MIT) and ring ionospheric trough (RIT) during the storm recovery phase was studied. Eight events out of 120 storms/substorms were analyzed in detail. The RIT is observed at latitudes of the residual (decaying) magnetospheric ring current L = 2.7-4.0, that is, RIT is a midlatitude trough in contrast to subauroral MIT. The RIT was first discovered and investigated by the Soviet satellites Kosmos-1809 and Kosmos-900, but its dynamics during the storm was mainly published in Russian and turned out to be inaccessible to a broader scientific community. The RIT is a nighttime phenomenon; it formed most frequently in the morning sector and less in the evening. In the after midnight sector, the RIT can be clearly observed for 1.5 days. The MIT and RIT cannot be clearly distinguished either by position or by form. Moreover, the MIT can transform abruptly into the RIT and vice versa. The most effective method of troughs separation is to analyze the prehistory of their development. Thus, the dynamics of both troughs during the disturbance is quite complex: a joint structure, only RIT or MIT, or both troughs simultaneously can be observed. [ABSTRACT FROM AUTHOR]

The quiet time characteristics of equatorial electrojet (EEJ), counter electrojet (CEJ), and solar quiet (Sq) day geomagnetic field over two decades (1980–2002) are established using Principal component analysis (PCA) from equatorial (Ettaiyapuram) and off‐equatorial (Hyderabad) Magnetic Observatories, India, over sunspot cycles 21–23. The patterns of normal field shows that the diurnal amplitudes were strong during equinox compared to other seasons. Varying contributions of abnormal field in different Lloyd's seasons were evident in different phases of sunspot cycle. The diurnal amplitudes have reduced from the 21st to the 23rd sunspot maxima following the trend of weakening of sunspot cycle. Analysis of seasonal means shows evening CEJs were more pronounced when compared to morning and afternoon in different phases of sunspot cycle. The abnormal field variations have a strong correlation with the occurrence of afternoon CEJs during solar minima; a correlation of seasonal occurrences of CEJs with phases of sunspot cycles is revealed. Key Points: Reduced EEJ, CEJ, and Sq amplitudes were observed following the weakening of sunspot cycle 21–23Occurrence of CEJs were found to be predominant in D season almost in all phases of sunspot cycle, when compared to the other two seasonsLT dependence of CEJ has a linear relation in minima and ascending, while it is nonlinear in descending and maxima phases of sunspot cycle [ABSTRACT FROM AUTHOR]

O2 aurora is one kind of important molecular aurorae that is not fully understood yet. It is hard to be investigated due to the contamination by nightglow. In this work, we studied O2 aurora based on the observations of O2 emission at 1.27 μm from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument during the nighttime over 18 years. The horizontal structure of O2 auroral zone is given for the first time, and the vertical profile of O2 auroral volume emission rate is obtained after removing O2 nightglow contamination. The O2 auroral intensity and the peak height and peak volume emission rate of the vertical profile averaged over the O2 auroral zone are studied in detail. For Kp Levels 1–5, the O2 auroral peak height varies between 104 and 112 km and is negatively correlated with the auroral intensity and peak volume emission rate. The O2 auroral intensity varies between 0.14 and 5.97 kR, and the peak volume emission rate varies between 0.97 × 102 and 41.01 × 102 photons cm−3 s−1. Furthermore, the dependences of the O2 aurora on geomagnetic and solar activities are analyzed. The O2 auroral intensity and peak volume emission rate exponentially increases with increasing Kp index, whereas the peak height decreases with increasing Kp index. The O2 auroral intensity and peak volume emission rate under solar minimum condition are larger than those under solar maximum condition. The peak height under solar minimum condition is lower than that under solar maximum condition. Plain Language Summary: Aurora is one of the most magnificent phenomena in the Earth's atmosphere. The types of aurora are abundant. Among a wide variety of aurorae, O2 aurora is one kind of important molecular aurorae. However, we still do not understand it clearly. It is hard to study O2 aurora using observations from ground and by nadir sounding due to the strong interference by O2 airglow. The limb sounding by satellite can provide a global distribution of the profiles of atmospheric emissions. This is beneficial for separating O2 aurora from O2 airglow and investigating vertical distribution of O2 aurora. We study O2 aurora based on the excellent limb sounding of O2 emission at 1.27 μm by the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) satellite. O2 aurora is derived after removing the interference from nightglow, and statistical structures of O2 auroral zone at different geomagnetic activity levels are displayed for the first time. We find that the structures of the O2 aurora change with geomagnetic and solar activities. The results provide further understanding of the horizontal and vertical structures of O2 aurora. The results are helpful for the verification of the excitation mechanisms of O2 aurora and the development and validation of O2 auroral models. Key Points: Statistical structures of nighttime O2 auroral zone are given for the first time by using TIMED/SABER limb observations over 18 yearsPeak heights of vertical profiles of O2 auroral emission are between 104 and 112 km for Kp Levels 0–5O2 auroral peak emission rate (peak height) increases (decreases) with geomagnetic activity and decreases (increases) with solar activity [ABSTRACT FROM AUTHOR]

This work conducts a statistical study of the subauroral polarization stream (SAPS) feature in the North American sector using Millstone Hill incoherent scatter radar measurements from 1979 to 2019, which provides a comprehensive SAPS climatology using a significantly larger database of radar observations than was used in seminal earlier works. Key features of SAPS and associated electron density (Ne), ion temperature (Ti), and electron temperature (Te) are investigated using a superposed epoch analysis method. The characteristics of these parameters are investigated with respect to magnetic local time, season, geomagnetic activity, solar activity, and interplanetary magnetic field (IMF) orientation, respectively. The main results are as follows: (1) Conditions for SAPS are more favorable for dusk than near midnight, for winter compared to summer, for active geomagnetic periods compared to quiet time, for solar minimum compared to solar maximum, and for IMF conditions with negative By and negative Bz. (2) SAPS is usually associated with a midlatitude trough of 15–20% depletion in the background density. The SAPS‐related trough is more pronounced in the postmidnight sector and near the equinoxes. (3) Subauroral ion and electron temperatures exhibit a 3–8% (50–120 K) enhancement in SAPS regions, which tend to have higher percentage enhancement during geomagnetically active periods and at midnight. Ion temperature enhancements are more favored during low solar activity periods, while the electron temperature enhancement remains almost constant as a function of the solar cycle. (4) The electron thermal content, Te × Ne, in the SAPS associated region is strongly dependent on 1/Ne, with Te exhibiting a negative correlation with respect to Ne. Key Points: Key features of North American SAPS and associated Ne, Ti, and Te were analyzed using four decade Millstone Hill IS radar measurementsNorth American SAPS climatology in terms of MLT, season, geomagnetic activity, solar activity, and IMF condition was comprehensively studiedBoth ion and electron temperatures exhibit moderate enhancement around SAPS, with similar geomagnetic but different solar activity dependence [ABSTRACT FROM AUTHOR]