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Auroral particle precipitation is usually assumed to be equally strong for both signs of the By component of the interplanetary magnetic field (IMF). This is also the case in most currently used precipitation models, which parameterize solar wind driving by empirical coupling functions. However, recent studies have showed that geomagnetic activity is significantly modulated by the signs and amplitudes of IMF By and the Earth's dipole tilt angle Ψ. This so called explicit By dependence is not yet included in any current precipitation models. In this paper, we quantify this By dependence for auroral electron precipitation for the first time. We use precipitation measurements of the Defense Meteorological Satellite Program (DMSP) Special Sensor J instruments from years 1995–2022. We show that the dawnside electron precipitation at energies 13.9–30 keV is greater at auroral latitudes for opposite signs of By and Ψ in both hemispheres, while the dusk sector is mostly unaffected by By and Ψ. For energies below 6.5 keV the By dependence is strong poleward of the auroral oval in the summer hemisphere, also exhibiting a strong dawn‐dusk asymmetry. We also show that By dependence of precipitation modulates ionospheric conductance. Plain Language Summary: Electron precipitation into the Earth's ionosphere creating the aurora borealis is driven by the magnetic field carried by the solar wind. The electron precipitation is known to increase the most, when the magnetic field of the solar wind points to south. The currently used models assume equally strong electron precipitation for eastward and westward solar wind magnetic fields. However, in this paper we show that the electron precipitation is different for duskward and dawnward magnetic fields, and that this magnetic field dependence varies with the season. We show that the electron precipitation is stronger on the dawnside in both hemispheres during the northern hemisphere winter when the solar wind magnetic field points duskward. In northern hemisphere summer the dependence on the magnetic field direction is opposite. We show that the solar wind magnetic field modulates the ionospheric conductivity in a similar way. Key Points: IMF By and Earth's dipole tilt angle Ψ (season) modulate auroral electron precipitationPrecipitation is stronger for 13.9–30 keV at auroral latitudes on the dawnside for both hemispheres, when By and Ψ have opposite signsBy drives a strong dawn‐dusk difference for energies 1.4–6.5 keV in the summer hemisphere [ABSTRACT FROM AUTHOR]

The analytical theory is presented that describes the propagation of power lines emission (PLE) with frequency of 50/60 Hz in the heights range from the Earth surface to the magnetosphere. Validation of the theory is made by the comparison with earlier published results of numerical modeling. It is shown that the actual source of emission is a magnetic dipole formed by the power line current and by the secondary image current in the ground. The emission is propagating to the lower boundary of ionosphere, where its main part is reflected back, but some of the energy (a few percent) penetrates into the ionosphere. There it is transformed into a quasi‐flat whistler wave. The generation of current in ground and the reflection from the ionosphere are the main factors that reduce the emission into space. In the ionosphere wave fronts propagate approximately vertically, and the energy propagates in a certain direction that depends on the geomagnetic field inclination. Thus, the ionosphere acts as a focusing system that collects PLE into a unidirectional beam. The PLE intensity does not change with altitude within the total range of ionospheric heights. In the magnetosphere PLE is transformed to both magnetosonic and Alfvén waves and the emission splits into two rays: one propagates along the wave vector and the other one—along the geomagnetic field lines. A set of analytical solutions is presented allowing determining the change in PLE parameters with altitude depending on the source parameters and ionospheric conditions. Plain Language Summary: The Earth's electromagnetic environment is influenced by both natural and human activities. Currently, there is a noticeable increase in human‐induced activity in the near space, and its potential environmental effects are hard to predict. Among the various forms of electromagnetic waves in the extra‐low frequency range that travel from Earth into space, the most common is the emission from power lines operating at industrial frequencies of 50/60 Hz. Recently, satellites have been able to detect this emission, offering a promising opportunity for remotely observing and assessing the characteristics of terrestrial sources of electrical energy, including those that are hidden. This paper presents an analytical theory that describes the propagation of this emission from the Earth's surface to the magnetosphere. Initially, the emission behaves like a conventional radio wave, traveling through the neutral atmosphere at nearly the speed of light. However, as it ascends and undergoes multiple transformations in the ionosphere layers, it transitions into slow plasma waves of the magnetohydrodynamic nature. The paper provides a set of analytical solutions that enable the determination of emission parameters based on source characteristics and conditions along the wave's propagation path. Key Points: The analytical theory of the propagation of 50/60 Hz electromagnetic emission from the Earth surface to the magnetosphere is presentedAt ionospheric heights this emission turns into whistler‐wave, and then, in the magnetosphere, into magnetohydrodynamic wavesThe generation of image current in ground and the reflection from the bottom of the ionosphere are the factors that reduce energy emission to space [ABSTRACT FROM AUTHOR]

Universal Time (UT) variations in many magnetospheric state indicators and indices have recently been reviewed by Lockwood and Milan (2023, https://doi.org/10.3389/fspas.2023.1139295). Key effects are introduced into magnetospheric dynamics by the eccentric nature of Earth's magnetic field, features that cannot be reproduced by a geocentric field model. This paper studies the UT variation in the occurrence of substorm onsets and uses a simple Monte‐Carlo model to show how it can arise for an eccentric field model from the effect of the diurnal motions of Earth's poles on the part of the geomagnetic tail where substorms are initiated. These motions are in any reference frame that has an X axis that points from the center of the Earth to the center of the Sun and are caused by Earth's rotation. The premise behind the model is shown to be valid using a super‐posed epoch study of the conditions leading up to onset. These studies also show the surprising degree of preconditioning ahead of the growth phase that is required, on average, for onset to occur. A key factor is the extent to which pole motions caused by Earth's rotation influence the near‐Earth tail at the relevant X coordinate. Numerical simulations by a global MHD model of the magnetosphere reveal the effect required to generate the observed UT variations and with right order of amplitude, albeit too small by a factor of about one third. Reasons why this discrepancy may have arisen for the simulations used are discussed. Plain Language Summary: Earth's magnetic field is eccentric in that the main magnetic (dipole) axis does not pass through the center of the Earth. This introduces a wobble into many aspect of near‐Earth space (the "magnetosphere") as Earth rotates. Many consequences of this have been noted in previous papers. This paper investigates the effect of the eccentricity on the phenomenon of magnetospheric substorms. It is shown that the explosive releases of energy stored in the tail of the magnetosphere are more likely to start ("onset") at some Universal Times (and therefore geographic longitudes) than others and an explanation of why is provided. Key Points: Universal Time (UT) effects in the magnetosphere are caused by the eccentric nature of Earth's intrinsic magnetic fieldThere is a UT dependence of the open flux (and hence also the integrated magnetopause reconnection voltage) needed to trigger substorm onsetGrowth phases that lead to substorm onset show considerable preconditioning by prior reconnection [ABSTRACT FROM AUTHOR]

Rapid increases in solar wind dynamic pressure, known as solar wind pressure pulses, compress the Earth's magnetosphere and can rapidly restructure the electrodynamics within. The propagation of pressure pulse effects into the magnetosphere is known as a geomagnetic sudden commencement (SC). SCs can be further subdivided into compressions which are rapidly followed by a geomagnetic storm (a sudden storm commencement, SSC) and those which are not (a sudden impulse, SI). In this paper, SSCs and SIs are compared and contrasted, and we examine in particular the differences between the pressure pulses that drive SSCs/SIs, and explore the physical conditions of the magnetosphere before pressure pulse arrival. Firstly, it is shown that SSCs are more likely to be driven by pressure pulses with higher magnitude and/or shorter rise time. Secondly, the magnetosphere is primed by stronger driving conditions and higher geomagnetic activity prior to SSCs than SIs. Finally, there is a solar cycle dependence in the occurrence and magnitude of solar wind pressure pulses. Plain Language Summary: The solar wind's dynamic pressure controls the size of the cavity the Earth's magnetic field forms in space. When the pressure increases rapidly, this is known as a solar wind pressure pulse. Pressure pulses can affect currents and magnetic fields within the Earth's space environment, and can have serious space weather implications. In this paper, the differences between pressure pulses that are followed by geomagnetic storms (large releases of energy in the Earth's system), and those that are not are investigated. The state of the Earth's system before the arrival of the pressure pulse is also considered, and its contribution to the resulting space weather effects. It is shown that larger pressure pulses are more likely to trigger a geomagnetic storm. Additionally, analysis presented shows that the Earth's system may already be active prior to the triggering of these storms. Finally, the occurrence and strength of pressure pulse events follows solar activity. Key Points: There is a solar cycle dependence to the occurrence and magnitude of sudden commencements (SCs)Sudden storm commencements (SSCs) are driven by higher magnitude and/or shorter duration pressure pulsesThe magnetosphere is primed by stronger solar wind/interplanetary magnetic field and higher geomagnetic activity for SSCs [ABSTRACT FROM AUTHOR]

Quantification of energetic electron precipitation caused by wave‐particle interactions is fundamentally important to understand the cycle of particle energization and loss of the radiation belts. One important way to determine how well the wave‐particle interaction models predict losses through pitch‐angle scattering into the atmospheric loss cone is the direct comparison between the ionization altitude profiles expected in the atmosphere due to the precipitating fluxes and the ionization profiles actually measured with incoherent scatter radars. This paper reports such a comparison using a forward propagation of loss‐cone electron fluxes, calculated with the electron pitch angle diffusion model applied to Van Allen Probes measurements, coupled with the Boulder Electron Radiation to Ionization model, which propagates the fluxes into the atmosphere. The density profiles measured with the Poker Flat Incoherent Scatter Radar operating in modes especially designed to optimize measurements in the D‐region, show multiple instances of close quantitative agreement with predicted density profiles from precipitation of electrons caused by wave‐particle interactions in the inner magnetosphere, alternated with intervals with large differences between observations and predictions. Several‐minute long intervals of close prediction‐observation approximation in the 65–93 km altitude range indicate that the whistler wave‐electron interactions models are realistic and produce precipitation fluxes of electrons with energies between 10 keV and >100 keV that are consistent with observations. The alternation of close model‐data agreement and poor agreement intervals indicates that the regions causing energetic electron precipitation are highly spatially localized. Plain Language Summary: Establishing how electromagnetic waves in the magnetosphere push high‐energy electrons into a funnel directed toward Earth along magnetic field lines is a critically important step to predict how electrons are lost into the atmosphere during geomagnetic storms. Wave‐electron interaction models predict the number of electrons that are funneled toward Earth from a set of in‐situ spacecraft measurements. As the electrons hurl into the upper atmosphere, ionization models predict how many electrons would be released by the neutral atmosphere due to the bombardment from funneled electrons. Electron density measurements with a radar especially tuned to optimize detection of electron densities in the upper atmosphere can be compared to predicted electron densities to determine the validity of the electron loss models. This paper reports a comparison for an interval of time when a Van Allen Probes spacecraft is measuring the waves and electrons at a location that would guarantee that the electrons would fall near the radar's location in Poker Flat, Alaska. The comparison shows that the models considered predict the correct number of electrons in multiple instances, thus establishing an important step in verifying the validity of the models of electron loss during geomagnetic storms. Key Points: Comparison between observed and modeled density for electron precipitation due to wave‐particle interactions in the magnetosphereComparison verifies the validity of D‐region electron density predicted by pitch‐angle diffusion models of wave‐particle interactionObserved electron profiles are obtained with an incoherent scatter radar mode especially designed to optimize measurements in the D‐region [ABSTRACT FROM AUTHOR]

In this paper, we statistically investigate an artifact in Langmuir Probe (LP) observations of Swarm satellites. A small peak of electron density (Ne) is frequently found in the Swarm data around the dayside dip equator. On the contrary, they appear in neither the Total Electron Content data of the Swarm/Global Positioning System Receivers nor COSMIC‐2 in‐situ measurements at similar altitudes but with low orbit inclination. Arguably, this peak does not represent natural ionospheric irregularities but is likely to result from artifacts. The phenomena are found regardless of the season, solar activity, and the velocity direction of the satellite (ascending and descending). They predominantly occur when the magnetic declination is close to zero, that is, when the Swarm ram direction and the Earth's magnetic field are aligned under sunlight. Hence, we attribute the phenomenon to intensified secondary electrons escape when the geomagnetic field lines are normal to conducting surfaces that emit secondary electrons. Since the magnitude of the artifact is only a few percent of the large‐scale background, it does not have a serious impact on the value of the Swarm/LP data in scientific research. Nevertheless, future efforts to determine the exact cause of the artifacts will contribute to improving the reliability and quality of plasma density and temperature measured by Swarm/LP. Plain Language Summary: In this paper, we statistically investigate artifacts found in Langmuir Probe (LP) observations of Swarm satellites and search for their possible causes. LP is an instrument that measures the density and temperature of charged particles (so‐called "plasma") around the satellite. Unexpectedly, small peaks of plasma density often occur around the dayside dip equator in the Swarm/LP data, while they are not at all observed in other similar equipment, such as Swarm Global Positioning System Receivers or COSMIC‐2 satellites' ion probes at similar altitudes. For this reason, we deem this peak an artifact. This phenomenon occurs in all seasons, solar activity, and satellite movement directions. But, it predominantly appears when the ram direction of Swarm is nearly aligned with the Earth's magnetic field under sunlight. Hence, we consider that it originates from enhanced secondary electrons escape under a particular magnetic field configuration. The artifacts do not have a deleterious impact on scientific research because they are only a few percent of the background. However, efforts to determine the exact cause of artifacts are warranted for improving the reliability and quality of LP data. Key Points: Swarm encounters peaks of electron density at the dayside dip equator under zero magnetic declination, which is arguably an artifactThe artifacts occur for all seasons and solar activity, and for both ascending/descendingWe suggest that the artifacts are associated with enhanced secondary electrons escape under favorable magnetic field configurations [ABSTRACT FROM AUTHOR]

Exospheric temperature is one of the key parameters in constructing thermospheric models and has been extensively studied with in situ observations and remote sensing. The Global‐scale Observations of the Limb and Disk (GOLD) at a geosynchronous vantage point provides dayglow limb images for two longitude sectors, from which we can estimate the terrestrial exospheric temperature since 2018. In this paper, we investigate climatological behavior of the exospheric temperature measured by GOLD. The temperature has positive correlations with solar and geomagnetic activity and exhibits a morning‐afternoon asymmetry, both of which agree with previous studies. We have found that the arithmetic sum of F10.7 (solar) and Ap (geomagnetic) indices is highly correlated with the exospheric temperature, explaining ∼64% of the day‐to‐day variability. Furthermore, the exospheric temperature has good correlation with thermospheric parameters (e.g., neutral temperature, O2 density, and NO emission index) sampled at various heights above ∼130 km, in spite of the well‐known thermal gradient below ∼200 km. However, thermospheric temperature at altitudes around 100 km is not well correlated with the GOLD exospheric temperature. The result implies that effects other than thermospheric heating by solar Extreme Ultraviolet and geomagnetic activity take control below a threshold altitude that exists between ∼100 and ∼130 km. Plain Language Summary: In the terrestrial thermosphere, which is the topmost layer of the collisional atmosphere surrounding the Earth, temperature generally increases with altitude unlike in the troposphere near the surface. However, the thermospheric temperature does not increase indefinitely, but approaches an asymptote, which is referred to as the exospheric temperature, the temperature of the outermost layer of the atmosphere. The Global‐scale Observations of the Limb and Disk (GOLD) instrument, launched into a geosynchronous orbit in 2018, regularly observes atmospheric glow from the Earth's limb, from which we can estimate the terrestrial exospheric temperature (Texo). The GOLD Texo is higher in the afternoon than in the morning and generally increases as solar radiation strength or geomagnetic field variability increases. In this paper, we show that a simple arithmetic sum of the solar and geomagnetic indices can explain a large part of the day‐to‐day Texo variability. Also, Texo has good (poor) correlation with thermospheric parameters such as temperature and density above ∼130 km (at ∼100 km). The behavior suggests that factors other than solar radiation and geomagnetic energy deposition, both of which are correlated well with Texo, take control below a threshold altitude that is between ∼100 and ∼130 km. Key Points: We report on the climatology of exospheric temperature (Texo) measured by NASA‐GOLD in 2018–2021Texo is highly correlated with Ap and F10.7, whose linear combination can explain ∼64% of the Texo day‐to‐day variability in 2018–2021Texo has positive correlation with Swarm thermospheric mass density, GOLD disk temperature, GOLD O2 density, and TIMED/SABER TCI [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]

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]

Geomagnetic conjugate mid‐latitude nighttime ionospheres are frequently simultaneously populated with electrified nighttime medium‐scale traveling ionospheric disturbances (MSTIDs). Earlier observations and theoretical analysis have underscored the ionosphere E‐F coupling and the postulation of coupled conjugate hemispheres, playing a pivotal role in the formation of electrified MSTIDs. In this paper, the conjugate MSTIDs are studied to elucidate the causes and effects of E‐F coupling in the interhemispheric coupled ionosphere. The hemisphere‐coupled ionospheres over Japan and Australia are observed and analyzed using total electron content (TEC) measurements, supplemented with multi‐source observations from ionosondes, Ionospheric Connection Explorer (neutral wind), Constellation Observing System for Meteorology, Ionosphere, and Climate (electron density), and Swarm (magnetic field). A double‐thin‐shell model is introduced to analyze the ionospheric responses in E and F regions during the coupling process. For the first time, observation results provide the evidence that F‐region geomagnetic conjugate irregularities in both hemispheres are mainly driven by the Es layers in the summer hemisphere. The Es layer in the summer hemisphere subsequently triggers local E‐F coupling and inter‐hemispheric coupling. In the winter hemisphere, Es layers show amplitude reduction or even dissipation during the interhemispheric coupling process. Furthermore, thermospheric winds, non‐equipotential magnetic field lines, and background TEC are presumed candidates for the inter‐hemispheric asymmetry in MSTIDs amplitudes and growth rates. Key Points: The F‐region geomagnetic conjugate medium‐scale traveling ionospheric disturbances (MSTIDs) in both hemispheres are mainly driven by the Es layers in the summer hemispherePreexisting Es layers in the winter hemisphere show intensity reduction or even dissipation after the interhemispheric couplingThe amplitude and growth rate of hemispheric conjugate MSTIDs are smaller in the winter hemisphere [ABSTRACT FROM AUTHOR]

In this paper, we provide statistical evidence that the level of solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling is weaker under radial (Sun‐Earth component dominant) interplanetary magnetic field (IMF) conditions than non‐radial IMF conditions. This is performed by analyzing auroral electrojet activity (using SuperMAG auroral electrojet indices) in the sunlit and dark ionospheres for long‐duration (at least 4 hr) radial IMF events and comparing against the same for long‐duration azimuthal (dusk‐dawn component dominant) IMF events. We show that the north‐south IMF component (IMF Bz) plays a crucial role in controlling the level of auroral electrojet activity as a negative half‐wave rectifier even for both IMF orientation categories. However, it is found that the magnitudes of the auroral electrojet indices are generally lower for radial IMF than for azimuthal IMF under similar sets of solar wind (radial bulk velocity and number density) and IMF Bz conditions, regardless of whether these indices are derived in the sunlit or dark regions. Moreover, the efficiency of coupling functions is lower for radial IMF than for azimuthal IMF, implying that increased coupling strength due to the azimuthal IMF component alone cannot well explain weaker auroral electrojets during radial IMF periods. Lastly, the contribution of the radial IMF component itself to auroral electrojet activity is also lower compared to the azimuthal IMF component. Our results suggest that the level of SW‐M‐I coupling characterized by auroral electrojet activity can be modulated by the radial IMF component, although the effect of this component is weaker than the other two IMF components. Plain Language Summary: The ionospheric electric current that flows along the auroral oval, called the auroral electrojet, is one of the manifestations of solar wind energy transfer into the magnetosphere‐ionosphere system via magnetic reconnection processes between the interplanetary and terrestrial magnetic field lines. Previous studies mainly focused on the role of interplanetary magnetic field (IMF) orientation in the plane perpendicular to the Sun‐Earth line (approximately solar wind flow direction) in controlling auroral electrojet activity, based on the dayside magnetopause magnetic field topology viewed from the Sun. In this study, we turn our attention to auroral electrojet activity under radial (nearly parallel to the Sun‐Earth line) IMF conditions by statistically examining the SuperMAG auroral electrojet indices for radial IMF intervals and comparing them with those for non‐radial IMF intervals under similar sets of interplanetary conditions. Our statistical results reveal that both the auroral electrojet intensity and the coupling efficiency between interplanetary and auroral electrojet parameters are lower under radial IMF conditions than non‐radial IMF conditions. These findings indicate that the radial component of the IMF can modulate the level of solar wind‐magnetosphere‐ionosphere coupling, although its effect is weaker than the other IMF components. Key Points: Auroral electrojet intensity is typically lower for radial interplanetary magnetic field (IMF) than for azimuthal IMF under similar IMF Bz and solar wind conditionsIncreased coupling strength due to IMF By alone cannot well explain weaker auroral electrojets during radial IMF compared to azimuthal IMFThe level of solar wind‐magnetosphere‐ionosphere coupling can be modulated by the radial IMF component as well as the other two IMF components [ABSTRACT FROM AUTHOR]

Surface waves on Earth's magnetopause have a controlling effect upon global magnetospheric dynamics. Since spacecraft provide sparse in situ observation points, remote sensing these modes using ground‐based instruments in the polar regions is desirable. However, many open conceptual questions on the expected signatures remain. Therefore, we provide predictions of key qualitative features expected in auroral, ionospheric, and ground magnetic observations through both magnetohydrodynamic theory and a global coupled magnetosphere‐ionosphere simulation of a magnetopause surface eigenmode. These show monochromatic oscillatory field‐aligned currents (FACs), due to both the surface mode and its non‐resonant Alfvén coupling, are present throughout the magnetosphere. The currents peak in amplitude at the equatorward edge of the magnetopause boundary layer, not the open‐closed boundary as previously thought. They also exhibit slow poleward phase motion rather than being purely evanescent. We suggest the upward FAC perturbations may result in periodic auroral brightenings. In the ionosphere, convection vortices circulate the poleward moving FAC structures. Finally, surface mode signals are predicted in the ground magnetic field, with ionospheric Hall currents rotating perturbations by approximately (but not exactly) 90° compared to the magnetosphere. Thus typical dayside magnetopause surface modes should be strongest in the East‐West ground magnetic field component. Overall, all ground‐based signatures of the magnetopause surface mode are predicted to have the same frequency across L‐shells, amplitudes that maximize near the magnetopause's equatorward edge, and larger latitudinal scales than for field line resonance. Implications in terms of ionospheric Joule heating and geomagnetically induced currents are discussed. Plain Language Summary: Waves on the boundary of the magnetosphere, the magnetic shield established by the interplay of the solar wind with Earth's magnetic field, play a controlling role on energy flow into our space environment. While these waves can be observed as they pass over satellites in orbit, due to the small number of suitable satellites available it would be helpful to be able to detect these waves from the surface of the Earth with instruments that measure the northern/southern lights, motion of the top of our atmosphere, or magnetic field on the ground. However, we do not currently understand what the signs of these waves should look like in such instruments. In this paper we develop theory and use computer simulations of these boundary waves to predict key features one might expect to measure from the ground. Based on these predictions, we also discuss how the waves might contribute to the hazards of space weather. Key Points: Theory and global simulations of magnetopause surface waves' effects on the aurorae, ionosphere, and ground magnetic field are investigatedWe predict poleward‐moving periodic aurora, convection vortices, and ground pulsations, with larger latitudinal scales than Alfvén modesAmplitudes of all signals peak near the projection of the inner/equatorward edge of the magnetopause rather than the open–closed boundary [ABSTRACT FROM AUTHOR]

Releasing diffuse artificial clouds into the space environment using a rocket or spacecraft can modify the natural plasma state. This uses space itself as a laboratory for studying plasma phenomena that cannot be reproduced on the ground. The Space Measurement of A Rocket‐released Turbulence (SMART) mission will inject a beam of barium neutral vapor into the ionosphere and perpendicular to the Earth's magnetic field. Barium atoms will be photoionized, forming an ion ring distribution that is unstable to lower hybrid waves. Large amplitude lower hybrid waves nonlinearly scatter to whistler and magnetosonic waves that can propagate out to the magnetosphere. This paper details the theory, modeling, and simulation we used to design this release experiment to optimize the energy in the plasma waves under a variety of constraints. A product of this analysis is quantitative predictions for some in situ and remote measurements. Hydrocode simulations of barium vapourization and acceleration via shaped charge provide the initial state of the neutral beam. We optimize the apogee, orientation, and position of the payload and instruments. Cloud dynamics are simulated with a direct simulation Monte Carlo technique, which includes photoionization with metastable barium states, collisions with the neutral background atmosphere, optical line emission, gravity, and electromagnetic forces. We predict the plasma density measured by the instrument payload and the remotely measured optical intensity. We also examine how the plasma wave growth differs at the measurement location compared to the bulk of the barium cloud. Key Points: A variety of models and simulations are combined to optimize plasma wave energy in a neutral beam injection experimentModeling provides quantitative predictions of measurements (e.g., plasma density, beam velocity, total barium produced, optical intensity)We discuss the many details that arose between concept and implementation of the experiment, and how these affected mission design [ABSTRACT FROM AUTHOR]

In this paper, the internal structuring of Polarization Jet (PJ)/Subauroral Ion Drift (SAID) is studied. It is shown that PJ/SAID consists of several Polarization Jet Strata (PJS) of electron density and temperature. A new type of phenomena has been discovered called Stratified Subauroral Ion Drift (SSAID), which is a PJ/SAID containing small‐scale PJS. Internal small‐scale structure of SSAIDs is measured during various geomagnetic events using data from NorSat‐1, DMSP F‐17 and F‐18 satellites. High resolution measurements on the Norsat‐1 satellite make it possible to detect and study small‐scale structures inside the PJ/SAID, that is, SSAID. The data from DMSP satellites is used to determine explicitly the presence of PJ/SAID during the considered geomagnetic events. Results show that SSAID consist of 2–4 large PJS and from a few units to several tens of small PJS. Large PJS have a size of 0.2°–0.3° latitude and small PJS have a size of <0.1°. Several suggestions regarding possible physical mechanisms of production of SSAID and PJS are presented. Plain Language Summary: In the subauroral zone, narrow fluxes of fast subauroral ion drifts are observed near the projection of the plasmapause at heights of the F‐layer of the ionosphere, which are most noticeable during geomagnetic activity against the background of large‐scale plasma convection, called polarization jet (PJ) or subauroral ion drifts (SAID). Despite the importance of using a variety of ground‐based observation facilities for studying and analyzing PJ/SAID properties, as well as developing analytical models and numerical modeling, in situ observations are most valuable. These observations can be obtained only during the satellite flyby through the subauroral zone during the PJ/SAID development. Small‐scale processes inside PJ/SAID were practically not studied and many open questions remain here due to lack of high‐resolution satellite measurements. In this work, using NorSat‐1 satellite high sampling rate data, we report that there are Stratified Subauroral Ion Drifts (SSAIDs) which consist of small‐scale Polarization Jet Strata (PJS) of electron density and temperature. Key Points: We explore the internal structuring of subauroral polarization jets/ion drifts (SSAID ‐ Stratified Subauroral Ion Drift)SSAIDs consist of 2–4 large Polarization Jet Strata (PJS) and from a few units to several tens of small PJSLarge PJS have a size of 0.2°‐0.3° latitude and small PJS have a size of <0.1° [ABSTRACT FROM AUTHOR]

The ionospheric electron density is expected to enhance after sunrise during geomagnetic quiet days. In this paper, we report an anomalous large‐scale ionospheric TEC depletion structure occurring in morning hours over low latitudes in China under relatively quiet geomagnetic conditions. The depletion structure, which occurred over a wide longitude range of >35°, developed toward higher latitudes at the speed of ∼250 m/s. A statistical analysis using the data in 2018–2020 showed that such large‐scale daytime TEC depletion mainly occurred in summer (mostly with Kp < 3), with large intensities at ∼16–30°N. The geomagnetic ΔH measurements at the stations near and away from the magnetic dip equator showed positive values, which rules out the possible contribution from a westward electric field. Observations from multiple ionosondes revealed a net downward plasma transport during the TEC depletion. An enhanced poleward neutral wind during the early morning hours, which may induce downward plasma transport, was surmised to play a leading role in driving the large‐scale morning TEC depletion. Plain Language Summary: Under geomagnetic storms, the daytime ionosphere may undergo significant plasma density depletion over wide longitude and latitude regions. Possible driving mechanisms of large‐scale plasma density depletion include the suppression of the fountain effect by westward equatorial electrojets, the upward plasma diffusion caused by increasing electron temperature, and the decreased O/N2 ratio transported by perturbed neutral circulation. In this paper, we report cases of anomalous large‐scale plasma density depletion structures in morning hours of geomagnetic quiet days over low latitudes in China. The large‐scale plasma density depletion structure, which produced TEC reduction of ∼4 TECu, covered >3,500 km in the east‐west direction and developed poleward. Based on the observations from multiple ionosondes and magnetometers at equatorial and low latitudes, it is unlikely that the possible driving sources are from geomagnetic activities. We surmise that there could be an enhanced poleward neutral wind over a wide longitude region which induced downward plasma transport and produced the depletion structure. Key Points: A daytime large‐scale TEC depletion structure covering ∼35° in longitude under relatively quiet geomagnetic conditions was reportedThe daytime large‐scale depletion structure with poleward development mainly occurred in summer with large intensities at 16–30°NIt is surmised that enhanced poleward neutral wind could be a possible driver for the daytime large‐scale depletion structure [ABSTRACT FROM AUTHOR]

"Ultra‐fast" Kelvin waves (UFKWs) serve as a mechanism for coupling the tropical troposphere with the mesosphere, thermosphere and ionosphere. Herein, solutions to the linearized wave equations in a dissipative thermosphere in the form of "Hough Mode Extensions (HMEs)" are employed to better understand the vertical propagation of the subset of these waves that most effectively penetrate into the thermosphere above about 100 km altitude; namely, UFKWs with periods ≲4 days, vertical wavelengths (λz) ≳30 km, and zonal wavenumber s = −1. Molecular dissipation is found to broaden latitude structures of UFKWs with increasing height while their vertical wavelengths (λz) increase with latitude. Collisions with ions fixed to Earth's magnetic field ("ion drag") are found to dampen UFKW amplitudes, increasingly so as the densities of those ions increase with increased solar flux. The direct effect of ion drag is to decelerate the zonal wind. This leads to suppression of vertical velocity and the velocity divergence, and related terms in the continuity and thermal energy equations, respectively, that lead to diminished perturbation temperature and density responses. Access is provided to the UFKW HMEs analyzed here in tabular and graphical form, and potential uses for future scientific studies are noted. Plain Language Summary: In earth's atmosphere, Kelvin waves (KWs) are eastward‐propagating oscillations with periods of days to weeks that are centered on the equator and confined to low latitudes. They are forced by the spatial‐temporal variability of the heat of condensation ("latent heating") that is released when rising moist air forms rain droplets, mainly in the tropics. As with many atmospheric waves, they propagate vertically and grow exponentially with height in concert with the decrease in atmospheric density and pressure. The KWs that survive the trip from near the surface to about 100 km altitude are called "ultra‐fast" Kelvin waves, or "ultra‐fast" Kelvin waves (UFKWs). Just above 100 km, they reach maximum amplitudes where their exponential growth is curtailed by the viscosity of this part of the atmosphere. Here they interact with ionized particles (the ionosphere) and generate electric fields that ultimately drive ionospheric variability at higher altitudes (>200 km), thus presenting an element of "space weather" to navigation and communications systems. In this paper we model a set of UFKWs to better understand how their amplitudes, vertical and latitudinal structures change as they propagate above 100 km, and in so doing advance our knowledge of the physical processes underpinning near‐earth space weather. Key Points: "Ultra‐fast" Kelvin wave (UFKW) with periods less than about 4 days can effectively penetrate above 100 km altitudeDissipation broadens UFKW latitude structures with increasing height and lengthens vertical wavelengths with increasing latitudeIon drag dampens UFKW amplitudes with increasing efficiency at higher solar activity levels [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]

We present a new model of the near‐Earth magnetospheric field produced by electric currents in the inner magnetosphere and the associated induced magnetic field. The model is designed to track hourly variations of these fields and accounts for their local time asymmetries. It is built by applying spherical harmonic analysis to vector measurements from the ground observatory network at low and mid‐latitudes. The primary and induced fields are separated with an approach in the time domain that uses a a priori radially‐symmetric electric conductivity model of the Earth. The model coefficients are computed at one‐hour time steps between 1997 and 2022. This model is shown to be consistent to within a few nT with previously developed indices which track the magnetospheric ring current. It is also validated against data from the Swarm, CHAMP and Øersted satellites. The fit to satellite data is comparable to that of the CHAOS‐7.15 model for geomagnetically quiet times, and improved by up to 20% on some components for geomagnetically moderate and active times. We attribute these differences mostly to a better representation of local time asymmetries, both on average and during individual geomagnetic storms. This model can be used in various applications, such as investigating the properties of the magnetospheric field and its sources and separating the magnetospheric field from the fields of other sources in geomagnetic field modeling. Plain Language Summary: Geomagnetic field modeling aims at building data‐based mathematical representations, or models, of the various contributions to the total Earth's magnetic field measured at or near the Earth's surface. One of such contributions is the magnetic field generated by electric currents in the inner magnetosphere, including the so‐called ring current. In this paper, we present a new model of the near‐Earth magnetic field generated in the inner magnetosphere based on data collected in ground magnetic observatories. The model covers the 1997–2022 time span, includes improved representations of the local time asymmetries of the field and of the effect of electrical induction in the Earth's mantle, and is validated against data collected in low Earth orbits by the Swarm, CHAMP and Oersted satellites. We find that the model provides an improved representation of the magnetic field generated in the inner magnetosphere during periods of moderate and high geomagnetic activity, including magnetic storms. Key Points: A new ground‐data based model of hourly variations of the primary inner near‐Earth magnetospheric and associated induced field is presentedComparison with satellite data shows up to 20% performance improvement compared to the CHAOS‐7 model for moderate and active timesThe model accounts for variations of the field with local time and can be used to study geomagnetic storms [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]

Geomagnetic disturbances caused by solar storms have the potential to create large‐scale geomagnetically induced currents in long conductors at the Earth's surface. These may disrupt the operation of electric power grids and cause blackouts. Ocean‐continent boundary regions are of particular concern because of the sharp contrast between the higher ocean electrical conductivity compared to the lower continental conductivity. This contrast may generate high‐amplitude geoelectric fields and cause power grids in coastal regions to be more vulnerable to space weather hazards. Previously, analytical calculations were used to estimate geomagnetically induced currents at ocean‐continent boundaries. However, for the analytical equations to be solvable, the physics and geometries of the problem were simplified. As a result of these simplifications and due to a lack of published measurements examining this issue in coastal regions, it is difficult to know for sure whether there are unique hazards to electric power grids at ocean‐continent boundaries. In this paper, a grid‐based, time domain modeling approach is used to solve the complete Maxwell's equations, which permits accommodation of (1) the complete physics of the propagating electromagnetic fields from disturbed ionospheric currents through the air and into the lithosphere and even into the ocean via the skin effect and (2) more realistic coastal geometries. Using this more robust approach, in the variety of scenarios studied in this paper, only a local enhancement of the electric fields was observed, which are expected to only pose potential hazards to power grids only a local scale near ocean‐continent boundaries. Key Points: Electromagnetic fields across ocean‐continent boundaries are solved via FDTD while accounting for a finite‐depth ocean and topographyAnalytical approaches are found to provide a close approximation to the more realistic FDTD results for the range of studied scenariosSurface electric field enhancements are highly localized along the coast, even for low frequencies and shallow oceans [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]

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]

At mid‐ and high‐latitudes, energetic protons of magnetospheric origin precipitate down to the upper atmosphere, manifested as one of the most important coupling processes between the ionosphere and magnetosphere. The equatorward boundary of the energetic proton precipitation, or called isotropic boundary (IB), is usually believed to coincide with the boundary in the magnetosphere where field line curvature scattering of protons transits from strong to weak. In this study, we statistically examine the global distribution of the IB for 39 and 115 keV ring current protons as detected by the NOAA/Polar Operational Environmental Satellites in 2013–2018 and propose an empirical model of the IB as a function of solar wind conditions, magnetic local time, and geomagnetic activity index. The IB is well ordered by SYM‐H index and interplanetary magnetic field Bz, moving to lower latitudes with more negative SYM‐H index and Bz values, and with larger AE index and solar wind pressure. Energy dispersions of the IB are also explored, consistent with the scenario of field curvature scattering in the magnetosphere. This empirical model can be used as an indirect measure of the strength of the magnetospheric stretching and may be applied in magnetosphere‐ionosphere mapping studies. Plain Language Summary: At mid‐ and high‐latitudes, considerable energetic particles can be deposited onto the Earth's upper atmosphere, reflecting the coupling process between the ionosphere and magnetosphere. This study statistically examined a low‐latitude boundary where proton precipitation flux suddenly decreases along with the drop of trapped flux. Such a boundary is called isotropic boundary (IB). It often corresponds to the magnetospheric boundary where the curvature radius of a geomagnetic field line becomes comparable to the proton's gyro‐radius. This paper proposes an empirical model of IB in response to interplanetary and geomagnetic conditions. Such a model can be of great significance in magnetosphere‐ionosphere mapping studies and can be used to estimate the stretching of the global magnetospheric configuration. Key Points: An empirical model of isotropic boundary (IB) is proposed as a function of geomagnetic activity and solar wind conditionsIB latitude is positively correlated with the SYM‐H index and interplanetary magnetic field Bz, but negatively with AE index and solar wind pressureIB is located more equatorward with higher‐energies, consistent with field line curvature scattering [ABSTRACT FROM AUTHOR]

The ionosphere exhibits enhanced semidiurnal lunitidal (M2) perturbations during sudden stratospheric warming (SSW) events, of which the manifestation and mechanism are not well documented and understood. This paper studies the latitudinal and inter‐hemispheric variations of the ionospheric M2 perturbations during the 2021 major Arctic SSW with total electron content (TEC) data in the American, Asian‐Australian, and African‐European sectors. Results show that the M2 perturbations in the three sectors all enhanced during the SSW, with the strongest M2 perturbations in the Northern and Southern Hemispheres at about 10°∼15°N and 25°∼30°S geomagnetic latitudes, respectively. Also, clear M2 perturbations in the three sectors all extended to middle latitudes during the SSW only in the Southern Hemisphere. Comparatively, the inter‐hemispheric asymmetry of the magnitude of the M2 perturbations during the SSW was more complex. The inter‐hemispheric asymmetry of the crest locations of the low‐latitude M2 perturbations in the three sectors during the SSW was probably due to the F‐region meridional wind effect on the diffusion process of the low‐latitude plasma fountain. Meanwhile, the longitudinal differences were also obvious. The TEC M2 amplitude in the American sector was obviously larger than those in the Asian‐Australian and African‐European sectors during the SSW. The M2 characteristics in the southern middle latitude in the American sector were significantly different from those in the other two sectors during the SSW, which may be related to the differences of the combined effects of the local wind and geomagnetic configuration. In addition, a moderate geomagnetic activity occurred during the 2021 SSW, which brings difficulties to the analysis of the SSW‐related TEC variation. Key Points: The latitudinal differences of the ionospheric semidiurnal lunitidal (M2) perturbations during the 2021 sudden stratospheric warming were studied with total electron contentThe low‐latitude M2 perturbations exhibited similar latitudinal and inter‐hemispheric variations in the American, Asian and African sectorsThe M2 perturbation in the southern middle latitude was obviously different in the American from those in the Asian and African sectors [ABSTRACT FROM AUTHOR]

The gravity‐driven and pressure‐gradient currents coexist in the ionosphere, and their effects are significant in there, rather than at the outside of the ionosphere; and can be important while studying the ionospheric currents using low‐Earth‐orbiting (LEO) satellite measurements. Maute and Richmond (2017, https://doi.org/10.1002/2017JA024841) (MR17) have demonstrated that above the F region peak, directions of these two coexisting currents are opposite and the net magnetic effects along the ambient magnetic field are nonsignificant. In the view of the diamagnetic corrections being applied to the LEO magnetic field measurements to account for the pressure‐gradient currents, it is imperative to examine the proposition of MR17. In the present paper, we have estimated the gravity and pressure‐gradient currents, using altitude profiles of electron density obtained from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)‐1 satellite cluster. In order to get the latitudinal profiles of magnetic field variations at a fixed local time (LT) using COSMIC data, it is required to combine either different days at a fixed longitude or all the longitudes on a fixed day, thus compromising with either days or longitudes. It is found that the net magnetic field is significant in the low‐latitude region, which increases with solar flux and decreases with altitude. The magnetic field effects show strong LT dependence and are significant in the noon to evening sector. The comparison of the present estimates with the diamagnetic corrections emphasizes that correcting for only one current can introduce unviable errors and thus supports the suggestion of MR17. Plain Language Summary: In addition to the ionospheric‐dynamo currents, the currents due to gravity and plasma pressure‐gradients flow in the Earth's ionosphere, whose contribution in the ground magnetic field measurements is negligible compared to that of ionospheric‐dynamo. However, considering the magnetic field due to gravity and pressure‐gradient currents present in the ionosphere may be important while studying the ionospheric currents using low‐Earth‐orbiting (LEO) satellite measurements. Above the F region peak where LEO satellites generally fly, the directions of these two coexisting currents are opposite and the net magnetic field along the ambient magnetic field is nonsignificant. In the view of the diamagnetic corrections being applied to the LEO magnetic field measurements to account for the pressure‐gradient currents, it is imperative to compute the magnetic field effects of these currents using actual observations of ionospheric electron densities. In the present paper, we have estimated the gravity and pressure‐gradient currents, using altitude profiles of electron density obtained from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)‐1 satellite cluster. The magnetic variations due to these currents are estimated at different heights, latitudes, solar fluxes, and local times. Key Points: COSMIC data have limitations in obtaining good latitudinal coverage of plasma densities on a given day at a fixed local time and longitudeThe magnetic field variations due to gravity and pressure‐gradient currents increase with solar flux and reduce with altitudeThese effects are stronger at equator to low‐latitude region and in the noon to evening sector and negligible in other local time sectors [ABSTRACT FROM AUTHOR]

In this paper, we address the issue of interchange instability excitation criterion in the magnetospheric plasma in the presence of the velocity shear using a magnetohydrodynamic approach. We conducted our analysis for the arbitrary β plasma configuration using a WKB approximation for wavelengths, λ, much smaller than the characteristic plasma inhomogeneity scale, L, and found two branches of waves. These two wave branches can exist due to the plasma velocity shear and one branch can be unstable for arbitrary angles between the plasma entropy parameter and magnetic flux tube volume gradients. The implications of these results for magnetospheric physics as well as comparison of our results with corresponding analysis of other authors are discussed. Key Points: Criterion of interchange instability development in the magnetospheric plasma is derivedThe role of the velocity shear in interchange instability excitation is discussedThe possible impact of the velocity shear on the plasma sheet interchange stability is considered [ABSTRACT FROM AUTHOR]

A stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all‐sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR‐arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring‐current ions is the most plausible mechanism for the SAR‐arc generation. Plain Language Summary: A stable auroral red (SAR) arc is an aurora with an optical red emission at latitudes slightly lower than the auroral zone. SAR arcs have been considered to occur due to the spatial overlap between the low‐energy plasmaspheric electrons and the high‐energy ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the upper atmosphere to cause the red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetospheric source region of SAR arcs. In this paper, we report the first quantitative evaluation of a SAR arc using an all‐sky imager at Nyrölä, Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions in the inner magnetosphere. The electromagnetic waves associated with the SAR arc were not observed. These observations suggest that the heating of plasmaspheric electrons by ring‐current ions is the most plausible mechanism for the SAR‐arc generation. This result provides direct evidence of the previous theoretical expectation on the generation mechanism of red aurora at lower latitudes. Key Points: The conjugate measurements of a detached SAR arc on 28 March 2017 are analyzed using observations from the Arase satelliteSwarm and GNSS‐TEC data show that the electron density decreased and the electron temperature increased in the ionosphere above the SAR arcThe observed plasmas and electromagnetic fields suggest that Coulomb collision is the most plausible mechanism for the SAR‐arc generation [ABSTRACT FROM AUTHOR]

In this paper, we first investigate the effects of a postevening Weddell Sea Anomaly (WSA)‐like plasma patch on a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID) over Xinglong, China (40.4°N, 117.6°E; Mlat. ~30.4°N). We found that some of this kind of WSA‐like plasma patches moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they traveled westward into China. During 2012–2016, these plasma patches frequently occurred during the period of May–August with a monthly occurrence rate of over 20%, causing nighttime plasma density enhancements (NPDEs) in midlatitudes of China. Over 50% of these structures were accompanied by concurrent MSTID and Es. We propose that an intense polarization electric field (PEF) associated with an MSTID/Es from the more northern regions of EIA could frequently drive these plasma patches poleward. On the night of 27 June 2014 (Kp < 2−), either an MSTID and an intense Es layer (the maximum foEs >12 MHz) occurred at the onset of the poleward motion of a WSA‐like plasma patch, lending credence to this possibility. The observed plasma patch was pushed poleward to interact with the observed MSTID, causing some poleward extending C‐shaped airglow depletions/enhancements of the MSTID in a transition region where the ionosphere changed from a collapse region to an uplifted one. We attributed those poleward extending airglow depletions to the interaction between the MSTID and the plasma patch because of the secondary gradient drift instability and those of airglow enhancements to the redistribution of plasma. Plain Language Summary: This study found that postevening Weddell Sea Anomaly (WSA)‐like patchy plasma structures frequently occurred during May–August of 2012–2016 in the East Asian regions. Among these plasma patches, a small portion moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they moved westward into China. On the magnetically quiet night of 27 June 2014 (Kp < 2−), such a plasma patch already moved westward and poleward to the midlatitude region of China, affecting the evolution/propagation of a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID). This paper, therefore, investigates the interaction between a midlatitude MSTID and a poleward moving WSA‐like plasma patch. We mainly investigate the possible physical mechanisms behind this kind of plasma patches and how they could affect the propagation/evolution of MSTIDs over midlatitudes of China. Key Points: Poleward extending C‐shaped airglow depletions/enhancements of MSTID were caused by interaction between an MSTID and a postevening WSA‐like plasma patchWestward and northward moving WSA‐like plasma patches frequently caused nighttime midlatitude plasma enhancements over Xinglong, ChinaAn intense PEF associated with an MSTID/Es from the more northern regions of EIA could frequently cause poleward motion of the WSA‐like plasma patch [ABSTRACT FROM AUTHOR]

The geomagnetic field is composed of a variety of sources that act on a wide range of timescales and amplitudes. The separation of magnetic storm effects from quiet variations is needed to accurately quantify impacts of space weather events. The extraction of such quiet contributions within geomagnetic measurements is achieved by the determination of baselines, which, ideally, is done by a simple algorithm which captures quiet sources suitably well, while being applicable to an extensive network of magnetic observatories independent of the period of time. In this work, we apply signal filtering techniques on the horizontal components of geomagnetic field measurements from low‐ and mid‐latitude observatories to determine baselines. The variations within the baseline are investigated for magnetically quiet periods between 1991 and 2019, focusing on long‐term trends, seasonal and local time dependencies, and day‐to‐day variability. The analysis confirms that the contributing quiet sources include the secular variation and the solar quiet (Sq) current system. The non‐negligible day‐to‐day variability, that is typical for Sq in low‐ and mid‐latitudes, is embedded within the baseline. Thus, the filter approach extracts quiet magnetic field variations well. Comparisons with other baseline methods show good agreements. We conclude that the filter approach can be used to determine baselines automatically during magnetically quiet periods without the need of further apriori information and is applicable on a wide network of magnetic observatories. It marks the first step for deriving magnetic indices for (near) real‐time space weather applications. Plain Language Summary: The Earth's intrinsic magnetic field is generated by the motion of molten rock within its interior and interacts with the constant flow of charged particles coming from the Sun. Measurements of the geomagnetic field strength on the surface not only include the intrinsic magnetic field but also phenomena that arise due to this interaction. Some of these phenomena show regular variations without major effects and some, like solar storms, are able to disrupt the geomagnetic field, affecting technological systems. In order to quantify how harmful disruptive events are, it is important to determine the regular variations first. In this paper, we determine the regular variations within the signal (baselines) by applying signal filtering techniques on geomagnetic field measurements. Our analysis shows that regular variations during undisturbed days in low‐ and mid‐latitude ranges are captured accurately. Key Points: A basic signal filtering approach is used to determine geomagnetic baselines during quiet periodsThe baselines capture the secular variation and solar quiet current systems accuratelyThe baseline method is applicable for low‐ and mid‐latitude magnetic observatories and does not require any apriori information [ABSTRACT FROM AUTHOR]

This paper presents updated methods for locating the Poleward and Equatorward Auroral Luminosity Boundaries (PALB and EALB) directly from IMAGE Far UltraViolet (FUV) images of the Northern Hemisphere auroral oval. Separate boundaries are determined from images measured at different FUV wavelengths. In addition, new methods for indirectly estimating the Open‐Closed magnetic field line Boundary (OCB) and the Equatorward Precipitation Boundary (EPB) locations are presented; these new boundaries are derived from a combination of the auroral luminosity boundary estimates with statistical latitudinal offsets derived from comparisons with low‐altitude spacecraft Particle Precipitation Boundaries (PPBs). Subsequently, we derive new circle model fits for all these boundary data sets, as well as new quality control criteria for these model fits. The suitability of circle fits for each of the data sets is discussed, and the OCB and PALB circle fits are validated against the Convection Reversal Boundary (CRB), as measured by low‐altitude in situ spacecraft. All the new boundary data sets, covering the epoch May 2000 to October 2002, are freely available online. Plain Language Summary: The ability to measure and model near‐Earth space, its response to driving forces in the interplanetary solar wind, and the impact of these forces on the Earth's upper atmosphere is the basis for the understanding and forecasting of space weather. Because there is a strong physical connection between the Earth's magnetosphere (the region of near‐Earth space dominated by the Earth's magnetic field) and ionosphere (the ionized region of the upper atmosphere) near the poles, observations of the ionosphere provide information about the magnetospheric state. Dynamic processes within the magnetosphere accelerate electrons and ions within the space environment giving them the energy to precipitate in the upper atmosphere. These high‐energy charged particles excite atmospheric atoms and molecules, creating aurora. The light from the aurora can be observed at ultraviolet wavelengths by space‐based imagers. In this study we identify the higher and lower latitude edges of the aurora and determine their relationship to important particle boundaries. Circles are then fitted to the spatial boundary variations to make sure they are defined across all longitudes. These data sets can be used to identify and track the spatial extent and dynamics of magnetospheric regions and boundaries. Key Points: Improved poleward and equatorward auroral luminosity boundaries are determined from IMAGE FUV measurementsNew Open‐Closed magnetic field line Boundary (OCB) and Equatorward Precipitation Boundary (EPB) data sets are derived from these observationsNew circle model fits are derived for all the boundary data sets [ABSTRACT FROM AUTHOR]

This study presents a number of previously unreported auroral features that form shortly after the IMF Bz ${\mathrm{B}}_{\mathrm{z}}$–component turns sharply northward. The auroral images were captured by the Boreal Aurora Camera Constellation (BACC) All‐sky camera at the Kjell Henriksen Observatory, located in Longyearbyen, Svalbard. These observations are different from those previously reported. There are several differences: (a) The Eastward‐Moving Auroral Forms (EMAFs) do not originate from type 1 or type 2 aurora. The EMAFs originate from a diffuse vertical auroral form in the far west part of the ASC image. Vertical refers to the auroral form oriented in the north‐south direction in the frame of the ASC image. (b) The EMAFs form crescent shapes after they begin to drift eastward. (c) The EMAFs are discrete auroral forms; previously reported ionospheric signatures of high‐latitude reconnection are described as patches of brightness that form within a pre‐existing auroral band. (d) The EMAFs are observed to first move eastward and then slightly poleward during their journey. The movements of previously reported Ionospheric signatures of high‐latitude reconnection are described as expanding patches of brightness within a pre‐existing auroral band. (e) Some of the EMAFs brighten again after fading, during their northeastward drift across the field of view. (f) The EMAFs begin their eastward motion aligned in the north‐south direction. During their northeast drift, they begin to tilt toward the east. Some of the EMAFs keep tilting until they are aligned in the east‐west direction. Plain Language Summary: Magnetic reconnection is a process which combines antiparallel magnetic field lines from different sources. Since magnetic field lines permeate our universe, there are many places where magnetic reconnection can occur. The solar‐terrestrial environment provides a great laboratory to study magnetic reconnection. Different orientations of the interplanetary magnetic field (IMF) allow it to connect to different regions of the Earth's geomagnetic field. Information from a magnetic reconnection event is transmitted to the ionosphere via the terrestrial magnetic field and is manifested in dazzling auroral displays. This paper presents ionospheric signatures of magnetic reconnection at high‐latitudes which are extremely different than past studies: (a) the origin of the observations in this study are different, (b) the ionospheric signatures form crescent shapes after they begin to drift eastward, (c) the ionospheric signatures are discrete auroral forms, (d) The ionospheric signatures are observed to first move eastward and then slightly poleward during their journey, (e) some of the ionospheric signatures brighten again after fading, during their northeastward drift across the sky, (f) during their northeast drift, they begin to tilt toward the east. Some of the EMAFs keep tilting until they are aligned in the east‐west direction. Key Points: Discrete auroral forms for northward IMF are aligned in the north‐south direction, propagating eastwardThese discrete auroral forms rotate clockwise during their eastward drift, some becoming aligned in the east west directionHigh‐latitude reconnection is a possible explanation for these discrete dayside auroral forms since the IMF clock angle is less than 90° [ABSTRACT FROM AUTHOR]

In this paper, the interaction between equatorial to low‐latitude postmidnight ionospheric irregularities and large‐scale traveling ionospheric disturbances (LSTIDs) on 14 December 2015 during strong geomagnetic disturbances in China is studied by using multiple ground‐based observations. Postmidnight irregularities initiated near the magnetic equator and extended northward to higher latitudes (∼28°N) until the local early morning period (∼05 LT), which is unusual in the Northern Hemisphere winter. Multibeam observations by the very high frequency coherent scatter radar at Fuke (19.5°N, 109.1°E; dip latitude 14.4°N), Hainan Island, China, showed that the meter‐scale field‐aligned irregularities were still in the evolutionary phase during the postmidnight period (∼04 LT) instead of the fossil structures that must already be in their decay phase. Meanwhile, the emergence of the postmidnight ionospheric irregularities was accompanied by the passage of LSTIDs, as revealed by total electron content measurements. Southward and northward LSTIDs, with velocities of ∼550–570 m/s and periods of ∼80–100 min, that originated from opposite hemispheres met at ∼27°–28°N, and the LSTIDs might produce localized perturbation during their passage to initiate the instability for the postmidnight irregularity development. The height of the ionospheric F‐layer measured by the collocated Digisonde at Fuke was suggested to be modulated by the overshielding penetration electric field and storm‐related atmospheric gravity waves (AGWs). This interesting case gives insight into the important role of storm‐related AGWs in providing seeding sources for plasma bubble development and reveals unique characteristics of coupling between LSTIDs and plasma bubbles during the postmidnight sector. Key Points: Low‐latitude postmidnight ionospheric irregularities and opposite‐propagating large‐scale traveling ionospheric disturbances (LSTIDs) were observed simultaneouslyThe quasiperiodic F‐layer height uplifting might be caused by the storm‐related atmospheric gravity waves and overshielding penetration electric fieldThe LSTIDs produced localized perturbations, initiating the development of instabilities for seeding postmidnight equatorial plasma bubbles [ABSTRACT FROM AUTHOR]

The solar wind is a continuous outflow of charged particles from the Sun's atmosphere into the solar system. At Earth, the solar wind's outward pressure is balanced by the Earth's magnetic field in a boundary layer known as the magnetopause. Plasma density and temperature differences across the boundary layer generate the Chapman‐Ferraro current which supports the magnetopause. Along the dayside magnetopause, magnetic reconnection can occur in electron diffusion regions (EDRs) embedded into the larger ion diffusion regions (IDRs). These diffusion regions form when opposing magnetic field lines in the solar wind and Earth's magnetic field merge, releasing magnetic energy into the surrounding plasma. While previous studies have given us a general understanding of the structure of the diffusion regions, we still do not have a good grasp of how they are statistically differentiated from the non‐diffusion region magnetopause. By investigating 251 magnetopause crossings from NASA's Magnetospheric Multiscale (MMS) Mission, we demonstrate that EDR magnetopause crossings show current densities an order of magnitude higher than regular magnetopause crossings—crossings that either passed through the reconnection exhausts or through the non‐reconnecting magnetopause, providing a baseline for the magnetopause current sheet under a wide range of driving conditions. Significant current signatures parallel to the local magnetic field in EDR crossings are also identified, which is in contrast to the dominantly perpendicular current found in the regular magnetopause. Additionally, we show that the ion velocity along the magnetopause is highly correlated with a crossing's location, indicating the presence of magnetosheath flows inside the magnetopause. Plain Language Summary: The magnetopause is a dynamic boundary layer created through the interaction of the solar wind with Earth's magnetic field. This boundary is supported by a current sheet and acts as the "entry gate" of the solar wind's energy into the magnetosphere through a process called magnetic reconnection where energy previously stored in the magnetic field is released into the surrounding magnetopause plasma. The reconnection process is initiated in localized diffusion regions, which form in the magnetopause's current sheet during specific solar wind conditions. In this paper, we clarify what makes the diffusion regions stand out from the background magnetopause current sheet by utilizing data from NASA's Magnetospheric Multiscale mission. Our analysis reveals that the diffusion regions have stronger currents than the background magnetopause and that a significant portion of this current becomes parallel to the local magnetic field. Key Points: Electron diffusion region (EDR) crossings show current densities an order of magnitude higher than regular magnetopause crossingsEDR crossings contain significant current components parallel to the local magnetic fieldIon velocity along the magnetopause is highly correlated with an event's location—indicating the presence of magnetosheath flows [ABSTRACT FROM AUTHOR]

Ionospheric responses to geomagnetic storms can cause irregular plasma structures and scintillation of Global Navigation Satellite System (GNSS) signals. In this paper, we investigate time lags between the detection of GNSS signal scintillation at northern hemisphere auroral latitudes and the onset of 15 geomagnetic storms that occurred in 2015–2017. The results show that the time lags between the detection of ground‐based GNSS scintillations and the observed sudden change in solar wind parameters are between tens of minutes and 15 hr. This time lag consists of two segments. The first segment is about 30–80 min, which is the lag between observed disturbances in the geomagnetic field and solar wind disturbances detected by orbiting spacecrafts around the L1 point. The second segment is between the observed GNSS signal scintillation and the storm sudden commencement (SSC). This second lag segment varies in the range of about 10–830 min and highly depends on the storm onset time and geomagnetic locations of GNSS signal propagation paths. Longer time lags over 450 min were observed with the signal ionospheric piercing point at 60–70° geomagnetic latitudes (MLAT) on the dayside, while shorter lags of 10∼450 min were observed with the signal IPPs at 68–81° MLAT and on the nightside at the time of SSC. The lag time variations can be explained by ionospheric irregularity production and transport processes associated with a variety of auroral and polar cap phenomena in response to solar wind coupling to the magnetosphere and ionosphere. Key Points: Time lags between storm sudden commencement and scintillation detections by ground‐based Global Navigation Satellite System receivers at northern auroral latitudes were surveyedThe time lags varied with geomagnetic latitude and magnetic local time of satellite‐receiver line‐of‐sight signal paths assumed at ionospheric piercing pointsShorter time lag was detected by the observations close to aurora cusp as well as those on the nightside at the storm onset [ABSTRACT FROM AUTHOR]

Acoustic gravity waves (AGWs) have a significant impact on the thermosphere‐ionosphere (T‐I) system by increasing ionisation variability, transporting energy, velocity, momentum, and creating additional horizontal gradients in the T‐I, all of which influence electromagnetic wave propagation and cause changes in the geomagnetic field. Ground magnetic disturbance measurements can thus map the global and local spatial distribution of T‐I current systems. The Hunga‐Tonga volcano eruption released a tremendous amount of mass and energy into the atmosphere on 15 January 2022, causing AGWs. Furthermore, the related lightning strikes disrupt the Global electric circuit. In this study, we look at (a) global geomagnetic disturbances caused by AGW constrained by VTEC amplitudes at two different sectors, (b) Schuman resonances (SR) parameters derived from modulations of ELF observations caused by the intense lightning phenomena that accompanied the volcanic eruption. Our analysis shows that (a) registration timing of geomagnetic disturbance at each observatory indicates the different modes of propagation between 180 and 350 m/s, indicating low‐frequency AGW components, (b) noticeable changes in SR frequency modes (7–21 Hz) occurred in the total power between 04:15 and 05:45 UT hr at Patiyasar, India, linked to heightened lightning activity, (c) velocities are found to be similar at similar distances in the northern and southern hemispheres from Hunga‐Tonga, (d) time difference between the SR and geomagnetic disturbance signature at observatories is found to have no relation with distance from the eruption, (e) fluctuations in VTEC at the Indian and Russian sectors show that the geomagnetic disturbances are linked to ionospheric perturbations. Plain Language Summary: This paper presents a detailed analysis of global geomagnetic disturbances caused by Hunga‐Tonga volcanic eruption by examining 20 geomagnetic observatories located at various distances from Hunga‐Tonga, as well as observations related to lightning strikes that caused disturbance in various Schumann resonance (SR) modes. Furthermore, this study found an increase in time delay registration between the SR modes and geomagnetic field disturbances at each observatory with increase in distance from Hunga‐Tonga. Key Points: Geomagnetic disturbance registration timing at each observatory shows propagation modes between 180 and 350 m/sChanges in Schumann resonance (SR) frequency modes (7–21 Hz) between 04:15 and 05:45 UT hours in Patiyasar were associated with increased lightning activityObservations indicate that the time difference between the SR and geomagnetic disturbance signatures increases with distance from eruption [ABSTRACT FROM AUTHOR]

Although many substorm‐related observations have been made, we still have limited insight into propagation of the plasma and field perturbations in Pi2 frequencies (∼7–25 mHz) in association with substorm aurora, particularly from the auroral source region in the inner magnetosphere to the ground. In this study, we present conjugate observations of a substorm brightening aurora using an all‐sky camera and an inner‐magnetospheric satellite Arase at L ∼ 5. A camera at Gakona (62.39°N, 214.78°E), Alaska, observed a substorm auroral brightening on 28 December 2018, and the footprint of the satellite was located just equatorward of the aurora. Around the timing of the auroral brightening, the satellite observed a series of quasi‐periodic variations in the electric and magnetic fields and in the energy flux of electrons and ions. We demonstrate that the diamagnetic variations of thermal pressure and medium‐energy ion energy flux in the inner magnetosphere show approximately one‐to‐one correspondence with the oscillations in luminosity of the substorm brightening aurora and high‐latitudinal Pi2 pulsations on the ground. We also found their anti‐correlation with low‐energy electrons. Cavity‐type Pi2 pulsations were observed at mid‐ and low‐latitudinal stations. Based on these observations, we suggest that a wave phenomenon in the substorm auroral source region, like ballooning type instability, play an important role in the development of substorm and related auroral brightening and high‐latitude Pi2, and that the variation of the auroral luminosity was directly driven by keV electrons which were modulated by Alfven waves in the inner magnetosphere. Plain Language Summary: One of the most frequent disturbances in the Earth's magnetosphere is called substorm. Through Earth's magnetic field line, the development of a magnetospheric substorm is projected onto Earth's high‐latitude ionosphere as auroral evolution. During a substorm, the energy in the tail of the magnetosphere is released to the earth, and quite a part of this energy is transmitted as different types of wave phenomena. These wave oscillations can be observed by magnetometers on the ground and satellites in the magnetosphere. It is still uncertain what mechanisms trigger these substorm‐related waves, how these waves correlate with other substorm phenomena, and how they transmit and dissipate, especially for the wave phenomena at ∼4–7 Earth radii away from the Earth. This paper presents a case study of such substorm event, where we show the variation of magnetic field and auroral luminosity on the ground, and the electromagnetic wave that is called as Pi2 pulsations and oscillation of plasma flux in the inner magnetosphere. We demonstrate that there are one‐to‐one correspondences in these oscillations. We suggest the possibility that a ballooning plasma instability triggers the observed oscillations in particle flux, as well as the electromagnetic wave that modulates the auroral luminosity. Key Points: Observations of plasma and field characteristics near the source region of a substorm auroral brightening at L ∼ 5Pi2, auroral luminosity variation and fluctuations of in‐situ electromagnetic field and particle flux were observed during substorm onsetCorrespondence between these fluctuations suggest waves in the magnetosphere like ballooning instability control auroral intensity [ABSTRACT FROM AUTHOR]

This paper studies extremely low frequency (ELF) whistler‐mode waves' behavior within small‐scale magnetic field irregularities in the Earth's magnetosphere, known as magnetic ducts. Based on the magnetic fields' magnitude inside and outside these ducts, they are categorized as high‐magnetic ducts and low‐magnetic ducts (LBD). Using the whistler‐mode dispersion relation analysis, our primary focus is to show that LBDs are prone to leak electromagnetic energy outside the duct. We further investigate the hypothesis that whistlers can propagate within LBDs without any signal loss when the width of the duct corresponds to an integer multiple of the perpendicular wavelengths of the waves inside it. This condition offers a straightforward and effective method for identifying non‐leaking eigenmodes of LBDs. Our analysis of this non‐leaking condition reveals that every LBD possesses a finite number of non‐leaking eigenmodes directly proportional to the duct's width and the magnitude of the ambient magnetic field within it. The analytical results are then validated using two‐dimensional, time‐dependent simulations of the electron‐Magnetohydrodynamics model. Also, we model the non‐leaking propagation of an ELF whistler‐mode wave observed inside the LBD by the NASA Magnetospheric Multiscale mission satellites. Key Points: Magnetospheric Multiscale mission satellites observe whistler‐mode waves inside the low‐magnetic ducts (LBD)We provide an analytical explanation for the occurrence of leaking ducts and show that the LBD are prone to leakSimulations confirm our analytical prediction of the most trapped extremely low frequency wave in the LBD, provided by least degree of leakage [ABSTRACT FROM AUTHOR]

Total electron content (TEC) is an important parameter that describes the features of the ionosphere. The International GNSS Service (IGS) has been providing IGS Global TEC maps using analysis algorithms. However, collecting the completed data is difficult because of the lack of ground receivers, and the processing to obtain the completed IGS TEC maps is time‐consuming. The fast development of deep learning brings an effective way to solve these problems. Among the various deep learning methods, the generative adversarial network (GAN) exhibits great potential in recovering missing data. In this paper, we fill the missing data of the global IGS TEC maps using pix2pixhd, which is a novel deep learning method based on GAN. Differing from the traditional GAN, pix2pixhd has two generators and three discriminators. The network enhances the ability of our model to complete images with large‐scale missing areas. The result demonstrates that our model generates the ionospheric peak structures at low latitudes well, while behaving badly (the average correlation coefficient: 0.6857) around the edge of the ionospheric peak region. Comparing different scales of the missing data areas, our model has the best performance with 0%–15% missing data. With the large scale of missing data areas (30%–45% and >45%), the performance is still satisfactory. In addition, the completion effect of our model is slightly affected by geomagnetic and solar activity. Our work demonstrates a new possibility for the application of deep learning to a broader field of geosciences, particularly for problems of missing observational data. Key Points: We make a model to complete International GNSS Service total electron content maps using pix2pixhd based on generative adversarial networkThe completion effect of our model with the large scale of missing data areas is still satisfactoryOur model generates the ionospheric peak structures well while it behaves slightly badly around the edge of ionospheric peak region [ABSTRACT FROM AUTHOR]

The location of the polar cap boundary is typically determined by using low‐orbit satellite measurements in which the boundary is identified by its unique signature of a sharp decrease in energy and particle flux poleward of the auroral oval. In principle, this decrease in precipitating particles should appear as a concomitant sharp change in auroral luminosity. Based on a few events, Blanchard et al. (1995, https://doi.org/10.1029/94JA02631) suggested that a dramatic gradient in redline aurora may also be an indicator of the polar cap boundary. In recent years, advances in capabilities and the deployment of ground‐based all‐sky imagers have ushered in a new era of auroral measurements. Auroral imaging has moved well beyond the capabilities of the instrumentation in the previous study in terms of both spatial and temporal resolution. We now have access to decades of optical data from arrays spanning a huge spatial range, enabling a fresh examination of the relationship between redline aurora, particle precipitation, and the polar cap open closed boundary. In this study, we use data from the Defense Meteorological Satellite Program satellites in conjunction with the University of Calgary's Redline Emission Geospace Observatory (630.0 nm) data to assess the viability of automated detection of the 2‐dimensional polar cap boundary. Our results exhibit good agreement between the optical and particle polar cap boundary and suggest that a luminosity in redline emission could serve as a reasonable proxy for the location of the electron poleward boundary, while providing both high temporal and spatial resolution maps of the open‐closed boundary. Plain Language Summary: Named for its shape, the auroral oval is an oval‐shaped region located near the north and south poles where the aurora is formed. Studying the location of the equatorward and poleward limits of the auroral oval is fundamental to evaluate the dynamics and responses of the near‐Earth space environment. In particular, the poleward boundary corresponds to a region in which Earth's magnetic field transitions from closed (connected to Earth on both sides) to open field lines (connected to the solar wind on one side); because of this topological change in magnetic field, the location of this boundary is fundamental to understand mass and energy transport in the whole magnetosphere‐ionosphere system. While satellite data can determine the exact location of this boundary, they only provide point measurements. Auroral images are capable of detecting the luminosity around this boundary; however, many qualitative studies consider the region immediately poleward of this luminosity as the oval's poleward boundary. In this paper, we analyze the location of this boundary in a case study by using satellite data combined with auroral imaging data. Key Points: Polar cap boundary determined with Defense Meteorological Satellite Program particle data allows for examination of the boundary in optical redline dataWe find good agreement between the auroral boundary determined by auroral luminosity and those determined by particle fluxLuminosity method can be applied during active geomagnetic conditions, providing high spatial and temporal resolution polar cap boundary [ABSTRACT FROM AUTHOR]

Magnetospheric Multiscale (MMS) observations during an extended crossing of the Earth's dayside magnetopause show evidence of multiple reconnections at the boundary. This crossing occurred when the Interplanetary Magnetic Field (IMF) was southward and had a significant BY component. Approximately 2 hr after this crossing, the Twin Rocket Investigation of Cusp Electrodynamics‐2 rockets were launched into the northern hemisphere cusp and observed overlapping cusp ion injections. These overlapping injections are also evidence of multiple reconnections at the magnetopause. Observations more than 2 hr apart do not constitute conjunction between the spacecraft and the rockets. However, the IMF conditions during the magnetopause crossing and the cusp traversal were very similar. Therefore, had the magnetopause crossings and cusp traversals occurred at the same time, the observations would have been similar. Thus, these two events illustrate the link between multiple reconnections at the magnetopause and overlapping cusp ion injections. Plain Language Summary: Magnetic reconnection is a fundamental process in plasmas that interconnects magnetic field lines on either side of a current layer and converts magnetic energy into particle energy. It is the primary process that interconnects the magnetic field of the Sun with the magnetic field of the Earth at the current layer interface called the magnetopause. Ions and electrons from the solar wind follow these interconnected field lines into the Earth's magnetospheric cusps. Magnetic reconnection at the magnetopause occurs along long lines, called X‐lines. This paper discusses observations at the magnetopause and in the cusp that indicate that there are actually multiple X‐lines at the magnetopause and these multiple X‐lines produce a specific signature in the cusp called overlapping ion injections. Key Points: Magnetospheric multiscale observations often show evidence of multiple reconnection at the magnetopauseTwin Rocket Investigation of Cusp Electrodynamics‐2 observations of overlapping cusp ion injections are also evidence of multiple reconnections at the magnetopauseThere is a strong link between multiple reconnections at the magnetopause and overlapping cusp ion dispersions [ABSTRACT FROM AUTHOR]

Numerical simulations using the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) are performed to elucidate the effects of the interplanetary magnetic field (IMF) on the middle thermosphere composition during a "geomagnetically quiet" period from the day of year (DOY) 110–111 in 2019 (when the Auroral electrojet (AE) index never exceeded 300 nT and the Kp never exceeded 2). In particular, this paper aims to examine how the Global‐scale Observations of the Limb and Disk (GOLD) mission observed daytime thermospheric O and N2 column density ratio (∑O/N2) depletion at mid‐latitudes originated under such a "geomagnetically quiet" condition. A comparison of electric potential, Joule heating rate per unit mass, ion velocity, neutral temperature and winds in the middle thermosphere (∼160 km) between real IMF and without the IMF east‐west component (By) indicates that a By dominant condition can enhance their strengths under this "geomagnetically quiet" condition. Consequently, ∑O/N2 depletion with a stronger magnitude (30% compared with ∼8% without By) and larger disturbed area was introduced in the post‐midnight sector at high‐latitudes due to strong and localized upwelling associated with the enhanced Joule heating rate per unit mass. The ∑O/N2 depletion was transported equatorward and corotated from local post‐midnight to early morning, and was observed by GOLD at middle latitudes during daytime. Plain Language Summary: This study utilizes numerical simulations with a climatological tide in the lower boundary to investigate how thermospheric composition disturbance is produced and transported to mid‐latitudes during a geomagnetically quiet period (DOY110‐112, 2019, maximum Kp = 1.7, AE < 300 nT). Through comparing parameters from simulations under real IMF and By = 0, it is found that the IMF By, which was dominant over this period, can significantly enhance the ionospheric potential, Joule heating rate per unit mass, ion drift and neutral winds compared with Bz only condition. As a result, the geomagnetic activity with AE < 300 nT can trigger stronger depletion (30%) of the column density ratio of thermospheric O and N2 (∑O/N2) in the polar region. Then the accelerated meridional wind transports the ∑O/N2 depletion equatorward to mid‐latitudes. The depletion is also transported from local post‐midnight to local early morning. This study shows the crucial role of dominant By during "quiet time". Additionally, it suggests that the response of ∑O/N2 to geomagnetic activity during the "quiet period" is similar to the one during geomagnetic storms. Key Points: IMF with dominant By enhances ion convection pattern, Joule heating rate and neutral windsStrong thermospheric composition disturbance can be generated and propagate into mid‐latitudes with dominant By during "quiet" timeResponse of thermospheric composition to geomagnetic activity with AE < 300 nT is similar to its response to geomagnetic storms [ABSTRACT FROM AUTHOR]

This paper reports a rare interaction between two fronts of nighttime medium scale traveling ionospheric disturbances (MSTIDs) and associated phenomena observed over geomagnetic low‐mid latitude transition region on a geomagnetically quiet night (Ap = 5) of 21 May 2020. The event was observed for nearly 8 hours in O(1D) 630.0 nm airglow images captured by an all‐sky imager installed at Hanle (32.7°N, 78.9°E; Mlat. ∼24.1°N), Leh Ladakh, India and supplemented by a digisonde and a global positioning system receiver located within the imager's field of view (FOV). The images revealed a slow‐moving MSTID front being followed by a fast‐moving MSTID front, both traveling southwestward. The third front of the MSTID caught up with the second one, initiating the interaction. As a consequence, a plasma channel was created during the interaction and the MSTIDs' fronts began decaying. Subsequently, they became thin strips and later dissipated within the FOV of the imager. The polarization electric field of the merged region of the two interacting MSTIDs' fronts played a key role in creating the plasma channel. In the later hours of the night, another MSTID with a rotating front was observed, which is a rarely reported phenomenon. The horizontal gradient in the westward wind is a plausible reason for the observed rotation. This study elucidates the complex nature of geomagnetic low‐mid latitude transition region. Plain Language Summary: Medium scale traveling ionospheric disturbances (MSTIDs) are plasma depleted and rich bands propagating in the mid latitude ionospheric F region. Nighttime MSTIDs in northern hemisphere are usually aligned in the northwest to southeast direction and propagate southwestward. Despite the extensive study of MSTIDs, a few rarely reported phenomena, such as their interaction and rotation, are not well understood. A better understanding of these phenomena is crucial because of their effects on radio communication and navigation. On the night of 21 May 2020, we observed southwestward propagating multiple plasma depleted and rich MSTIDs' fronts in 630.0 nm all‐sky airglow images. The characteristics (horizontal velocity, orientation with respect to geomagnetic N‐S, and wavelength) of the observed fronts of the MSTIDs were different. As the MSTIDs' fronts propagated in the imager's field of view, the faster front caught up with the slower one, resulting in the interaction. Later on, MSTIDs' fronts decayed into thin strips and eventually disappeared. In about 1 hour after the disappearance of thin strips, another MSTID started propagating southwestward. However, its orientation and horizontal velocity decreased gradually over time. Using multi‐instrument data sets, we have explored possible mechanisms behind these phenomena. Key Points: The interaction of Medium scale traveling ionospheric disturbances (MSTIDs)' fronts resulted in creating a plasma channel followed by the decay of the MSTIDsPolarization electric field played a key role in creating the plasma channelA significant change in the orientation of one of the MSTIDs' fronts was observed [ABSTRACT FROM AUTHOR]

We conduct a global survey of magnetosonic waves and compute the associated bounce‐ and drift‐averaged diffusion coefficients, taking into account colocated measurements of fpe/fce, to assess the role of magnetosonic waves in radiation belt dynamics. The average magnetosonic wave intensities increase with increasing geomagnetic activity and decreasing relative frequency with the majority of the wave power in the range fcp < f < 0.3fLHR during active conditions. In the region 4.0 ≤ L* ≤ 5.0, the bounce‐ and drift‐averaged energy diffusion rates due to magnetosonic waves never exceed those due to whistler mode chorus, suggesting that whistler mode chorus is the dominant mode for electron energization to relativistic energies in this region. Further in, in the region 2.0 ≤ L* ≤ 3.5, the bounce‐ and drift‐averaged pitch angle diffusion rates due to magnetosonic waves can exceed those due to plasmaspheric hiss and very low frequency (VLF) transmitters over energy‐dependent ranges of intermediate pitch angles. We compute electron lifetimes by solving the 1D pitch angle diffusion equation including the effects of plasmaspheric hiss, VLF transmitters, and magnetosonic waves. We find that magnetosonic waves can have a significant effect on electron loss timescales in the slot region reducing the loss timescales during active times from 5.6 to 1.5 days for 500 keV electrons at L* = 2.5 and from 140.4 to 35.7 days for 1 MeV electrons at L* = 2.0. Plain Language Summary: The modeling and forecasting of the Earth's radiation belts, composed of charged particles trapped by the Earth's magnetic field, is important for mitigating damage to satellites during extreme solar events. Interactions with plasma waves in the magnetosphere can energize and scatter these charged particles, depending on the intensity of the plasma wave and local plasma conditions. One such plasma wave is the magnetosonic wave, which can contribute to both electron energization and loss to the atmosphere. In this paper, we study the role of magnetosonic waves using simultaneous measurements of their intensity and local plasma conditions and find a reduction in the loss timescales of high energy electrons in the near‐Earth region when magnetosonic waves act in combination with other plasma waves. Key Points: Pitch angle diffusion rates for magnetosonic waves exceed those of hiss and VLF transmitters over energy‐dependent ranges of pitch anglesMagnetosonic waves have a significant effect on electron loss timescales in the region 2.0 ≤ L* ≤ 3.5Magnetosonic waves reduce the loss timescales during active times from 140.4 to 35.7 days for 1 MeV electrons at L* = 2.0 [ABSTRACT FROM AUTHOR]

Multifrequency scintillation observations show that strong amplitude scintillations on very high frequency (VHF) signals are accompanied by weak L‐band scintillations near the dip equator and strong L‐band scintillations in equatorial ionization anomaly (EIA) region. For several decades this has been attributed to higher ambient plasma density in the EIA region. Recent work suggests that occurrence of stronger L‐band scintillations in the EIA region requires that the intermediate‐scale (~100 m to few km) ionospheric irregularity spectrum in this region be significantly shallower than that in the equatorial region. However, this has not been established so far. Signal frequency dependence of amplitude scintillations is characterized by the frequency exponent that determines the dependence on signal frequency of the S4 index: the standard deviation of normalized intensity fluctuations. In this paper, theoretical calculations of the frequency exponent for VHF and L‐band signals are carried out using different irregularity spectra and compared with observations in order to characterize power‐law irregularity spectra in different latitude zones. Intermediate‐scale irregularities in the equatorial and low‐latitude region, which are aligned with the geomagnetic field, are described by a two‐dimensional model. Model simulations show that the frequency exponent derived from S4 indices for VHF and L‐band signals is determined only by the characteristics of the irregularity spectrum and hence can be utilized to identify the nature of the power‐law irregularity spectrum. Frequency exponents computed using VHF and L‐band observations near the dip equator and EIA regions show distinct patterns, which clearly indicate steep and shallow irregularity spectra, respectively, in these regions. Key Points: Simulated scintillations show that signal frequency dependence of S4 is determined by irregularity spectrum, not ambient plasma densityFrequency exponent for VHF and L‐band indicates steeper power‐law irregularity spectrum near dip equator than at 10°N geomagnetic latitudeShallow irregularity spectrum found in equatorial ionization anomaly region is critical for strong L‐band scintillations recorded there [ABSTRACT FROM AUTHOR]

The effect of radial interplanetary magnetic field (IMF BX ${\mathrm{B}}_{\mathrm{X}}$) has been ignored in the solar wind‐magnetosphere‐ionosphere (S‐M‐I) coupling. We present a statistical study of IMF Bx effects on the Heppner‐Maynard Boundary (HMB) midnight latitude calculated from SuperDARN measurements between January 2002 and December 2017. The HMB represents the equatorward extent of the ionospheric convection pattern and can be used as a proxy for the equatorward boundary of the auroral oval. That is, the HMB expands toward lower latitude when the level of S‐M‐I coupling is enhanced. It is found that the averaged HMB midnight latitudes during both sunward and antisunward radial IMFs are around 64–65° magnetic latitude (MLAT). HMB midnight latitude is negatively correlated with the magnitude of IMF Bx as well as solar wind speed, while no significant trend is found between HMB midnight latitude and IMF Bz or By during radial IMF. For antisunward radial IMF, the relationship between upstream parameters and HMB is more dependent than in sunward radial direction. As the first long‐term statistical study focused on HMB during radial IMF conditions, this work provides observational evidence that the IMF Bx plays an important role in the coupling process and the traditional coupling function could not be applicable in the case of radial IMF. Plain Language Summary: The magnetic reconnection between solar wind and magnetosphere can drive a convection structure in the high‐latitude ionosphere. Heppner and Maynard developed a method to calculate the equatorward boundary of the ionospheric convection, which is called Heppner‐Maynard Boundary (HMB). HMB could be used as the monitor of the interaction process between the solar wind and geomagnetic field. In this paper, we report the effects of the radial (sunward or antisunward) interplanetary magnetic field (IMF) on HMB in the northern hemisphere. For normal cases, the Bz component o IMF is the most important. But our statistical results indicate that there is a decreasing trend between the strength of IMF Bx and HMB latitudes at midnight. No significant trend is found between HMB midnight latitude and IMF Bz or By during radial IMF. Moreover, the results provide observational evidence the Bx term should be considered in the coupling function during radial IMF. Key Points: The HMB midnight latitude has a negative trend on the IMF Bx, especially for negative IMF BxThe averaged values of HMB during radial IMFs are larger than those for substorm periods and a long‐term period of 17 yearsThe traditional coupling function may not be applicable in the case of radial IMF [ABSTRACT FROM AUTHOR]

Using ground‐based magnetic field measurements and numerical simulations from the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM), a first paper (Zhang, Yamazaki, et al., 2021, doi: https://doi.org/10.1029/2021JA029681; under review) introduced the potential roles of disturbance dynamo electric field due to subauroral polarization streams (SAPS) on the equatorial electrojet (EEJ) during a moderate geomagnetic storm on 1 June 2013. Our second study investigated the temporal responses of equatorial electrojet to SAPS. At noon, the residual EEJ (ΔEEJ) owing to SAPS flows westward, that is, counter equatorial electrojet (CEJ). The temporal variation of CEJ excited by the dynamo electric field was basically consistent with that by SAPS, and the effects of zonal wind were larger than those of meridional wind. The relative time delay of CEJ and SAPS was related to the propagation time of disturbance wind from mid‐latitudes to low‐latitudes. It took 2–3 hr for SAPS‐related disturbance wind to propagate to the equatorial region and change the polarity of EEJ. The influence of meridional winds on the temporal variations of ΔEEJ is related to the generation of eastward currents at mid‐latitudes, which can accumulate the positive charges at dusk terminator and then generate a westward electric field at lower latitudes. Plain Language Summary: The equatorial electrojet (EEJ) represents a ribbon of intense electric current flowing in the ionospheric E region (approximately 110 km) on the dayside along the dip equator. Its behaviors are controlled by the daytime ionospheric electric field and ionospheric conductivity. However, the temporal variations of EEJ in response to the subauroral polarization streams (SAPS), an interesting and important physical phenomenon at subauroral latitudes, are seldom explored and poorly understood. The understanding of EEJ behaviors and their associated physical drivers during SAPS periods can contribute to the modeling and forecasting of the equatorial space environment, and the understanding of the coupling between ionosphere‐thermosphere systems at high‐latitudes and dip equator. Key Points: ubauroral polarization streams‐induced counter equatorial electrojet (CEJ) shows a 2–3 hr delay with respect to SAPSThe time delay of CEJ attributes to the traveling atmospheric disturbances in zonal windsThe effects of meridional winds on CEJ depend greatly on the geomagnetic declination [ABSTRACT FROM AUTHOR]

The formation of the evening time F3 layer remains elusive. One of the hypotheses for the formation of the evening time F3 layer relies on the height gradient in the vertical plasma drift. In this paper, we test the validity of the hypothesis using Jicamarca incoherent scatter radar observations of height profiles of electron density and vertical plasma drift. Results show that the evening F3 layer could occur when the plasma drift is moderate and the height gradient in the vertical plasma drift is small or insignificant. Given that the production in the evening hours is negligible, these results call for interpretation for understanding the formation of the evening F3 layer. We propose that in addition to the known role of vertical plasma drift, the height variation in the recombination loss of electrons plays an important role in the formation of evening F3 layer over the geomagnetic equator. Key Points: First simultaneous observations of height profiles of electron density and vertical drift linked with evening time F3 layer are presentedResults show that the evening time F3 layer could occur when the height gradient in vertical drift is small or insignificantThe roles of height varying vertical drift and recombination loss are discussed [ABSTRACT FROM AUTHOR]

Newly built Jiamusi (JME) radar and Hokkaido East (HOK) radar data reveal statistical features of the mid-latitude ionospheric irregularities during quiet times. In this paper, the distribution features and seasonal variations of the scattering occurrence rate and convection velocity of the ionospheric, F-region irregularities between 45°N and 60°N magnetic latitude (MLAT) during geomagnetic quiet times are studied by using one-year observation data from the JME radar and HOK radar. The results show that the scattering occurrence rate is strongest in summer and weakest in winter in this region, and the highest value always appears on the dusk side, especially in summer and the equinox. Line-of-sight (LOS) velocity also shows significant seasonal variation. It is stronger in winter and the equinox than in summer, which is in good agreement with previously reported observations from six midlatitude Super Dual Auroral Radar Network radars in the North American sector. An interesting new finding is that on the night side, LOS velocity decreases first and then increases as the MLAT increases from 45°N to 56°N, particularly in summer and the equinox. This implies that there is a velocity reversal region during the geomagnetic quiet time in regard to night side, mid-latitude, ionospheric convection. Plain Language Summary The distribution features and seasonal variations of the scattering occurrence rate and convection velocity of the F-region irregularities between 45° to 56° magnetic latitude during the geomagnetically quiet time are studied by using one-year observation data from the Jimausi radar and the Hokkaido East radar. [ABSTRACT FROM AUTHOR]

A two‐dimensional particle‐in‐cell (PIC) simulation is performed to study the formation of the pitch angle distribution of energetic electrons at dipolarization fronts (DFs) driven by magnetic reconnection. The energetic electrons at DFs are originated from the lobe region, and experience a two‐step acceleration process at the reconnection x‐line and the DFs, respectively. Three kinds of pitch angle distributions commonly observed in the magnetotail, Pancake, Rolling pin, and Cigar distributions, are formed in sequence during the propagation of the DFs. In the early stage, Pancake distributions are formed through betatron acceleration. During this stage, the flux of energetic electrons with pitch angles around 0° and 180° is low because these electrons have no time to be reflected many times at the DFs to obtain sufficient Fermi acceleration. However, the electrons with pitch angles around 90° are difficult to be trapped around the DFs for a long time, and their flux saturates quickly; while the electrons with pitch angles around 0° and 180° can be trapped inside the closed field lines and they get continuous Fermi acceleration during the propagation of the DFs. Therefore, in the later stage, the flux of energetic electrons with pitch angles around 0° and 180° gradually increases and at last exceeds that of energetic electrons with pitch angles around 90°, forming Rolling pin and Cigar distributions in sequence. Plain Language Summary: Energetic electrons in the magnetotail usually show anisotropic pitch angle distributions, such as Pancake, Cigar, and Rolling‐pin distributions. All of the three kinds of pitch angle distributions are frequently observed to be related to dipolarization fronts. Pancake distribution is characterized by the enhancement of energetic electron flux with pitch angles around 90°; Cigar distribution is characterized by the enhancement of energetic electron flux with pitch angles around 0° and 180°; Rolling‐pin distribution is characterized by the enhancement of energetic electron flux with pitch angles around 0°, 90°, and 180°. It has been long accepted that electrons with pitch angles around 90° are generated by betatron acceleration, and electrons with pitch angles around 0° and 180° are generated by Fermi acceleration. However, the differences in the conditions to form the three kinds of distributions are far less understood. In this paper, we study when and where these distributions can be formed, and the detailed acceleration processes that lead the energetic electrons to form these distributions. Key Points: The energetic electrons at the dipolarization fronts (DFs) experience a two‐step acceleration process at the reconnection x‐line and DFsPancake, Rolling pin, and Cigar distributions are formed in sequence during the propagation of the DFsThese distributions are resulted from the different time scales of betatron and Fermi mechanisms at the DFs [ABSTRACT FROM AUTHOR]

A quasi‐semidiurnal type pattern was observed earlier in the diurnal UT variation of the geomagnetic storms studied using mainly Kyoto Dst (disturbance storm‐time) index. However, the pattern has been argued as apparent due to uneven longitude distribution of the four Dst observatories. Unlike earlier studies, this paper investigates the diurnal UT variation of the storms automatically identified in six available indices including Kyoto Dst, USGS (United States Geological Survey) Dst, SymH (symmetric‐H), RC (ring current), Dcx (corrected extended Dst), and AER (Atmospheric and Environmental Research) in 50, 50, 36, 21, 5, and 7 years, respectively. The indices are derived using 4, 4, 12, 14, and 15 ground observatories (with maximum longitude separations of ∼120°, 120°, 70°, 110°, and 50°) and four DMSP (Defense Meteorology Satellite Program) satellites, respectively. The UT distribution of the storm intensity (minimum value of an index during the storm main phase) in all indices shows a striking quasi‐semidiurnal type variation with maxima around 06–08 UT and 21–23 UT and minima around 03–05 UT and 13–15 UT. Similar quasi‐semidiurnal variation is also observed in the computed values of the main energy input in the ring current. The variation correlates well with the variations of the dipole tilt angles μ and θ involved in the equinoctial hypothesis and Russell‐McPherron (RM) effect, respectively. These observations indicate that the quasi‐semidiurnal variation is real. Plain Language Summary: Large disturbances in the geomagnetic field lasting form several hours to several days are known as geomagnetic storms. The variations of the occurrence and intensity of the storms with solar activity and season have been understood thanks to the works of a large number of scientists. The variation of the storms with the time‐of‐day studied using mainly the low latitude geomagnetic activity index Dst has shown a quasi‐semidiurnal pattern. The pattern, however, has been argued as apparent due to the uneven longitude distribution of the four magnetic observatories used for deriving Dst. The present study investigates the diurnal UT variation of the storms using six available indices. The results show similar striking quasi‐semidiurnal patterns in the UT distribution of the storm intensity in all indices and computed value of the main energy input in the ring current. The quasi‐semidiurnal pattern also correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect. These observations indicate that the quasi‐semidiurnal variation is real. Key Points: A striking quasi‐semidiurnal pattern is observed in the UT distribution of the geomagnetic storm intensity in six low latitude indicesSimilar pattern exists in the UT variation of the computed value of the main energy input in the ring currentThe quasi‐semidiurnal pattern correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect [ABSTRACT FROM AUTHOR]