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The properties of chorus waves have been extensively studied, which are important to understand chorus generation and wave‐particle interactions between chorus and energetic electrons. Observations reveal that there exists a distinct day‐night asymmetry for the properties of chorus waves, such as the sweep rate and duration, which is predicted to be relevant to the asymmetric configuration of Earth's background magnetic field. In this paper, using the one‐dimensional (1‐D) particle‐in‐cell (PIC) simulation model with a compressed/stretched dipole magnetic field, we study the dependences of the properties of rising‐tone chorus waves on the background magnetic field. It is demonstrated that the saturated amplitude and the chorus sweep rate increase while the chorus duration decreases with an increase of the compression factor ξ, which represents the compression/stretch degree of the field line. Moreover, the threshold of exciting chorus waves in the compressed dipole field is generally lower than that in the stretched dipole field. These results are useful to understand the chorus excitation and the day‐night asymmetry of chorus properties. Key Points: A one‐dimensional particle‐in‐cell model is used to study how the configuration of the magnetic field controls the properties of chorus wavesThe amplitude and frequency sweep rate increase with increasing compression factor ξ, while the duration varies oppositelyThe threshold of exciting chorus waves is lowered by a large compression factor or a large field curvature [ABSTRACT FROM AUTHOR]

Typhoon is one of the major hazards in ocean coastal areas, but traditional techniques are inadequate to monitor typhoons due to limited or high‐cost observations, like radio sounding and meteorological radar. Previous studies have found that typhoons can cause ionospheric disturbances, but the relationship and characteristics are still unclear. In this paper, about 400 stations observations of the Global Positioning System (GPS) network in Taiwan are used to extract ionospheric disturbances during multiple typhoons. The detailed characteristics of the ionospheric disturbances are investigated using a fourth‐order Butterworth filter following the 2016 Nepartak, 2019 Lekima, 2019 Mitag, and 2020 Hagupit typhoons. The results show that significant ionospheric disturbances were observed during the typhoons, and the larger disturbances are mostly located 400–1200 km far from the typhoon eye. The estimated horizontal propagation velocity of the ionospheric disturbances is about 127–194 m/s. The locations of the ionospheric disturbances between the typhoon eye and the landfall site are related to the typhoon path. The azimuth distribution of the ionospheric disturbance around the typhoon eye is affected by the GPS elevation angles. At 500–700 km from the typhoon eye, the mean ionospheric disturbances are 0.17 TECU (TEC Unit) and 0.15 TECU for super typhoon Nepartak and Lekima, and 0.13 TECU and 0.18 TECU for typhoon Mitag and Hagupit. The higher the intensity of the typhoon is, the greater the magnitude of the ionospheric disturbance is. Key Points: Significant ionospheric disturbances are observed following multiple typhoons from GPS‐derived Total Electron Content dataThe directivities of positive and negative ionospheric disturbances are found in different azimuthsThe magnitude of ionospheric disturbances is related to the typhoon intensity with a positive proportion [ABSTRACT FROM AUTHOR]

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

The magnetosheath and near‐Earth solar wind emit X‐rays due to charge‐exchange between the extended atmosphere and highly ionized particles in the solar wind. These emissions can be used to remotely sense the dynamic processes in this region. The Solar Wind Magnetosphere Ionosphere Link Explorer mission will carry out these measurements. In a previous paper, we looked at the effect of photon counting statistics on determining the location of the magnetopause and bow shock. In this paper we explore, through simulations, the more challenging question of orbital viewing geometry bias when the model and the emissions do not match each other exactly. Our simulations conclude that while care must be taken to avoid false minima in the fitting, there is very little to no orbital bias in extracting the position and large‐scale shape of the magnetopause and bow shocks from 2‐D X‐ray images from the future Solar Wind Magnetosphere Ionosphere Link Explorer mission. Key Points: A 3‐D model fit to 2‐D images can extract magnetopause and bow shock boundaries even when the model is not an exact fit to the observationsOrbital bias produced model/observation differences is within the science goal of the SMILE mission for large part of the orbitWhen using the simplex method, the initial guess must be chosen carefully to avoid false minima [ABSTRACT FROM AUTHOR]

The plasma sensor on the sixth H‐II Transfer Vehicle (HTV) measured the depletion of electron density during some maneuver operations for attitude or altitude control. In this paper, we first discuss the on‐orbit observation of electron density depletion during the entire lifetime of the HTV. On‐orbit data showed that both altitude and attitude control maneuvers caused the depletion effect. The geocentric coordinates and universal time were also shown to confirm the depletion effect around the HTV by using other techniques, such as total electron content based on the Global Navigation Satellite System network. In the latter part of the paper, we discuss the cause of the depletion effect based on past literature. To analyze the interaction between thruster plume gas and ionospheric plasma, computational fluid dynamics simulations were conducted to analyze the number density distribution of molecules around the HTV. It was found that the combination of charge exchange and dissociative recombination possibly caused the depletion of electron density during altitude control maneuvers, due to neutral particles being ejected from the thrusters and a time delay in the reaction response. During attitude control maneuvers, the electrons of ionospheric plasma were pushed away from the plasma sensor by dense neutral clouds. Plain Language Summary: The electron current sensor on the H‐II Transfer Vehicle that transports supplies to the International Space Station observed the depletion of electron density during maneuver operations in orbit. The present study showed the measured data and discussed the causes of electron density depletion. This discussion was supported by computational fluid dynamics and simple numerical analyses. The present study found that the distance between the thruster and monitor, and the time to chemical reaction are important parameters to clarify the cause of electron density depletion. Key Points: The plasma sensor on the HTV observes electron density depletion during maneuver operations in the ionosphereComputational Fluid Dynamics simulations were conducted to clarify the number density distribution of neutral particles emitted from thrustersDissociative recombination and the push away effect are possible causes of electron density depletion [ABSTRACT FROM AUTHOR]

Earthquake (EQ) prediction is an essential topic in recent geophysics and disaster management. In order to explain EQ precursory phenomena, especially the ionospheric perturbations, a few hypotheses have been proposed as the mechanism of lithosphere‐atmosphere‐ionosphere coupling. The most reliable seems to be the atmospheric gravity wave (AGW) hypothesis, because a lot of subionospheric very low frequency (VLF) observations and also ground surface measurements support this hypothesis. However, no direct signature of AGW activity in the middle atmosphere has been yet obtained. So in this study, we have used the ERA5 reanalysis data to investigate the tempo‐spatial evolution of stratospheric AGW activity before a recent huge seismic event of the 2016 Kumamoto EQ on 15 April (Mw7.2). It was found that the AGW potential energy is enhanced significantly during about 1 week before the EQ. The enhancement was mainly distributed around the EQ epicenter and expanded toward eastern Japan. The AGW activity was highest around 11 April, then it calmed down and returned to the background level right before the EQ. These results were compared with the subionospheric VLF observations during the same period, and then the tempo‐spatial evolutions of the lower ionospheric perturbation are found to be very consistent with the stratospheric behavior. The present paper reports on the first finding that the abnormal AGW activity in the stratosphere is detected before a major EQ, and the coincidence of stratospheric AGW activity with VLF subionospheric perturbations provides further support to the AGW hypothesis of the lithosphere‐atmosphere‐ionosphere coupling process. Key Points: This paper reports on the first investigation on the stratospheric AGW activity before a major earthquake (EQ)The stratospheric AGW activity was significantly enhanced around the EQ epicenter during about 1 week prior to the EQThe tempo‐spatial coincidence of AGW activity and VLF subionospheric perturbations further supports the AGW hypothesis of LAIC process [ABSTRACT FROM AUTHOR]

In the Earth's magnetotail, magnetic reconnection releases stored magnetic energy and drives magnetospheric convection. The rate at which magnetic flux is transferred from the reconnection inflow to outflow regions is determined by the reconnection electric field Er, which is often referred to as the unnormalized reconnection rate. To better quantify the efficiency of reconnection, this electric field Er is often normalized by the characteristic Alfvén speed and the reconnecting magnetic field. This parameter is generally called the normalized or dimensionless reconnection rate R. In this paper, we employ a two‐dimensional fully kinetic simulation to model a magnetotail reconnection event with weak geomagnetic activity (<200 nT of the AE index) observed by the Magnetospheric Multiscale (MMS) mission on 11 July 2017. We obtain R and Er from direct measurements in the diffusion region and indirect measurements of the rate at the separatrix using a recently proposed remote sensing technique. The measured normalized rate for this MMS event is R ∼0.15–0.2, consistent with theoretical and simulation models of fast collisionless reconnection. This corresponds to an unnormalized rate of Er ∼2–3 mV/m. Based on quantitative consistencies between the simulation and the MMS observations, we conclude that our estimates of the reconnection rates are reasonably accurate. Given that past studies have found Er of the order ∼10 mV/m during strong geomagnetic substorms, these results indicate that the local Er in magnetotail reconnection may be an important parameter controlling the amplitude of geomagnetic disturbances. Key Points: Reliable reconnection rates are obtained based on virtual observations in a fully kinetic simulation of an MMS tail reconnection eventThe normalized rates obtained from the simulation and MMS data are 0.15–0.2, indicating the occurrence of fast reconnectionThe observed unnormalized rate is 2–3 mV/m, while higher rates were observed in other events with stronger geomagnetic activities [ABSTRACT FROM AUTHOR]

Jupiter has a strong magnetic field, and a huge magnetosphere is formed through the solar wind-Jupiter interaction. The generated magnetosphere-ionosphere system is reproduced based on the 9-component Magnetohydrodynamics (MHD) and the current conservation in the ionosphere. Assuming Io plasma emission rate 1.4 t/sec, this paper reproduces self-consistently global magnetic configuration, generations of the field-aligned current (FAC) and aurora, formation of the Io plasma disk at 8-20 RJ, plasma corotation, instability in the plasma disk, transition from the Io plasma disk to the plasma sheet at 20-150 RJ, and the plasmoid ejection. The rotating Io plasma in the disk forms instabilities that promotes radial diffusion. H+ is supplied from the ionosphere along high-latitude magnetic field lines and mixed with heavy ions around 15-20 RJ. Beyond 20 RJ, mixed plasma diffuses further outward by the centrifugal force that can exceed magnetic tension. In the ionosphere, the main oval occurs at 13.7°-15.5° colatitude. The Io disk is inner side of magnetic field lines traced from the low-latitude edge of the main oval. Along magnetic field lines, the main oval is mapped from the outer edge of the Io disk to the entire plasma sheet accompanying rotation delay. Due to the corotation limit, convection is accompanied by plasmoid ejection. Back reaction of plasmoid ejection affects even transport process in the Io disk. The downward FAC occurs in the polar cap showing variability. The region of externally driven Dungey convection seems quite narrow. [ABSTRACT FROM AUTHOR]

This study investigates the ionospheric total electron content (TEC) responses in the 2‐D spatial domain and electron density variations in the 3‐D spatial domain during the annular solar eclipse on 14 October 2023, using ground‐based Global Navigation Satellite System (GNSS) observations, a novel TEC‐based ionospheric data assimilation system (TIDAS), ionosonde measurements, and satellite in situ data. The main results are summarized as follows: (a) The 2‐D TEC responses exhibited distinct latitudinal differences. The mid‐latitude ionosphere exhibited a more substantial TEC decrease of 25%–40% along with an extended recovery time of 3–4 hr. In contrast, the equatorial and low‐latitude ionosphere experienced a smaller TEC reduction of 10%–25% and a faster recovery time of 20–50 min. The minimal eclipse effect was observed near the northern equatorial ionization anomaly crest region. (b) The ionospheric electron density variations during the eclipse were effectively reconstructed by TIDAS data assimilation in the 3‐D domain, providing important altitude information with validity. (c) The ionospheric electron density variations showed a notable altitude‐dependent feature. The eclipse led to a substantial electron density reduction of 30%–50%, with the maximum depletion occurring around the ionospheric F2‐layer peak height (hmF2) of 250–350 km. The post‐eclipse recovery of electron density exhibited a relatively slower pace near the F2‐layer peak height than that at lower and higher altitudes. Plain Language Summary: On 14 October 2023, the Great American annular solar eclipse traversed North, Central, and South America with dense observational network in place, presenting a valuable opportunity for exploring the eclipse‐induced ionospheric responses from mid‐latitude to equatorial regions. This paper presents a comprehensive analysis of the 2‐D ionospheric TEC and 3‐D electron density responses during the eclipse, utilizing dense ground‐based GNSS observations, a new TEC‐based ionospheric data assimilation system (TIDAS), and ionosonde and satellite data sets. The TIDAS data assimilation system provides accurate and reliable regional ionospheric electron density reconstruction, which can effectively reproduce the electron density variations during the eclipse in the 3‐D domain with important altitude information and high‐fidelity details. This multi‐instrumental and data assimilation study highlights the latitudinal and altitudinal dependencies of the eclipse‐induced ionospheric responses, advancing the current understanding of how a solar eclipse event impacts the ionosphere. Key Points: The TEC response showed latitudinal variances, with a 25%–40% decrease in midlatitudes but only a 10%–25% reduction in the equatorial regionThe Ne response showed altitudinal dependencies, with a larger depletion and a slower recovery near the F2 peak height than below and aboveThe NmF2 exhibited a 30%–50% reduction, and the hmF2 exhibited a 20–30 km decrease in the recovery phase after the maximum obscuration [ABSTRACT FROM AUTHOR]

Short large‐amplitude magnetic structures (SLAMS) frequently appear near the bow shock. Inside steepening SLAMS, whistler‐mode waves are coherently generated at their leading edges. These waves are crucial for electron dynamics, energy conversion and magnetic reconnection near the shock. Nevertheless, the characteristics and generation of the whistler‐mode waves inside SLAMS are still unclear. In this study, we conducted a statistical analysis of whistler‐mode waves within SLAMS near the Earth's bow shock to investigate their properties and generation mechanisms. We found that these waves are mainly excited by electron temperature anisotropy with 99% of them falling below the nonlinear resonant threshold. The combination of a low background magnetic field and intense steepening at SLAMS' leading edge makes it the main source region of whistler‐mode waves and may induce the nonlinear resonance of whistler‐mode waves. Plain Language Summary: On the quasi‐parallel side of the bow shock, ultra‐low frequency waves and nonlinear structures such as shocklets and SLAMS are commonly observed, which are crucial for the particle dynamics and energy conversion near the shock. Whistler‐mode waves are coherently generated at the leading edge or Earthward side of these nonlinear structures and may subsequently evolve into the shocklets. These waves are crucial for electron dynamics, energy dissipation and energy cascade near the shock. In addition, recent studies have discovered that the propagation of whistler‐mode waves can trigger magnetic reconnection processes inside SLAMS. Nevertheless, the characteristics and generation of the whistler‐mode waves inside SLAMS are still unclear. A comprehensive understanding of whistler‐mode waves inside SLAMS is still missing. In this paper, based on statistical results, we found that most of the whistler‐mode waves are locally generated and trapped inside the SLAMS. The nonlinear resonance condition of whistler‐mode waves is only satisfied at the leading edge of SLAMS with significant steepening. Key Points: Whistler‐mode waves are locally excited and trapped inside the SLAMSMost of the whistler‐mode are excited by electron temperature anisotropy resulting from SLAMS steepeningThe significant steepening seems essential for the nonlinear resonance of whistler‐mode waves at the leading edge of SLAMS [ABSTRACT FROM AUTHOR]

We present a statistical analysis of electrostatic solitary waves observed aboard Magnetospheric Multiscale spacecraft in the Earth's magnetosheath. Applying single‐spacecraft interferometry to several hundred solitary waves collected in about 2‐minute interval, we show that almost all of them have the electrostatic potential of positive polarity and propagate quasi‐parallel to the local magnetic field with plasma frame velocities of the order of 100 km/s. The solitary waves have typical parallel half‐widths from 10 to 100 m that is between 1 and 10 Debye lengths and typical amplitudes of the electrostatic potential from 10 to 200 mV that is between 0.01% and 1% of local electron temperature. The solitary waves are associated with quasi‐Maxwellian ion velocity distribution functions, and their plasma frame velocities are comparable with ion thermal speed and well below electron thermal speed. We argue that the solitary waves of positive polarity are slow electron holes and estimate the time scale of their acceleration, which occurs due to interaction with ions, to be of the order of one second. The observation of slow electron holes indicates that their lifetime was shorter than the acceleration time scale. We argue that multi‐spacecraft interferometry applied previously to these solitary waves is not applicable because of their too‐short spatial scales. The source of the slow electron holes and the role in electron‐ion energy exchange remain to be established. Plain Language Summary: Earth's magnetosheath is a highly turbulent medium and an ideal natural laboratory for the analysis of plasma turbulence. Spacecraft measurements showed that high‐frequency electric field fluctuations in the Earth's magnetosheath are predominantly electrostatic and consist, particularly, of electrostatic solitary waves with bipolar parallel electric fields. The properties of these electrostatic fluctuations have been largely unaddressed and, moreover, the results of previous studies were inconsistent. In this paper, we present a statistical analysis of electrostatic solitary waves observed aboard Magnetospheric Multiscale in the Earth's magnetosheath. We revealed that most of the solitary waves are Debye‐scale structures with the electrostatic potential of positive polarity and typical amplitudes between 0.01% and 1% of local electron temperature. We demonstrated that the solitary waves must be electron holes, purely kinetic structures produced in a nonlinear stage of various electron‐streaming instabilities. Even more critical is that these structures are slow; their plasma frame velocities are well below electron thermal speed but coincide with the velocities of the bulk of ions. While the source of electrostatic fluctuations in Earth's magnetosheath could not be revealed, the finding that these fluctuations can be slow implies they can facilitate efficient energy exchange between ions and electrons. Key Points: Statistical analysis of 645 solitary waves in the Earth's magnetosheath revealed that 630 of them are electron holesThe electron holes are associated with quasi‐Maxwellian ion velocity distribution functionsThe electron hole velocities are comparable with those of the bulk of ions and well below electron thermal speed [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves lead to rapid scattering of relativistic electrons in Earth's radiation belts, due to their large amplitudes relative to other waves that interact with electrons of this energy range. A central feature of electron precipitation driven by EMIC waves is deeply elusive. That is, moderate precipitating fluxes at energies below the minimum resonance energy of EMIC waves occur concurrently with strong precipitating fluxes at resonance energies in low‐altitude spacecraft observations. This paper expands on a previously reported solution to this problem: nonresonant scattering due to wave packets. The quasi‐linear diffusion model is generalized to incorporate nonresonant scattering by a generic wave shape. The diffusion rate decays exponentially away from the resonance, where shorter packets lower decay rates and thus widen the energy range of significant scattering. Using realistic EMIC wave packets from δf particle‐in‐cell simulations, test particle simulations are performed to demonstrate that intense, short packets extend the energy of significant scattering well below the minimum resonance energy, consistent with our theoretical prediction. Finally, the calculated precipitating‐to‐trapped flux ratio of relativistic electrons is compared to ELFIN observations, and the wave power spectra is inferred based on the measured flux ratio. We demonstrate that even with a narrow wave spectrum, short EMIC wave packets can provide moderately intense precipitating fluxes well below the minimum resonance energy. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are one of the most important plasma emissions in the near‐Earth space. When electrons experience an approximately constant EMIC wave phase in gyration, they resonate with these waves and are scattered to precipitate to the Earth's upper atmosphere. Such cyclotron resonance between electrons and EMIC waves are typically above 1 MeV of electron energy. However, spacecraft at low Earth orbit often observe that electrons in the hundreds of keV range, which are not in resonance with EMIC waves, precipitate simultaneous with those >1 MeV. Strongly modulated EMIC wave packets are promising in precipitating the sub‐MeV electrons through nonresonant interactions. Here, the theoretical model of nonresonant scattering is verified for realistic EMIC wave packets from self‐consistent computer simulations. EMIC wave power spectra are inferred from electron precipitation measurements by ELFIN. Short EMIC wave packets are shown to give a better agreement between the theoretical and observed precipitating‐to‐trapped flux ratios. Key Points: The theoretical model of nonresonant scattering is verified for wave packets derived from self‐consistent simulationsShort EMIC wave packets extend the energies of efficient scattering well below the minimum resonance energy, consistent with the theoryEMIC wave power spectra are inferred from ELFIN observations of relativistic electron precipitation, including nonresonant scattering [ABSTRACT FROM AUTHOR]

The Low lAtitude long Range Ionospheric raDar (LARID), which consists of two high frequency (HF) radars looking toward the east and west of Hainan Island, respectively, has been developed and installed at Dongfang (19.2°N, 108.8°E, dip lat. 13.8°N), China. This paper describes the system design of LARID and its first observational results of equatorial plasma bubble (EPB) irregularities. The antenna array of LARID is composed of a west‐looking array and an east‐looking array. Each array consists of 20 log‐periodic antennas for transmission and reception and 4 log‐periodic antennas for interferometry, and has a beam steering capability in the azimuth angles of ±24° due the boresight pointing east (or west). Observational results show that the LARID is capable of detecting backscatter echoes from EPB irregularities and the ground, with a distance of 4,000 km or more away from Hainan Island. Multiple EPB structures were continuously observed on 17 April 2023, with eastward drifts ranging between 70 and 130 m/s. Based on ray tracing simulations, the backscatter echoes of EPB irregularities were due to the 0.5‐hop and 1.5‐hop propagation modes. The distances between the successive EPB structures were estimated ranging between 500 and 900 km in longitude. The LARID observations, together with other instruments in the East and Southeast Asian sector, provided a clear picture of longitudinal variation of EPBs in 90–125°E. It is expected that the LARID will provide an important tool to study the generation and evolution of EPBs and their short‐term prediction in East and Southeast Asia. Key Points: A Low lAtitude long Range Ionospheric raDar (LARID) operating at 8–22 MHz has been developed and installed at Hainan Island, ChinaFirst long‐range observations of equatorial plasma bubble (EPB) irregularities and their zonal drifts by the LARID were presentedThe LARID is expected to play an important role in the study of EPB and its short‐term forecasting in East and Southeast Asia [ABSTRACT FROM AUTHOR]

Based on Global Coupled Ionosphere‐Thermosphere‐Electrodynamics Model, the solution of the 3‐dimensional current in the ionospheric region, the equivalent sheet current and filed‐aligned current are examined. The simulation study enables a comprehensive analysis of the effect of the geomagnetic field configuration, especially the non‐dipole component and tilt angle, on the ionospheric electrodynamics phenomena. Different geomagnetic field configurations are specified in the present work, including realistic geomagnetic field (RGF), tilted dipole geomagnetic field (TDGF) and zero‐declination dipole geomagnetic field (ZDGF). Our simulation focuses on the seasonal variation of Sq current, primarily governed by annual and semi‐annual variation. The modulation of the tilt angle of the geomagnetic field is globally distributed, whereas the modulation of the geomagnetic anomaly is localized. At mid‐latitudes, the annual mean and semi‐annual amplitudes of the Sq current are negatively correlated with the magnetic field strength, especially shown in geomagnetic anomaly area, while there is the opposite effect in the geomagnetic conjugate regions of the opposite hemispheres. The annual variation of Sq current system is more affected by the offset of the geomagnetic latitudes and geographical latitudes. The seasonal variation of the total Sq current is also modulated by the geomagnetic field. The annual mean, the annual and semi‐annual components of the total Sq current are negatively correlated with the magnetic field strength, while the annual variation is also controlled by the tilt angle of the geomagnetic field. The solar radiation affects the semi‐annual variation of the current more strongly than the annual variation. Plain Language Summary: Ionospheric currents and magnetic fields exert significant influences on energy and material transport within the ionosphere, playing a pivotal role in Earth's electromagnetic environment, which is the primary focus of ionospheric research. To investigate the impact of the geomagnetic field on the ionospheric current system, we employ a theoretical model (GCITEM) for simulation work. In this simulation, we consider both the realistic geomagnetic field configuration and two hypothetical idealized configurations of the geomagnetic field: one excluding non‐dipole terms and the other further aligning the geomagnetic axis with Earth's spin axis. The analysis of the longitude and the seasonal variation of the simulated currents under realistic magnetic field conditions has been validated through previous observation results. In comparison to simulation results obtained under hypothetical configurations, we discern and analyze the modulation and mechanism of non‐dipole terms and the tilt angle of geomagnetic field on the variability of the ionospheric current system, which are discussed in detail within this paper. This study facilitates the analysis and prediction of long‐term changes in the geomagnetic field's effects on ionospheric current generation and the impact on space's electromagnetic environment. Key Points: The geomagnetic field intensity exerts a significant influence on the Sq currents and exhibits a negative correlation with Sq currentThe annual mean and annual variation amplitude of Sq current are positively correlated with the geomagnetic anomaly at conjugate pointsThe tilt angle of the geomagnetic field affects the annual variation of Sq current globally, but hardly effects the semiannual variation [ABSTRACT FROM AUTHOR]

An ultralow frequency (ULF) wave was simultaneously observed in the ionosphere by the Super Dual Auroral Radar Network (SuperDARN) radar at Hankasalmi, Finland and on the ground by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers with close proximity to the radar. The onset time of the wave event was around 03:00 magnetic local time. Fourier wave analysis of the event suggests a wave period of about 1,340 s with an equatorward latitudinal and eastward longitudinal wave phase propagation, and an effective azimuthal wave number of 17 ± 1, in the intermediate range of those observed in ULF waves. This wave has been interpreted as resulting from drifting electrons of energies of 13 ± 5 keV in a drift resonance condition linked to energetic particle populations during a magnetospheric substorm. The latitudinal phase characteristics of this wave experienced temporal evolution, believed to be caused by additional injected particle populations associated with the same substorm driving the wave, which resulted in an observed loss of HF backscatter. This observation of a unique type of temporal evolution in the phase propagation characteristics of ULF waves enhances current understanding about the structure, dynamics and source of these types of ULF waves. Plain Language Summary: The ultralow frequency (ULF) waves provide information about the Earth's global magnetic field known as magnetosphere. These waves contribute to the circulation of mass, energy and momentum in the near‐earth space environment. However, the sources of these waves as well as the generation and propagation mechanisms behind them are not fully understood. In this paper, we have used Super Dual Auroral Radar Network (SuperDARN) radar and magnetometer observations to characterize a ULF wave event and investigate noticeable changes in the wave phase propagation over space and time during the wave event. The results show that the case‐study ULF wave is localized with very long period. We find that the wave appears to be driven by resonance with energetic electrons injected into the magnetosphere by a process in the magnetosphere known as a substorm. We also find that the wave event is particularly unusual in that its phase propagation characteristics appear to change during the event. The results enhance current understanding of changes in the characteristics of ULF waves over space and time using a multi‐instrument measurements. Key Points: The case‐study ultralow frequency wave, localized but with very long period, and with m number of ≈17 was simultaneously observed in the ionosphere by ground‐based radar and on the Earth's surface by ground‐based magnetometersThe wave phase propagates eastward and appears to be driven by resonance with energetic electrons injected into the magnetosphere during a recent magnetospheric substormThe wave event is particularly unusual in that its latitudinal phase propagation characteristics appear to change during the event [ABSTRACT FROM AUTHOR]

Ions in the F region ionosphere at 150–400 km altitude consist mainly of molecular NO+ and O2+ ${\mathrm{O}}_{2}^{+}$, and atomic O+. Incoherent scatter (IS) radars are sensitive to the molecular‐to‐atomic ion density ratio, but its effect to the observed incoherent scatter spectra is almost identical with that of the ion temperature. It is thus very difficult to fit both the ion temperature and the fraction of O+ ions to the observed spectra. In this paper, we introduce a novel combination of Bayesian filtering, smoothness priors, and chemistry modeling to solve for F1 region O+ ion fraction from EISCAT Svalbard IS radar (75.43° corrected geomagnetic latitude) data during the international polar year (IPY) 2007–2008. We find that the fraction of O+ ions in the F1 region ionosphere is controlled by ion temperature and electron production. The median value of the molecular‐to‐atomic ion transition altitude during IPY varies from 187 km at 16–17 MLT to 208 km at 04–05 MLT. The ion temperature has maxima at 05–06 MLT and 15–16 MLT, but the transition altitude does not follow the ion temperature, because photoionization lowers the transition altitude. A daytime transition altitude maximum is observed in winter, when lack of photoionization leads to very low daytime electron densities. Both ion temperature and the molecular‐to‐atomic ion transition altitude correlate with the Polar Cap North geomagnetic index. The annual medians of the fitted transition altitudes are 14–32 km lower than those predicted by the International Reference Ionosphere. Plain Language Summary: Ions in the F region ionosphere at 150–400 km altitudes consist mainly of molecular NO+ and O2+ ${\mathrm{O}}_{2}^{+}$, and atomic O+. Incoherent scatter radars are sensitive to the molecular‐to‐atomic ion density ratio, but its effect to the observed incoherent scatter spectra is almost identical with that of the ion temperature. It is thus very difficult to fit both the ion temperature and the fraction of O+ ions to the observed spectra. This causes bias to the fitted temperatures and leaves behavior of the F1 region ion composition poorly known. We apply a novel combination of inverse mathematics and chemistry modeling to analysis of EISCAT Svalbard incoherent scatter radar data, and solve for both the ion temperature and the fraction of O+ ions at the same time. We find that the ion composition is considerably different from a standard model, that it undergoes regular diurnal variations, and it is affected by geomagnetic activity. The typical variations can be qualitatively explained by known diurnal variations in ion temperature, solar photoionization, and ion chemistry. When the ionosphere above Svalbard is in almost complete darkness in mid‐winter, ion composition in the daytime ionosphere is considerably different from that observed during the other seasons. Key Points: We use novel data analysis techniques and chemistry modeling to fit atomic oxygen ion fractions to EISCAT Svalbard radar data during IPYWe characterize the F1 region ion composition dependence on local time, solar zenith angle, and geomagnetic activityThe molecular‐to‐atomic ion transition altitudes are 14–32 km lower than those predicted by the International Reference Ionosphere [ABSTRACT FROM AUTHOR]

This paper conducts a multi‐instrument analysis of a latitudinal plasma density peak at the middle latitudes during the early recovery phase of the April 2023 geomagnetic storm. The total electron content (TEC), peak density of the F layer, and the in situ plasma density from Swarm and Defense Meteorological Satellite Program (DMSP) satellites all capture this peak feature. This narrow latitudinal peak structure appeared around 50°N and extended from 40°E to 150°E in longitude with a prolonged duration of about 8 hr from sunset to midnight. This mid‐latitude peak reveals a noticeable equatorward motion and a slight westward shift. According to the plasma composition observations from DMSP satellites, this peak structure shows an O+ ions dominance, which means that this peak is more likely to be formed by an internal rather than an external source from the plasmasphere. Meanwhile, the middle latitude Fabry–Perot interferometer (FPI) observed strong equatorward thermospheric winds, and the peak height of the F layer presented a visible elevation, which suggests that the equatorward wind lifting caused the plasma density enhancement. Besides, the O/N2 ratio significantly decreased at lower and middle latitudes, and ion drift observations showed a distinct subauroral westward channel. Based on these simultaneous measurements, this structure's sharp equatorward and poleward boundaries might be related to the O/N2 ratio change and the subauroral polarization stream (SAPS) flow separately. Plain Language Summary: The ionospheric response to geomagnetic storm disturbances exhibits diverse plasma density structures. During the early recovery phase of the April 2023 geomagnetic storm, a distinct latitudinal plasma density peak is observed at middle latitudes. The formation of the mid‐latitude plasma density peak structure is not settled yet, even though some homologous structures have been reported. This study investigates the spatial and temporal features of this mid‐latitude peak structure and the dominant ion composition. The investigation is achieved by utilizing the total electron content, F layer parameters, and in situ plasma observations. Furthermore, the observations of neutral wind, thermospheric composition, and ion drift observations show a good correlation with the spatial‐temporal characteristics of this peak structure. These clues suggest that the formation of the mid‐latitude peak structure during the April 2023 geomagnetic storm may be attributed to a combined effect involving equatorward neutral winds, thermospheric composition change, and subauroral polarization stream flow. Key Points: A mid‐latitude peak is present in total electron content, F layer peak density, and in situ plasma density during the storm recovery phaseThe mid‐latitude peak covers 40°–150°E in longitude with a noticeable equatorward motion and slight westward shiftThe peak structure may be associated with the thermospheric equatorward wind, composition change, and subauroral polarization stream [ABSTRACT FROM AUTHOR]

As activity in Earth orbit continues to grow, it is important to characterize the environment of near‐Earth space. One means of remotely sensing lower thermospheric neutrals is by measurement of O and N2 density through the observation of far‐ultraviolet (FUV) airglow of atomic oxygen at 135.6 nm and the N2 Lyman‐Birge‐Hopfield (LBH) bands (~130–180 nm), as has been done on the Ionospheric Connection Explorer (ICON), Global‐scale Observations of the Limb and Disk (GOLD), and Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) missions. This technique is not without limitations, however, as the FUV measurements suffer from contamination by ionospheric emissions at low latitudes and auroral emissions excited by precipitating energetic electrons and protons at high latitudes. Previous work has shown the potential for making measurements of O and N2 density in the lower‐middle thermosphere using observations of extreme‐ultraviolet (EUV) airglow. This measurement approach has a potential advantage in that it does not have an inherent ionospheric emission that must be accounted for. Additionally, these emissions are primarily excited directly by solar UV rather than electron impact and thus have the potential to enable expansion of neutral density observations into the auroral zone and polar cap where the FUV measurement cannot be applied. This article demonstrates a new approach and algorithm designed to retrieve thermospheric O and N2 density from 150 to 400 km using measurements from the ICON EUV instrument. The retrieval results throughout 2020 are summarized and compared to measurements from ICON FUV, GOLD, and SWARM. Plain Language Summary: The region of space where most satellites orbit is almost empty ‐ but not completely empty. The small amount of nitrogen and oxygen gas present can cause satellites to gradually fall out of orbit through drag. A failure to understand and evaluate changes in the density of the upper atmosphere led to the loss of 38 Starlink satellites in February 2022, for example, This is one reason that it is important to understand how much gas is in near‐Earth space and how the amount changes over time, but orbit decay due to drag makes it difficult to make direct measurements. One common solution is to measure the amount of gas from afar, taking advantage of the fact that these gases emit small amounts of light in the far‐UV spectrum. There are some limitations to this approach, one of which being that charged particles in the same region can create an extra and unrelated signal, which requires correction. This paper presents a novel approach which uses more energetic extreme‐UV light as an alternative to the far‐UV. We show the results of this new algorithm and demonstrate that the measurements line up well with those from other instruments and spacecraft. Key Points: We present an algorithm concept for remote sensing thermospheric O and N2, novel in its use of EUV airglow measurementsComparison to related measurements during conjunctions with ICON FUV, GOLD, and Swarm are favorableRetrieval results from 2020 indicate cooler atmospheres than those predicted by MSIS [ABSTRACT FROM AUTHOR]

STEVE, Strong Thermal Emission Velocity Enhancement, was often observed by ground‐based imagers in visible wavelengths and rarely detected by global FUV imagers. We present a new event, and revisit two reported STEVE events, that were observed by the DMSP/SSUSI FUV imager at O 135.6 nm, N2 LBHS, and LBHL emissions. Coincident particle and plasma drift observations showed that the events were associated with high ion drift speed, low ion density and no energetic particle precipitation. This is consistent with earlier findings (e.g., MacDonald et al., 2018, https://doi.org/10.1126/sciadv.aaq0030; Gallardo‐Lacourt et al., 2018a, https://doi.org/10.1029/2018ja025368, 2018b, https://doi.org/10.1029/2018gl078509; Archer et al., 2019). In this paper, several new features are identified: (a) Detection rates of FUV STEVE are much lower than that of visible STEVE; (b) The STEVE on 27 March 2008 covers 3‐hr local time with a length up to 2,700 km around 60° magnetic latitudes; (c) Different widths of STEVE observed in O 135.6 nm and N2 LBH images indicate a large altitude range in the STEVE FUV emissions; (d) The true ion (assuming O+) drift speeds during the 27 March 2008 STEVE event could be up to 20 km/s, well above the DMSP SSIES sensor limit. The kinetic energy of the high speed ions is larger than the excitation potential of the observed FUV emissions; (e) The requirement of such extreme high ion drift speed explains why FUV STEVE have been rarely observed, compared to the visible STEVE; (f) The three FUV STEVE events occurred during moderate geomagnetic activity; (g) Theses events were conjugate in both hemispheres. Plain Language Summary: STEVE, Strong Thermal Emission Velocity Enhancement, was discovered by ground‐based imagers in visible wavelengths. We present examples of STEVE detected by FUV imager at O 135.6 nm, N2 LBHS (140–150 nm), and LBHL (165–180 nm) emissions. These FUV STEVE events were associated with high plasma drift speed and low plasma density in the ionosphere, and no energetic particle precipitation. These FUV STEVE events show several new features: (a) Detection rates of FUV STEVE are much lower than that of visible STEVE; (b) The STEVE length is up to 2,700 km; (c) There is a large altitude range of the FUV emission; (d) The true ion (assuming O+) drift speeds could be up to 20 km/s. Ions with such a high drift speed would carry sufficient kinetic energy to excite FUV emissions; (e) The FUV STEVE events occurred during moderate geomagnetic activity; and (f) The FUV STEVE events were conjugate in both hemispheres. Key Points: In addition to the visible emissions, STEVE is also observed in far ultraviolet emissionsThe STEVE on 27 March 2008 has a horizontal length up to 2,700 km and a large altitude extensionEstimated O+ ion drift speed for some STEVE events is ∼20 km/s and the ions' kinetic energy is greater than the FUV excitation potential [ABSTRACT FROM AUTHOR]

The latitudinal location of the Equatorial Ionization Anomaly (EIA) crest has seasonal variation, and there are disagreements on the interpretation of such seasonal characteristic in previous studies. Some studies suggested that this seasonal characteristic is determined by the seasonal characteristic of the equatorial electric field. Others suggested that this seasonal characteristic is determined by the seasonal changes of the thermospheric wind. The current paper uses Total Electron Content (TEC) data and the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) to analyze the seasonal variation of the northern EIA crest in the eastern Asian sector under low solar activity. Our results show that the monthly averaged latitudinal location of the northern EIA crest has a good linear relationship (r = 0.74) with the monthly averaged Equatorial Electrojet (EEJ) intensity, which is a commonly used proxy of the eastward electric field. However, TIEGCM simulations with and without F‐region wind indicate that such a relationship might be attributed to wind effects. Additionally, the linear relationship between the EEJ intensity and the northern‐southern EIA crest distance is not significant (r = 0.47) in the eastern Asian sector. Our results suggest that a good correspondence between the eastward electric field and the latitudinal location of the EIA crest is not assured annually, as the seasonally varying F‐region wind significantly influences EIA evolution. Key Points: Variations of the EIA crest latitude in the east Asian sector are analyzed with TEC data and TIEGCM simulationEEJ intensity and northern EIA crest latitude have a good linear relationship but might be attributed to F‐region meridional wind effectWind effect on the EIA evolution cannot be neglected on an annual timescale [ABSTRACT FROM AUTHOR]

Energetic electron dynamics in the Earth's radiation belts and near‐Earth plasma sheet are controlled by multiple processes operating on very different time scales: from storm‐time magnetic field reconfiguration on a timescale of hours to individual resonant wave‐particle interactions on a timescale of milliseconds. The most advanced models for such dynamics either include test particle simulations in electromagnetic fields from global magnetospheric models, or those that solve the Fokker‐Plank equation for long‐term effects of wave‐particle resonant interactions. The most prospective method, however, would be to combine these two classes of models, to allow the inclusion of resonant electron scattering into simulations of electron motion in global magnetospheric fields. However, there are still significant outstanding challenges that remain regarding how to incorporate the long term effects of wave‐particle interactions in test‐particle simulations. In this paper, we describe in details two approaches that incorporate electron scattering in test particle simulations: stochastic differential equation (SDE) approach and the mapping technique. Both approaches assume that wave‐particle interactions can be described as a probabilistic process that changes electron energy, pitch‐angle, and thus modifies the test particle dynamics. To compare these approaches, we model electron resonant interactions with field‐aligned whistler‐mode waves in dipole magnetic fields. This comparison shows advantages of the mapping technique in simulating the nonlinear resonant effects, but also underlines that more significant computational resources are needed for this technique in comparison with the SDE approach. We further discuss applications of both approaches in improving existing models of energetic electron dynamics. Key Points: We discuss two approaches to incorporate resonant effects into test particle simulation modelsDetailed approaches have been shown for continuous stochastic differential equations (SDEs) and the mapping technique, respectivelyIn contrast to continuous SDE, the mapping technique allows one to simulate nonlinear resonant effects [ABSTRACT FROM AUTHOR]

The turbulent foreshock region upstream of the quasi‐parallel bow shock is dominated by waves and reflected particles that interact with each other and create a large number of different foreshock transients. The structures with the enhanced magnetic field, Short Large Amplitude Magnetic Structures, and density spikes named plasmoids are frequently observed. They are one of the suggested sources of transient flux enhancements or jets in the magnetosheath. Using measurements of the Magnetospheric Multiscale Spacecraft (MMS) and OMNI solar wind database between the 2015 and 2018 years, we have found that there is a category of events exhibiting both magnetic field and density enhancements simultaneously and we introduce the term "mixed structures" for them. Consequently, we divided our set of observations into three groups of events and present their comparative statistical analysis in the subsolar foreshock. Based on our results and previous research, we discuss the properties, possible origin and occurrence rates of these events under different upstream conditions and their possible relation to the jet and plasmoid formation in the magnetosheath. Plain Language Summary: The solar wind is a plasma stream of charged particles that expands outward from the solar corona. The solar wind on its path first encounters the bow shock, whose structure and properties depend strongly on the angle between the interplanetary magnetic field and the shock normal, θBn. Most of the foreshock phenomena, which are the main subject of this study, are observed for θBn < 45°, that is, upstream of the quasi‐parallel bow shock. The transients like ULF waves, the Short Large Amplitude Magnetic Structures, plasmoids, magnetic holes, etc. are created by interaction of the incoming solar wind and ions reflected from the bow shock. In this paper, we use Magnetospheric Multiscale (MMS) Mission data inside the foreshock and solar wind OMNI database for a statistical study of the properties of these compressive foreshock structures. We show differences between the morphology of the magnetic field, the temperature anisotropy and the occurrence rate of the structures and discuss possible implications for magnetosheath jets. Key Points: Both paramagnetic and diamagnetic plasmoids exist in the foreshockThe occurrence rate of the foreshock compressive structures increases on highly decelerated solar wind in the foreshockVelocity inside the structures is deflected from the surrounding foreshock, but deflection inside the plasmoids is very low [ABSTRACT FROM AUTHOR]

We present a case study of plasma and magnetic field observations in the Martian magnetotail using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission during an orbit when the spacecraft was in the optical shadow, past the dusk terminator and downstream of the strongest crustal magnetic fields. In this region, we observed multiple magnetic field rotations (a signature of currents) closely associated with energized (up to 100 eV) electron populations. Several transitions between closed and draped magnetic topologies also occur in this region, which are likely to be caused by magnetic reconnection between the interplanetary magnetic field (IMF) and crustal magnetic fields. We also observe two regions of energized, counter‐streaming electrons, which are rare in the magnetotail, but twice as likely to occur downstream of strong crustal magnetic fields when they are near the evening terminator. Together, the multiple magnetic field rotations, topological changes, and counter streaming electrons suggest the presence of an electric potential structure similar to those observed above the auroral arc regions at Earth. Plain Language Summary: Mars does not have a global dipole magnetic field, but a fraction of the planet's crust carries strong remanent magnetization, which affects the plasma environment. When the magnetized solar wind encounters Mars, its magnetic field drapes around the conducting ionosphere and crustal magnetic fields and forms a magnetotail downstream of the planet. Reconnection between the solar wind magnetic field and crustal magnetic fields occurs often, resulting in a complex and changing configuration of open, closed, and draped magnetic field lines. Reconnection and reconfiguration of the magnetic field releases stored energy and drives currents, which energize the plasma. The energized plasma can precipitate onto the atmosphere and cause aurora. In this paper, we present a case study of plasma and magnetic field observations when the MAVEN spacecraft was downstream of strong crustal magnetic fields located near the evening terminator. Multiple magnetic field rotations, topological changes, and energized plasma indicate that reconnection gives rise to a complex magnetic environment downstream of the crustal fields. We found counter‐streaming electrons in the magnetotail, an observation which is thought to be a signature of the potential structure that energizes the auroral electrons, and are commonly observed at high altitudes above auroral regions at Earth. Therefore, we consider the observations in this study to be possible precursors to discrete auroral activity in Mars' crustal magnetic field regions. Key Points: The Martian magnetotail exhibits complex morphology and topology downstream of strong crustal magnetic field sourcesMultiple magnetic field rotations on spatial scales that are small compared to the magnetotail dimensions suggest multiple current regionsRare counter‐streaming electrons, are twice as likely to occur downstream of crustal fields when they are located near the dusk terminator [ABSTRACT FROM AUTHOR]

The Plasma Environment Modeling in the Earth's Magnetosphere (PEMEM) is a European Space Agency activity supporting the development of a new specification model for the spacecraft surface charging risk assessment. This paper presents a description of the basic model version: the PEMEM percentile model. The model is intended to be used for space missions with near‐equatorial orbits. The model is based on the Van Allen Probes particle measurements inside the geostationary orbit. The model's primary input is a planned spacecraft trajectory. It outputs statistical characteristics of the plasma environment which are expected to be encountered during a mission lifetime. These characteristics include differential electron and proton flux percentiles for a set of energies (percentile spectra), and percentiles of the integrated electron flux. The model covers the energy range of 1–100 keV for electrons and 40 eV–51 keV for protons. Since extreme spacecraft charging usually occurs in the eclipse, the same characteristics can be separately output for the periods when the spacecraft is shadowed by the Earth. Key Points: The new specification model of cis‐GEO near‐equatorial plasma environment aimed at the surface charging risk assessment is presentedThe model covers the energy ranges 1–100 keV for electrons and 50 eV–50 keV for protonsFor a given orbital scenario, the model outputs flux percentiles that are expected to be encountered during the mission lifetime [ABSTRACT FROM AUTHOR]

High energy electron precipitation from the Earth's radiation belts is important for loss from the radiation belts and atmospheric chemistry. We follow up investigations presented in Reidy et al. (2021, https://doi.org/10.1029/2020ja028410) where precipitating flux is calculated inside the field of view of the POES T0 detector using quasi‐linear theory and pitch angle diffusion coefficients (Dαα) from the British Antarctic Survey (BAS). These results showed good agreements at >30 keV for L* >5 on the dawnside but the flux were too low at higher energies. We have investigated the effect of changing parameters in the calculation of the precipitating flux to improve the results for the higher energies using comparisons of in situ flux and cold plasma measurements from GOES‐15 and RBSP. We find that the strength of the diffusion coefficients rather than the shape of the source spectrum has the biggest effect on the calculated precipitation. In particular we find decreasing the cold plasma density used in the calculation of Dαα increases the diffusion and hence the precipitation at the loss cone for the higher energies, improving our results. The method of calculating Dαα is also examined, comparing co‐located rather than averaged RBSP measurements. We find that the method itself has minimal effect but using RBSP derived Dαα improved our results over using Dαα calculated using the entire BAS wave data base; this is potentially due to better measurements of the cold plasma density from RBSP than the other spacecraft included in the BAS wave data base (e.g., THEMIS). Plain Language Summary: High energy particles trapped in the Earth's radiation belt can enter the atmosphere, known as particle precipitation, and collide with atmospheric particles, which can change the atmospheric chemistry. This input into our atmosphere is key to understanding the effects of space weather on our climate system variability but is difficult to quantify. Reidy et al. (2021, https://doi.org/10.1029/2020ja028410) calculated the precipitation that would be measured by a low‐Earth orbiting satellite using wave‐particle theory and diffusion coefficients from a radiation belt model. Diffusion coefficients describe the amount of diffusion of the trapped radiation belt particle population driven by different sources (e.g., chorus waves). Reidy et al. (2021, https://doi.org/10.1029/2020ja028410) found good agreement between the calculated and measured precipitation for lower energy particles but found there was something missing for the higher energies. This paper investigates the impact of changing certain parameters within the calculations, finding the cold plasma density to be key in improving the results at higher energies. Key Points: Decreasing the cold plasma density increases calculated electron precipitation at the higher energiesCalculated precipitation for a low Earth orbiting detector is weakly dependent on the source spectrumRBSP derived diffusion coefficients provide more diffusion for precipitating particles than those derived from a larger wave database [ABSTRACT FROM AUTHOR]

Plasma escape from the high‐latitude ionosphere (ion outflow) serves as a significant source of heavy plasma to the magnetospheric plasma sheet and ring current regions. Outflows alter mass density and reconnection rates, hence global responses of the magnetosphere. A new fully kinetic and semi‐kinetic model, KAOS (Kinetic model of Auroral ion OutflowS), is constructed from first principles which traces large numbers of individual O+ ion macro‐particles along curved magnetic field lines, using a guiding‐center approximation, in order to facilitate calculation of ion distribution functions and moments. Particle forces include mirror and parallel electric field forces, a self‐consistent ambipolar electric field, and a parameterized source of ion cyclotron resonance wave heating, thought to be central to the transverse energization of ions. The model is initiated with a steady‐state ion density altitude profile and Maxwellian velocity distribution and particle trajectories are advanced via a direct simulation Monte Carlo scheme. This outlines the implementation of the kinetic outflow model, demonstrates the model's ability to achieve near‐hydrostatic equilibrium necessary for simulation spin‐up, and investigates L‐shell dependent wave heating and pressure cookers scenarios. This paper illustrates the model initialization process and numerical investigations of L‐shell dependent outflows and pressure cooker environments and serves to advance our understanding of the drivers and particle dynamics in the auroral ionosphere. Key Points: A new model, KAOS (Kinetic model of Auroral ion OutflowS), is introduced for studying ionospheric outflows from first principlesWe quantify dependence of upflowing/outflowing ion distributions on parameters pertaining to the characterization of pressure cookersMagnetospheric potential structures may transport highly transversely energized ion distributions to ionospheric altitudes [ABSTRACT FROM AUTHOR]

The emission of artificial electron beams causes an active spacecraft charging effect that poses challenges for electron beam sounding experiments. Plasma contactors have been suggested as a potential solution to this issue based on theoretical and simulation studies, but the impact of these contactors on spacecraft potentials in complex space environments remains unexplored. This paper investigates the fast active charging effect of spacecraft under the action of plasma contactors in the geostationary Earth orbit environment by using a two‐dimensional Particle‐in‐Cell model. We analyze and compare the effects of the "ion emission" process of the plasma contactor and the "electron collection" process of the background plasma on the spacecraft potential at different beam and environment parameters, as well as the coupling effects when both processes occur simultaneously. Our findings indicate that the "ion emission" and "electron collection" processes are the primary factors in determining the spacecraft potential in relatively thin and dense plasma environments, respectively. When the two processes occur together, the contactor plasma cloud increases the "electron collection" return current by expanding the range of the positive potential area around the spacecraft, while the dense background plasma mediates and provides the return current to ensure that the plasma contactor can effectively suppress the active charging effect even after the ion sheath is present. These results suggest that a reasonable adjustment of the contactor parameters based on the relationship between the electron beam and the background environment can effectively extend the normal emission time of the electron beam. Key Points: When the total current of "ion emission" and "electron collection" exceeds the beam current, the spacecraft reaches peak potentialDuring the "ion emission" process, the diffusion of the space plasma cloud raises the "electron collection" return currentBy acting as a medium and supplying return current, dense plasma suppresses the upward trend of spacecraft potential following an ion sheath [ABSTRACT FROM AUTHOR]

This paper reports the first high‐resolution 4‐dimensional ionospheric disturbance caused by hurricane Matthew on 7 October 2016. The temporal as well as vertical and horizontal variations of the concentric traveling ionospheric disturbances (CTIDs) were reconstructed using 3‐dimensional computerized ionospheric tomography (3DCIT) technology, based upon the Global Navigation Satellite System data from the dense receiver network over North America. The frequency range of disturbances was determined by spectrum analysis, and a Butterworth band‐pass filter was used to de‐trend the total electron content (TEC) sequences to determine TIDs. A remarkable CTID segment was detected at a distance of 1,000–1,500 km from the hurricane eye at ∼5:40–6:10 UT on 7 October 2016, moving westward with the horizontal phase velocity of ∼153.4 m/s, the period of ∼30 min and the horizontal wavelength of ∼276.1 km. The positive and negative wavefronts dominated the CITD at different times during the event. From 4:00 to 8:00 UT, the altitudinal variation of the CTIDs in electron density exhibited clear downward phase progression predominately in the range of 150–400 km altitudes; however, the percentage of electron density disturbances was larger below 250 km. The inverted cone‐like geometry of CTID wavefronts was presented. The vertical phase velocities of the CTIDs ∼1,100 km away from the hurricane eye in the northwest direction near 88°W, 34°N were ∼203.7–277.8 m/s, and at the same location, the horizontal phase velocities at 300 km altitude were ∼149.1–181.5 m/s, slightly larger than those at 200 km altitude (∼145.1–178.5 m/s). Key Points: The propagation features of concentric traveling ionospheric disturbances (CTID) caused by Hurricane Matthew on 7 October 2016 were analyzedFirst 4‐dimensional CTIDs caused by hurricane were reconstructed using ionospheric tomographyThe horizontal phase velocities at different altitudes and the vertical phase velocities of the ionospheric disturbances were estimated [ABSTRACT FROM AUTHOR]

A broad statistical study addresses for the first time an evolution of ultra‐low frequency waves and/or fluctuations in the terrestrial foreshock around the Moon generated through the interaction between the back‐streaming particles reflected from the bow shock and the incoming solar wind. It is supposed that the waves propagate sunward but they are convected by the solar wind flow back toward the bow shock and their amplitudes grow. However, our study shows that waves/fluctuations could be growing as well as decaying toward the bow shock under the quasi‐radial interplanetary magnetic field. We demonstrate that the growth rate is positive and larger for compressive variations of the magnetic field strength and density than for components of the magnetic field. We show that even if a possible influence of the Moon and its wake is excluded, the growth rate is decreased by nonlinear effects. Plain Language Summary: The paper presents an extensive statistical study of fluctuations in the distant foreshock. The study is based on observations of the ARTEMIS spacecraft pair orbiting around the Moon. The study is motivated by already published values of wave growth rate in this region that are in some sense contradictory. The analysis is based on ratios of standard deviations of the ion density and magnetic field magnitude and its components measured by two spacecraft. The growth rate was computed during intervals when both spacecraft were inside the foreshock in positions ensuring that the wave propagation from the upstream to downstream spacecraft cannot be influenced by the Moon or its wake. Although the determined growth rates can have different signs in individual events, we demonstrate that the growth rate is positive and larger for compressive variations of the magnetic field strength and density than for components of the magnetic field in a statistical sense. We also show that the growth rate is decreased by nonlinear effects. Key Points: Ultra‐low frequency waves/fluctuations in the distant foreshock around the Moon are statistically analyzedThe fluctuations could be growing as well as decaying toward the bow shock under the quasi‐radial interplanetary magnetic fieldThe wave growth rate is positive and larger for compressive variations of the magnetic field strength and plasma density [ABSTRACT FROM AUTHOR]

Previous statistical studies of plasmaspheric hiss investigated the averaged shape of the magnetic field power spectra at various points in the magnetosphere. However, this approach does not consider the fact that very diverse spectral shapes exist at a given L‐shell and magnetic local time. Averaging the data together means that important features of the spectral shapes are lost. In this paper, we use an unsupervised machine learning technique to categorize plasmaspheric hiss. In contrast to the previous studies, this technique allows us to identify power spectra that have "similar" shapes and study their spatial distribution without averaging together vastly different spectral shapes. We show that strong negative correlations exist between the hiss frequency and bandwidth. Key Points: Unsupervised machine learning is used to categorize hiss power spectra of the electric field from Van Allen ProbesFrom the pre‐noon sector toward the afternoon sector the hiss frequency decreases while the bandwidth increasesWe discuss possible source mechanisms that are consistent with the spatial distribution of the hiss waves [ABSTRACT FROM AUTHOR]

Jupiter's magnetic field is tilted by ∼10° with respect to the planet's spin axis, and as a result the Jovian plasma sheet passes over the Galilean satellites at the jovigraphic equator twice per planetary rotation period. The plasma and magnetic field conditions near Ganymede's magnetosphere therefore change dramatically every ∼5 hr, creating a unique magnetosphere‐magnetosphere interaction, and on longer time scales as evidenced by orbit‐to‐orbit variations. In this paper, we summarize the typical magnetic field conditions and their variability near Ganymede's orbit as observed by the Galileo and Juno spacecraft. We fit Juno data from orbit 34, which included the spacecraft's close Ganymede flyby in June 2021, to a current sheet model and show that the magnetospheric conditions during orbit 34 were very close to the historical average. Our results allow us to infer the upstream conditions at the time of the Juno Ganymede flyby. Plain Language Summary: Ganymede is the only moon in the solar system with an intrinsic magnetic field. This field forms a bubble in space around the moon, called a magnetosphere, that is itself contained within Jupiter's magnetosphere. The magnetic field and plasma conditions within Ganymede's magnetosphere can be used to infer information about the satellite's atmosphere, ionosphere, and interior. It is therefore important to understand the interaction between Ganymede's magnetosphere and the Jovian environment in the same way that we study the effects of space weather on the Earth. Here we analyze Galileo magnetic field measurements from Jupiter's magnetosphere in the region near Ganymede's orbit to establish the typical magnetic field magnitude and direction. We discuss the average conditions as well as the nature of the variability that occurs due to dynamic processes occurring in Jupiter's magnetosphere. This information provides useful context for analyzing data from Juno's recent flyby of Ganymede, which we show occurred during typical magnetospheric conditions. Key Points: The magnetic field magnitude and direction upstream of Ganymede vary strongly with longitudeTemporal variations in the magnetosphere also influence Ganymede's upstream field conditionsJuno's Ganymede flyby occurred during typical magnetospheric conditions [ABSTRACT FROM AUTHOR]

The recalibrated FAST/TEAMS data is used to study the response of O+ and H+ outflow to energy inputs in the nightside aurora during the 24–25 September 1998 geomagnetic storm, the same storm studied by Strangeway et al. (2005), https://doi.org/10.1029/2004JA010829. In contrast to the cusp, the Poynting flux and electron precipitation energy input are not as well correlated on the nightside, so their effects on outflow can be differentiated. The O+ outflow shows a strong correlation with both the Alfvénic Poynting flux (r = 0.71) and the soft electron precipitation (r = 0.69), while the H+ outflow only correlates well with the electron number flux (r = 0.74). This indicates that the auroral H+ outflow is close to its limiting flux without additional wave acceleration, while the outflow for the heavier O+ ion is increased by additional wave acceleration. Plain Language Summary: Geomagnetic activity can cause electrons to precipitate into the ionosphere in the nightside auroral region. It can also deliver electromagnetic wave energy to the same region. The electron precipitation can both heat and further ionize the ionosphere, leading to ions moving up along the field line. The wave energy can further accelerate the ions. If the ions are accelerated enough by these processes, they will flow out along the magnetic field, escaping the ionosphere. This paper finds that the H+ outflow increases with increased precipitating electrons in the nightside aurora. The O+ outflow increases with both electron precipitation and wave acceleration. Key Points: Electron precipitation and Poynting flux are less correlated on the nightside than the cusp, so their effects on outflow can be distinguishedO+ outflow is correlated with both Poynting flux and electron precipitation, while H+ is only correlated with electron precipitationParameterization of the outflow dependence is consistent between the dayside cusp and the nightside auroral regions [ABSTRACT FROM AUTHOR]

This paper quantifies and interprets how thermosphere dissipation in the form of molecular viscosity, molecular thermal conductivity and collisions with ions ("ion drag") determines the height versus latitude structures of a variety of migrating and non‐migrating solar thermal tides, pole‐to‐pole up to 400 km altitude. This is done through computation of thermosphere Hough Mode Extensions (HMEs); that is, solutions to the linearized momentum, thermal energy, continuity, state and hydrostatic balance equations wherein the horizontal structure of troposphere forcing for each HME corresponds to the eigenfunction (Hough function) of Laplace's tidal equation for a particular tidal mode, and the background state and thermosphere dissipation are specified for nominal solar minimum, average and maximum activity conditions. The broad features revealed, different for each HME, include changes in vertical(horizontal) structure with latitude(height), degree of vertical penetration to the middle and upper thermosphere, changes in vertical wavelength (λz) with height due to the transition in background thermal gradient around the mesopause, and insights into role of ion drag in determining tidal amplitudes and their solar cycle variability. The HMEs are particularly useful for fitting measurements of tides, and for estimating tidal fields beyond those dependent variables actually fit and including those outside the fitting domain. The HMEs reported here were created as part of the Ionospheric CONnection (ICON) mission to serve as observation‐based lower boundary conditions for Thermosphere Ionosphere Mesosphere General Circulation Model‐ICON. Access to the full set of HMEs in tabular and graphical form is provided. Key Points: Dissipation broadens amplitude(phase) latitudinal(vertical) structures with increasing height(latitude)Vertical wavelength shortening occurs with the shift to positive background thermal gradient above the mesopauseMolecular dissipation and ion drag result in decreased amplitudes with increasing solar activity at 400 km [ABSTRACT FROM AUTHOR]

The magnetopause is treated as a boundary where the pressure of the incoming solar wind is balanced by the pressure of the geomagnetic field and the plasma pressure inside the magnetopause is often neglected. However, published studies of pressure balance at the magnetopause reveal an excess of the magnetosheath pressure. Moreover, our survey of about 50,000 THEMIS magnetopause crossings shows that about 1% of them exhibits even larger magnetic field in the magnetosheath than in the magnetosphere. A subsequent analysis of crossings observed in the subsolar region under a southward interplanetary magnetic field shows the pressure of the dense cold ion population as an important component of the total magnetospheric pressure. This component is too cold to be registered by standard ion analyzers but its density can reach 300 cm−3. The second effect connected with a presence of cold plasma population is a reduction of the reconnection rate that slows down the transport of the magnetic flux down the tail and leads to magnetic pile‐up in the subsolar magnetosheath. Plain Language Summary: The magnetopause is usually treated as a boundary where the pressure of the incoming solar wind is balanced by the pressure of the geomagnetic field and the plasma pressure inside the magnetopause is often neglected. However, we have found a large number of subsolar magnetopause crossings exhibiting larger magnetic field in the magnetosheath than in the adjacent magnetospheric region. Taking into account the contribution of the magnetosheath plasma, the pressure balance at the magnetopause seems to be violated. Statistical analysis revealed that these events are observed predominantly during geomagnetic storms when the cold plasma from the plasmasphere comes toward the magnetopause. This plasma is too cold to be registered by the ion spectrometers but its density is sufficient to keep the pressure balance across the magnetopause as we demonstrate in two case studies. Our analysis explains contradiction that can be found in several already published papers pointing out a lack of the magnetospheric pressure at the magnetopause. Key Points: About 1% of subsolar magnetopause crossing exhibits larger magnetic field in the magnetosheath than in the magnetosphereSuch magnetopause configuration is typical for periods of geomagnetic stormsThe cold dense plasma of plasmaspheric origin was identified as a component keeping the pressure balance across the magnetopause [ABSTRACT FROM AUTHOR]

Man‐made very low frequency (VLF) transmitter waves play a critical role in energetic electron scattering and precipitation from the inner radiation belt, a type of which is called wisp precipitation. Wisps exhibit dispersive energy‐versus‐L spectra due to the evolution of electron cyclotron resonance conditions with near‐monochromatic VLF transmitter waves. Here, we report on such observations of inner belt wisp precipitation events with full pitch angle resolution in the energy range of 50 to ∼500 keV as measured by Electron Loss and Fields Investigation (ELFIN) at L < ∼2 between March 2021 and April 2022. Statistical observations (82 events) reveal occasional (18 events) wisp precipitation events with local bounce‐loss‐cone electron flux enhancements, which provide new information compared with flux enhancements measured in previous studies only in the drift loss cone. Based on magnetic field and plasmaspheric density models, quasilinear theory, and detailed pitch angle distributions of wisps from ELFIN, we have estimated the wisp electron bounce‐averaged pitch angle diffusion coefficients to be of the order of 10−4 to 10−2 s−1. These are several orders of magnitude larger than the diffusion rates calculated from models using global statistical averages of VLF transmitter wave power. When using our estimated diffusion coefficients to deduce the associated local transmitter wave amplitudes near the equator, based on quasilinear calculations from a transmitter‐induced electron diffusion model, we find these wave amplitudes to be >1 mV/m. Although probable overestimates, such inferred wave amplitudes exceed the theoretical threshold amplitude for nonlinear interactions, strongly suggesting that it is necessary to include nonlinear effects for an accurate evaluation of energetic electron scattering by transmitter waves. Plain Language Summary: Man‐made ground‐based very low frequency (VLF) transmitter waves can scatter and precipitate energetic radiation belt electrons, which pose a threat to telecommunication and human assets in space. Therefore, understanding the controlling parameters and the efficiency of energetic electron scattering by VLF transmitter waves are of both theoretical and practical importance. Our paper reports statistical analyses of inner belt wisp precipitation events due to scattering by VLF transmitter waves as observed by the Electron Loss and Fields Investigation CubeSats. Occasional strong local wisp precipitation events have been found to occur at preferential longitudes east of the South Atlantic Anomaly in the Northern Hemisphere and were primarily observed in the morning magnetic local time. These local wisp precipitation events have been used to infer energetic electron diffusion rates and VLF transmitter wave amplitudes near the equator based on quasilinear diffusion theory. Such inferred wave amplitudes exceed the theoretical threshold amplitude for nonlinear interactions, strongly suggesting that it is necessary to include nonlinear effects for an accurate evaluation of energetic electron scattering by transmitter waves. Key Points: We report statistical observations of inner belt wisps with full pitch angle distributions measured by Electron Loss and Fields InvestigationLocal bounce‐loss‐cone wisp precipitation events allow inference of quasilinear diffusion rates and transmitter wave amplitudesInferred transmitter wave amplitudes exceed nonlinear interaction threshold amplitudes near the equator [ABSTRACT FROM AUTHOR]

Auroral streamers have been found to be a particularly important cause of magnetic perturbations within the auroral oval during substorms. We present a detailed investigation on the association of equatorward extending auroral streamers with ground magnetic perturbations at locations initially equatorward of the auroral oval using Time History of Events and Macroscale Interactions imagers and ground magnetometers. Based on the analysis of four events, we show that negative perturbations in the horizontal (H) component at initially subauroral latitudes were not seen prior to streamer onset, and the peak negative H‐bay at subauroral latitudes coincided with the equatorward extending auroral streamers. Although stations close to the poleward boundary of auroral oval showed modest H‐component perturbations in response to the substorm onset and poleward auroral intensifications, the abrupt and large responses within the oval were associated with the onset of streamers. Results confirm that ground magnetic perturbations that are traditionally perceived as signatures of magnetic substorm onset are not always an auroral substorm onset but can also be related to auroral streamers. These results are supported by the remarkable correlation of positive bay onset and sudden Pi2 enhancement with the onset of auroral streamers, irrespective of the classical substorm onset. In addition, the magnitude of positive bay and amplitude of Pi2 pulsations, particularly at subauroral latitudes, are enhanced during the events of equatorward extending streamers. In general, all streamer events were accompanied by dipolarization and electron flux enhancement at geosynchronous orbit that continue to strengthen during the emergence of bright streamers. Plain Language Summary: Large, rapid perturbations in the ground magnetic field at high latitudes have been considered a major space weather hazard as they can harm technological systems. Such magnetic perturbations are common at auroral latitudes during geomagnetic storms and substorms. However, it is a matter of research to comprehend under what conditions such perturbations can extend equatorward of the auroral oval. The ground magnetic perturbations at auroral latitudes are shown to be linked with auroral streamers. Auroral streamers are roughly north‐south oriented narrow bands of auroral luminosity. They originate near the poleward edge of the auroral oval and propagate equatorward. Auroral streamers are related to flow bursts in the plasma sheet. The equatorward motion of auroral streamer is related to the earthward motion of flow bursts in the plasma sheet. This paper shows that the abrupt magnetic perturbations at latitudes equatorward of the auroral oval (i.e., nominal subauroral latitudes) are linked with the equatorward extending auroral streamers. The observations suggest that as the streamers extended to lower latitudes, the auroral oval expands equatorward to reach previously subauroral latitudes. We found that auroral streamers are, in general, followed by abrupt decreases in the H (horizontal)‐component at auroral and subauroral latitudes. Key Points: The onset of abrupt negative perturbations in H‐component at auroral and part of subauroral latitudes coincided with the onset of auroral streamerThe peak negative bay in H‐component at subauroral latitudes coincided with the equatorward extending auroral streamersBright equatorward extended auroral streamers were associated with the peak enhancement in electron flux at geosynchronous orbit [ABSTRACT FROM AUTHOR]

Thin current sheets (TCSs) are the key structures in space and laboratory plasmas responsible for the accumulation and subsequent release of the stored magnetic energy. Spacecraft measurements in the near Earth's space revealed the complicated multilayer TCSs structure consisting from extremely thin and strong electron current embedded inside the thicker ion current layer. In this paper we present for the first time the analytical self‐consistent model of 1D super thin electron current sheet (STCS) supported by magnetized electrons within a much wider (but still very thin) proton structure carried by nonmagnetized (quasi‐adiabatic) particles. We take into account the dependence of electron STCS on anisotropy of electron distribution and magnetic field shear often existing in realistic TCS configurations. It is shown that the structure of the embedded STCS can be described by the simple nonlinear equation, which allows to estimate the scaling of STCS thickness due to coupling of electron and ion domains, although electron dynamics described by guiding center drift approximation is scale free. The theoretical results are in a good agreement with MMS spacecraft observations in the Earth's magnetotail. Key Points: The model of extremely thin current layer is constructed and investigatedThe thickness of embedded electron current layer is estimated in a wide range of parameters of the modelComparison of MMS observational data with theoretical results revealed a good agreement [ABSTRACT FROM AUTHOR]

This paper analyses magnetosphere‐ionosphere (M‐I) coupling from a perspective that is independent of inertial reference frame, and delineates how physical theories of M‐I coupling are affected by the principle of relativity. For the first time in the context of M‐I coupling, we discuss the literature from the 1970s on how the low‐velocity limit of the theory of special relativity is applied to electrodynamics. In most M‐I coupling theories, a particular low‐velocity limit applies, known as the "magnetic limit." Two important consequences of this literature are: (a) significant displacement currents in Maxwell's equations break the Galilean invariance of the equations and (b) magnetic fields are not generated by currents created by a net charge density in motion. We show how reference frame‐independent descriptions of M‐I coupling require that ion‐neutral relative velocities and ion‐neutral collisions are key drivers of the physics. Currents are independent of reference frame whereas electric fields depend on reference frame. Starting with the same momentum equations that are typically used to derive Ohm's law, it is possible to express the perpendicular ionospheric current as depending on collisions between ions and neutrals, and electrons and neutrals, without reference to electric fields. Ignoring the relative motion between ions and neutrals results in errors exceeding 100% for estimates of high latitude Joule heating during significant geomagnetic storms, when ion‐neutral velocity differences are largest near the initiation of large‐scale ion convection. Plain Language Summary: Interactions between the magnetized and ionized solar wind, the magnetospheric cavity surrounding Earth, and Earth's ionized upper atmosphere (ionosphere) can create rapid (∼1 km/s) large‐scale motions of the ionosphere during periods known as geomagnetic storms. We use the principle of relativity (PR) to gain insight into the complex physics of these interactions. Relativity states that the physics governing geospace must be independent of the velocity of an observer making measurements of the system. We write key equations governing interactions of the magnetosphere‐ionosphere system in terms of quantities that do not depend on the observer's motion. In doing so, we find that some previous theories had over‐emphasized the importance of a large‐scale electric field that grows during storms, while neglecting important physics related to collisions between the ionized portion of the atmosphere and the un‐ionized "neutral" component that contains much more mass. These collisional interactions create upper atmospheric heating and expansion, and cause large‐scale currents to flow between the ionosphere and magnetosphere, resulting in a multitude of impacts to our technological society. Using the PR, and isolating the physics that is independent of observer motion, led us to a deeper understanding of key physical processes during storms. Key Points: Relativistic transformations applied to electrodynamics are analyzed in the context of magnetosphere‐ionosphere (M‐I) couplingWe present an "Ohm's law" relating horizontal ionospheric currents to quantities that do not vary with inertial reference frameElectrodynamic theories of M‐I coupling that do not account for the relative motion of ions and neutrals are not quantitatively accurate [ABSTRACT FROM AUTHOR]

This paper reports the climatology of medium‐scale traveling ionospheric disturbances (MSTIDs) observed between September 2018 and August 2019 in the nighttime redline airglow intensity measurements from an all‐sky imager located in Sutherland, South Africa (32.4°S, 20.8°E; magnetic latitude: 40.7°S). The nighttime MSTIDs appeared as either single or multiple dark bands, commenced mostly before midnight, and predominantly occurred during the southern winter solstice. They primarily propagated westward with speeds of 31–127 m/s. The periods and wavelengths of the multi‐band MSTIDs were in the ranges of 26–155 min and 134–578 km, respectively, while the temporal and spatial scales of the single band MSTIDs were 11–30 min and 43–143 km, respectively. Analysis of neutral wind measurements from a co‐located Fabry‐Perot interferometer showed that the thermospheric wind was mostly northeastward in the evenings of MSTID occurrence and that the favored westward propagation of the MSTIDs was the least restricted propagation direction by the background wind. Based on data from nearby ionosondes, the MSTIDs were mostly accompanied by sporadic E and spread‐F structures. From the seasonal occurrences of MSTIDs and sporadic E as well as the MSTID propagation and prevalent wind directions, it is concluded that Perkins instability is the most likely source of the majority of the observed nighttime MSTIDs, while the remainder may be seeded by gravity waves. Key Points: Properties of dark band structures representing nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed over South Africa presented for the first timeMSTIDs prevalent in southern hemisphere winter and primarily propagated westwardOccurrence of MSTIDs is mostly accompanied by sporadic and spread‐F, while propagation is supported by favorable neutral wind circulation [ABSTRACT FROM AUTHOR]

Mirror modes (MMs) are ubiquitous in space plasma and grow from pressure anisotropy. Together with other instabilities, they play a fundamental role in constraining the free energy contained in the plasma. This study focuses on MMs observed in the solar wind by Solar Orbiter (SolO) for heliocentric distances between 0.5 and 1 AU. Typically, MMs have timescales from several to tens of seconds and are considered quasi‐MHD structures. In the solar wind, they also generally appear as isolated structures. However, in certain conditions, prolonged and bursty trains of higher frequency MMs are measured, which have been labeled previously as MM storms. At present, only a handful of existing studies have focused on MM storms, meaning that many open questions remain. In this study, SolO has been used to investigate several key aspects of MM storms: their dependence on heliocentric distance, association with local plasma properties, temporal/spatial scale, amplitude, and connections with larger‐scale solar wind transients. The main results are that MM storms often approach local ion scales and can no longer be treated as quasi‐magnetohydrodynamic, thus breaking the commonly used long‐wavelength assumption. They are typically observed close to current sheets and downstream of interplanetary shocks. The events were observed during slow solar wind speeds and there was a tendency for higher occurrence closer to the Sun. The occurrence is low, so they do not play a fundamental role in regulating ambient solar wind but may play a larger role inside transients. Plain Language Summary: Plasma strives to be in equilibrium with little to no free energy. However, this is often not the case, especially in close proximity to complex structures such as shock waves and interplanetary coronal mass ejections (ICMEs). The latter is an eruption of plasma from the Sun that propagates outward into the solar system. In the presence of some free energy, instabilities will arise to remove it, one example is the mirror mode (MM) instability. Instabilities such as these are of extremely high importance to plasma physics as they act as a feedback mechanism to the plasma. Nevertheless, there are many open questions regarding the MM instability, especially when their properties are different from the most common scenarios. Typically, MMs in the solar wind appear as dips that are isolated structures. However, this paper investigates MMs when they appear as sudden bursts of magnetic peaks and dips and typically have smaller temporal scales. These kinds of MMs have been called MM storms. This study aims to address at what distances from the Sun they arise, what types of solar wind structures they are associated with, quantify their physical properties, and understand what local plasma conditions are important. Key Points: Mirror mode (MM) storms predominantly occurred during slow solar windHeliospheric plasma sheet crossings were effective at setting up MM unstable conditionsSpatial scales of MM structures approached and were smaller than ion‐scales [ABSTRACT FROM AUTHOR]

In this paper, we have modified the International Reference Ionosphere model 2016 (IRI2016) with the Global Navigation Satellite System total electron content data observation in China by adjusting the ionospheric index (IG12) for given location and time. The capability of the modified IRI2016 to reproduce the F2 layer critical frequency foF2 is validated for high/low solar activity conditions and disturbed geomagnetic conditions for the time period 2014–2017 over low‐latitude ionosonde station located at Sanya in China. The analysis shows that the modified IRI2016 better reproduced the variability of the observed foF2 versus time and seasons. Also, the modified IRI2016 foF2 prediction exhibits lower root mean square error and higher correlation coefficients (R) over different solar activity periods, and significantly reduces the average percentage deviations (△foF2(%)) under enhanced geomagnetic activity periods. Subsequently, the 24 hr ahead forecast foF2 map over China constructed by the combination of the modified IRI2016 and the eXtreme Gradient Boosting algorithm is presented for the first time. The promising performance of the modified IRI2016 indicates that this combination method may be an effective way to predict the ionospheric parameters, though further validations are needed in the future work. Key Points: The International Reference Ionosphere model 2016 (IRI2016) is modified with the Global Navigation Satellite System data by adjusting the ionospheric index for given location and time in ChinaThe modified IRI2016 improves the foF2 prediction for both high/low solar activity conditions and disturbed geomagnetic conditionsThe 24 hr ahead forecast foF2 map over China is constructed for the first time to our knowledge [ABSTRACT FROM AUTHOR]

This paper describes magnetospheric waves of very long wavelength in thin magnetic filaments. We consider an average magnetospheric configuration with zero ionospheric conductance and calculate waves using two different formulations: classic interchange theory and ideal magnetohydrodynamic (MHD) ballooning. Classic interchange theory, which is developed in detail in this paper, is basically analytic and is relatively straightforward to determine computationally, but it cannot offer very high accuracy. The eigenfrequencies from the two formulations cover a range of a factor of seven and generally agree with a root-mean-square difference between the common logarithms of interchange and MHD frequencies to be ∼0.054. The pressure perturbations in the classic interchange theory are assumed constant along each field line, but the MHD computed pressure perturbations along the field line vary in a range ∼30% in the plasma sheet but are larger in the inner magnetosphere. The parallel and perpendicular displacements, which are very different in the plasma sheet and inner magnetosphere, show good qualitative agreement between the two approaches. In the plasma sheet, the perpendicular displacements are strongly concentrated in the equatorial plane, whereas the parallel displacements are spread through most of the plasma sheet away from the equatorial plane; and can be regarded as buoyancy waves. In the inner magnetosphere, the displacements are more sinusoidal and are more like conventional slow modes. The different forms of the waves are best characterized by the flux tube entropy PVγ. [ABSTRACT FROM AUTHOR]

Declaration (Stauning, 2022, https://doi.org/10.1029/2022JA030355) on “invalid polar cap south (PCS) index” is based on the following arguments: PCS index is calculated with use of incorrect “unified” PC derivation method; PCS index used in analyses is a preliminary index, which was not approved by IAGA and, therefore, it cannot be regarded as a correct index; PCN and PCS indices demonstrate, intermittently, large difference in value, which should be treated as evidence of the PCS index invalidity. The paper presents comments to these arguments. Conclusion is made that criticism of the PCS index, presented in Stauning (2022, https://doi.org/10.1029/2022JA030355), is based on groundless arguments. [ABSTRACT FROM AUTHOR]

The occurrence and intensity of the geomagnetic storms/activity in the weak solar cycle 24 (SC24) were studied mainly for low latitudes, with intensity being the maximum value of the activity. The impulsive strength of the activity giving its mean value during the main phase, which can better indicate the effect of the activity on utility systems, has not received much attention at any latitude. In this paper, we investigate the intensity and impulsive strength of the 179 and 85 clear geomagnetic activities (DstMin ≤ −50 nT) identified in the low, mid and high latitude indices (SymH, Kp, and AE) and corresponding solar wind velocity V, IMF Bz, and the product V × Bz in solar cycles 23–24 (1996–2019) for the first time. Compared to SC23, the total intensity and total impulsive strength in SC24 in all latitudes reduce by nearly equal amounts (∼60%) as the reduction in the number of activities (∼53%). The average intensity and average impulsive strength, however, reduce by nearly equal and largest amounts in low latitudes (∼23%), which is close to the reduction in the combination <−(V × Bz)MP > by ∼30%. At mid and high latitudes, the average intensity and average impulsive strength reduce by only small amounts (∼7.5%). Only the impulsive strength of the geomagnetic activity at low latitudes (IpsSymH) and the combination <−(V × Bz)MP> identify the two power outages happened in SC23. The correlation between IpsSymH and <−(V × Bz)MP> is also high (0.83) in SC23. [ABSTRACT FROM AUTHOR]

Particle precipitation is a loss mechanism from the radiation belts whereby particles trapped by the Earth's magnetic field are scattered into the loss cone due to wave-particle interactions. Energetic electron precipitation creates ozone destroying chemicals which can affect the temperatures of the polar regions, therefore it is crucial to accurately quantify this impact on the Earth's atmosphere. We use bounce-averaged pitch angle diffusion coefficients for whistler mode chorus waves, plasmaspheric hiss and atmospheric collisions to calculate magnetic local time (MLT) dependent electron precipitation inside the field of view of the Polar Orbiting Environmental Satellites (POES) T0 detector, between 26–30 March 2013. These diffusion coefficients are used in the BAS Radiation Belt Model (BAS-RBM) and this paper is a first step toward testing the loss in this model via comparison with real world data. We find the best agreement between the calculated and measured T0 precipitation at L* > 5 on the dawnside for the >30 keV electron channel, consistent with precipitation driven by lower band chorus. Additional diffusion is required to explain the flux at higher energies and on the dusk side. The POES T0 detector underestimates electron precipitation as its field of view does not measure the entire loss cone. We demonstrate the potential for utilizing diffusion coefficients to reconstruct precipitating flux over the entire loss cone. Our results show that the total precipitation can exceed that measured by the POES >30 keV electron channel by a factor that typically varies from 1 to 10 for L* = 6, 6.5, and 7. [ABSTRACT FROM AUTHOR]

The magnetospheric cusp plays an essential role in mass and energy transportation from the solar wind to the geo-space environment. Recent development of the X-ray imaging technique makes it possible to study the large scale properties of the cusp region from a global view. As the detected X-ray signals will be integrated along the line-of-sight (LOS), it is a challenging task to derive the information about the 3D cusp region from a 2D X-ray image. Based on global magnetohydrodynamic (MHD) code, this paper simulated and analyzed the X-ray image of cusp. An approach was developed to present the following aspects related to the morphology of cusp from the X-ray image: (1) The tangent direction of the cusp boundary is the direction with appreciable increase of local standard deviation in X-ray intensity, and (2) the direction running through the cusp center is the direction with peak X-ray intensity. Lack of knowledge on radial distribution of X-ray signal along the line-of-sight, exact position of cusp cannot be further derived without assumptions. Nevertheless, under special solar wind conditions such as zero Interplanetary Magnetic Field (IMF) By, it is reasonable to assume that the cusp center is in the noonmidnight meridian plane. Therefore, position of the cusp center can be reconstructed. Based on MHD simulations, the above conclusions have been validated with different viewing geometries and solar wind conditions. With reasonable assumptions, the large-scale cusp features in response to the solar wind variations can be clearly revealed by analyzing X-ray images. [ABSTRACT FROM AUTHOR]

This paper reports, for the “first time,” the delayed response of O1D 630.0 nm dayglow emissions over Trivandrum (8.5°N, 77°E, 0.5°N dip lat.), a geomagnetic dip equatorial station in India, to the noontime X‐class solar flare event of July 30, 2005. The dayglow measurements were made using a unique dayglow photometer operating at three wavelengths. The Equatorial Electrojet induced magnetic field, measured using a proton precession magnetometer, showed a magnetic spike having ∼90 nT enhancement during this flare with a time delay of ∼7.2 min. A noteworthy observation is that unlike to the conventional belief, the O1D 630.0 nm dayglow over the dip equator exhibited a fourfold enhancement during the noontime flare after a time delay of ∼45 min. Analysis of satellite measured electron density and modeling simulations indicate that the thermospheric O1D 630.0 nm dayglow emission over the dip equatorial region during a solar flare is primarily driven by the electrodynamics, rather than the direct solar control. This finding is new, unique and very important for the studies related to plasma‐neutral coupling and also for modeling studies on the equatorial thermosphere‐ionosphere region is concerned.Key Points: This paper discusses an unusual and delayed response of O1D 630.0 nm dayglow emissions over the geomagnetic equator, during an X‐class flareAnalysis revealed that the dayglow emission over the dip‐equator during a flare is primarily driven by the equatorial electrodynamicsThe equatorial fountain brings the flare‐enhanced ionization from the lower F‐region to the emission centroid and increases the dayglow [ABSTRACT FROM AUTHOR]

The paper is devoted to study of the field line resonance in the three-dimensionally inhomogeneous model of the magnetosphere taking into account the plasma inhomogeneity across the magnetic shells, along the field lines and in the azimuthal direction. The emphasis is made on the azimuthally small scale modes since they allow approximate analytical investigation. It was found that the Alfvén mode can propagate only in a channel bounded by the toroidal and poloidal surfaces where the wave's frequency matches the toroidal and poloidal eigenfrequency, correspondingly. The resonance surface is located inside the channel. It is a separatrix dividing energy flows from the toroidal and poloidal surfaces. The separatrix is a surface where the wave energy is accumulated. The approach to the separatrix surface is accompanied by decrease of the azimuthal wavelength up to zero, the crossing is accompanied by jump of the wave amplitude. The wave amplitude on the resonance surface remains finite. [ABSTRACT FROM AUTHOR]

High‐resolution electron and ion velocity distributions, f(v), which consist of N effectively disjoint beams, have been measured by NASA's Magnetospheric Multiscale Mission and in reconnection simulations. Commonly used standard velocity moments assume a single mean‐flow velocity for the entire distribution. This can lead to counterintuitive results for a multibeam f(v). An example is the standard thermal energy density moment (at a given space‐time point) of a pair of equal and opposite cold particle beams. This standard moment is nonzero even though each beam has zero thermal energy density. By contrast, a multibeam moment of two or more cold beams at a given position and time has no thermal energy. A multibeam moment is obtained by taking a standard moment of each beam and then summing over beams. In this paper we will generalize these notions, explore their consequences, and apply them to an f(v) which is a sum of tri‐Maxwellians. Both standard and multibeam energy moments have coherent and incoherent pieces. Examples of incoherent moments are the thermal energy density, the pressure, and the thermal energy flux (enthalpy flux plus heat flux). Corresponding coherent moments are the bulk kinetic energy density, the ram pressure, and the bulk kinetic energy flux. The difference between a standard incoherent moment and its multibeam counterpart will be defined as the "pseudothermal part" of the standard moment. The sum of a pair of corresponding coherent and incoherent moments is the undecomposed moment. Undecomposed standard moments are always equal to the corresponding undecomposed multibeam moments. Key Points: Standard velocity moments of effectively disjoint measured multibeam particle velocity distributions, f(v), can be misleadingSo‐called multibeam moments of multibeam f(v) are found by taking a standard moment of each beam and then summing over beamsStandard thermal moments (pressure, thermal flux, etc.) of multibeam f(v) contain pseudothermal parts which are absent in multibeam moments [ABSTRACT FROM AUTHOR]