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Journals occasionally solicit manuscripts for special collections, in which all papers are focused on a particular topic within the journal's scope. For the Journal of Geophysical Research: Space Physics, there have been 51 special collections from 2005 through 2018, with a total of 1,009 papers out of the 8,881 total papers in the journal over those years (11%). Taken together, the citations to special collection papers, as well as other metrics, are slightly higher than papers not in special collections. Several paper characteristics were examined to assess whether they could explain the higher citation and download values for special collection papers, but they cannot. In addition, indirect methods were conducted for assessing self‐citations as an explanation for the increased citations, but no evidence was found to support this hypothesis. It was found that some paper types, notably Commentaries and Technical Reports, have lower average citations but higher average downloads than Research Articles (the most common type of paper in this journal). This implies that such paper types have a different kind of impact than "regular" science‐result‐focused papers. In addition to having higher average citations and downloads, special collections focus community attention on that particular research topic, providing a deadline for manuscript submissions and a single webpage at which many related papers are listed. It is concluded that special collections are worth the extra community effort in organization, writing, and reviewing these papers. Plain Language Summary: Journals sometimes focus the attention of the research community by having a special collection, even an entire issue, devoted to a single topic. A reasonable question to ask is whether the extra effort of organizing, promoting, and maintaining the special collection is worthwhile. This paper examines paper impact in this journal, the Journal of Geophysical Research Space Physics, separating the special collection papers from those not in special collections. The short answer is, on average, yes, at least based on the metric of citations. Some characteristics of the paper were also assessed, such as the use of a colon in the title, the average author team size, the average number of references in each paper, and the paper type of the articles. None of these factors explains the higher average citations and downloads for papers in special collections. In this analysis, though, it was found that several paper types have lower‐than‐average citations but higher‐than‐average downloads, including Commentaries (personal perspectives articles) and Technical Reports (describing new methods or data sets). This implies that such papers are being read but perhaps not heavily referenced (yet). The overall conclusion is that special collections are worth the extra work. Key Points: JGR Space Physics published 51 special collections from 2006 to 2018, totaling 1,009 papers out of 8,881Average citations and downloads are slightly higher for papers in special collections compared to those not in collectionsPaper attributes thought to influence citations were analyzed, finding no statistically significant effect for special collection papers compared to others [ABSTRACT FROM AUTHOR]

Abstract: The number of citations to a refereed journal article from other refereed journal articles is a measure of its impact. Papers, individuals, journals, departments, and institutions are increasingly judged by the impact they have in their disciplines, and citation counts are now a relatively easy (though not necessarily accurate or straightforward) way of attempting to quantify impact. This study examines papers published in the Journal of Geophysical Research—Space Physics in the year 2012 (n = 705) and analyzes the characteristics of high‐citation papers compared to low‐citation papers. We find that high‐citation papers generally have a large number of authors (>5) and cite significantly more articles in the reference section than low‐citation papers. We also examined the gender and country of institution of the first author and found that there is not a statistically significant gender bias, but there are some significant differences in citation statistics between articles based on the country of first‐author institution. [ABSTRACT FROM AUTHOR]

Led by the Institute of Geology and Geophysics, Chinese Academy of Sciences, we have built a brand‐new modular active digital phased array, with all solid‐state transmission and digital receiving incoherent scatter radar (ISR) in Sanya (18.3°N, 109.6°E), a station in low latitude China called Sanya ISR (SYISR) since 2015. The development of SYISR involved the indoor design and development of key components, an outdoor prototype test, and the production and debugging of the entire array. The unique features of SYISR include a single‐channel directly connected T/R unit and antenna, a radar array monitoring and calibration network, environmental adaptability design and open architecture. The entire radar has 4,096 channels and 5,930 modules in total. All the technical indices, mainly including a >2 MW peak power, a 43 dBi antenna gain and a <120 K noise temperature, either meet or are superior to the designed value through an independent evaluation. Waveforms of single pulse, linear frequency modulation, Barker code, long pulse and alternating code have been implemented to meet multiple purposes. Four observational modes for ionospheric experiments, including zenith stare, perpendicular to geomagnetic field, meridian scan, and all sky scan, have been developed. We have implemented time domain decoding, frequency domain decoding, and statistical inversion methods in calculating the autocorrelation function and power spectra in signal processing. The preliminary experimental results on ionospheric parameters, plasma lines, irregularities and hard targets are reasonable and encouraging, which greatly enhances our confidence in achieving our scientific goals in the future. Plain Language Summary: The Earth's ionosphere (∼60–1,000 km above the sea level) is composed of dense plasma, which shows complicated variations versus altitude, geographic location, and solar activity level. When a ground‐based radio wave is transmitted onto these plasmas, it will generate stimulated radiation, with a small portion of them radiating back to the ground. If this backscattered radiation is collected by a ground‐based radar, a variety of ionospheric parameters, including plasma density, temperature, and movement speed, can be derived through a complicated signal processing algorithm. An ionospheric monitoring method called incoherent scatter radar (ISR) was therefore developed. However, since this backscattered radiation is very weak, the ISR should be built with sufficiently large power (usually megawatts) and apertures (usually hundreds of square meters). This leads to the high cost of building an ISR, with only a few having been built in the world thus far. In this paper, we describe a newly built ISR, called Sanya ISR (SYISR), in low latitude China by applying modern radar technologies, such as modular active digital phased arrays, all solid‐state transmitting and digital receiving. The development of SYISR followed the principle of a gradual and orderly progress, including indoor design, outdoor prototype testing and the entire radar construction and debugging. The radar technical indices have been evaluated, and all meet the designed value. We also developed a signal processing algorithm and a variety of observational modes. Finally, preliminary experimental results are shown, which are reasonable and encouraging. In the future, we will gradually achieve our scientific goals via effective data accumulation and analysis. Key Points: Sanya incoherent scatter radar (SYISR) is a newly built incoherent scatter radar in low latitude China with modular active electronic scanning, digital phased array, solid‐state transmitting, and digital receivingThis paper details the hardware, key parameters, data processing and preliminary experimental results of SYISRThe key parameters meet or are superior to the original design, and the preliminary observations are encouraging [ABSTRACT FROM AUTHOR]

Recently, our paper "Poststorm Thermospheric NO Overcooling?" has been published in JGR. Lei with colleagues (2012) who have proposed the "NO overcooling" concept have written Comments on this paper. Below is given our reply. Everywhere MP20 means the reference to the paper by Mikhailov and Perrone (2020). In the beginning to avoid misunderstanding, it is necessary to stress that in MP20 we did not touch on the well-documented process of the thermosphere NO cooling (e.g., Gordiets et al., 1982; Maeda et al., 1989; Mlynczak et al., 2018; Prölss, 2004, 2011; Roble, 1995; Weimer et al., 2011) which mainly takes place in the lower thermosphere. We only explained a decrease of neutral gas density at F2-layer heights during the recovery storm phase. The effect manifests seasonal dependence which is not explained by the "NO overcooling" mechanism. [ABSTRACT FROM AUTHOR]

Here, we extend the scope of the Gamayunov and Engebretson (2021, hereinafter Paper 1), https://doi.org/10.1029/2021JA029247 work by analyzing the low frequency ultra‐low‐frequency (ULF) wave power spectra in the Earth's inner magnetosphere during high speed stream (HSS) and quiet solar wind (QSW) driving conditions in the upstream solar wind (SW) and comparing our results to the results of Paper 1, where the statistics of ULF wave power spectra during coronal mass ejections (CMEs) are presented. The most important results of our statistical and comparative analyses are as follows. (a) During CMEs, HSSs, and QSW, the magnetic field power spectra of the transverse and compressional fluctuations are well approximated by power laws in the ∼mHz–Hz frequency range, where on average the parameters of power law fits during CMEs and HSSs are close, and those during QSW differ considerably from the respective parameters during CMEs and HSSs. (b) The dominance of the average compressional power over the average transverse power for the low frequency ULF waves during the 0 < SYM/H ≲ 25 nT geomagnetic conditions may serve as a proxy of HSSs in the upstream SW, whereas the opposite relation between the average powers is an indication of CMEs. (c) Independently of the SW driving conditions, a turbulent energy cascade from low frequencies in the ULF wave frequency range into the higher frequency range exists in the Earth's inner magnetosphere, supplying the nonthermal electromagnetic seed fluctuations needed for the growth of electromagnetic ion cyclotron waves (∼Hz) due to relaxation of unstable distributions of energetic magnetospheric ions. Key Points: Statistics of low frequency ultra‐low‐frequency wave power spectra during high speed streams and quiet solar wind are presented and compared to the statistics of the coronal mass ejection driven eventsA turbulent energy cascade in the Earth's inner magnetosphere exists independently of the solar wind driving conditionsA turbulent energy cascade supplies seed fluctuations needed for electromagnetic ion cyclotron wave growth due to instabilities of the energetic magnetospheric ions [ABSTRACT FROM AUTHOR]

The majority of Zhang et al. critical comments concern our method to retrieve aeronomic parameters from ionosonde foF1 observations. The main idea of the method was described by Mikhailov and Perrone (2016, https://doi.org/10.1002/2016JA022716), but it is seen that an additional clarification is required. The method has been slightly changed to meet numerous comments of referees, and its description is given here. The comments by Zhang et al. are analyzed and answered. Key Points: An improved method to extract a self‐consistent set of seven aeronomic parameters responsible for the midlatitude F1 region formation is presentedMean ion mass inferred from foF1 observations manifests negative trends that should correspond to Tn negative trend contrary to ISR resultsRoutine ISR method based on a fixed model of ion composition (crucial for the method) should inevitably result in wrong Tn long‐term trends [ABSTRACT FROM AUTHOR]

The Earth's magnetosheath and cusps are the sources of soft X‐rays. In the accompanying paper (Part 1) and this paper, we discuss the methods of finding the magnetopause position by analyzing the X‐ray images. We use the software developed for the Soft X‐ray Imager (SXI) on board the forthcoming Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission. We show how to find the maximum SXI count rate in noisy count maps. We verify the assumption that the maximum of the X‐ray emissivity integrated along the Line‐of‐Sight (Ix) is tangent to the magnetopause. We consider two cases using two MHD models and apply different methods of magnetospheric masking. Overall, the magnetopause is located close to the maximum Ix gradient or between the maximum Ix gradient and the maximum Ix depending on the method used. But since the angular distance between the maximum Ix gradient and the maximum Ix is relatively small (about 3°), the maximum Ix might be used as an indicator of the outer boundary of a wide magnetopause layer usually obtained in MHD simulations. Plain Language Summary: This is the second of two papers presenting the techniques to estimate the Earth's magnetopause location (the outer boundary of the magnetosphere) under the impact of the highly dynamic solar wind. Our knowledge of the overall shape of the magnetopause will be vastly improved when we start using X‐ray imagers to monitor large areas around this boundary as the solar wind varies. In this second paper of the series, we make use of the X‐ray emissions in the vicinity of the Earth simulated in the first paper for two case studies with vastly different incoming solar wind conditions. Here we examine different methods of how to extract the magnetopause shape and position from X‐ray maps of the type that will be returned by the X‐ray imager due to flying on the SMILE mission. Key Points: Different methods of finding the location and shape of the magnetopause from X‐ray images give consistent resultsMaximum integrated emissivity indicates the outer boundary of the magnetopauseThis work is made in preparation for the forthcoming Solar wind Magnetosphere Ionosphere Link Explorer mission [ABSTRACT FROM AUTHOR]

This commentary paper presents summaries of my research work on magnetospheric reconnection and on geomagnetic storms, to which I have devoted most of my professional career in space physics. Thus, the paper summarizes: (a) my early studies on the magnetospheric reconnection-electric field, involving observations and a model of the related polar cap-electric field during events with observed interplanetary driving parameters and (b) studies on several fundamental aspects associated with the origin of geomagnetic storms, especially leading to intense and extreme levels of activity, mostly associated with the arrival of geoeffective Coronal Mass Ejections (CMEs). The latter also involves studies on recurrent geomagnetic activity caused by interplanetary high-speed streams, among which “High-Intensity, Long-Duration and Continuous Auroral Activity (HILDCAAs for short)” events were shown to represent a special class. In the final Section, I have added some comments on recent issues dealing with fundamental concepts on reconnection at the Earth's magnetopause. [ABSTRACT FROM AUTHOR]

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 polar cap magnetic activity index (PC) was approved by IAGA as an index characterizing the solar wind energy input into the magnetosphere. Relation of the PC index to solar wind parameters and solar activity during 23/24 solar cycles has been studied in paper (Troshichev et al., 2021, https://doi.org/10.1029/2020JA028491). The paper was commented by Dr. Stauning, who states that results obtained in the study are based on erroneous data presented on web site http://pcindex.org. According to Dr. Stauning, the following invalidities served as a reason for this assertion: incorrect determination of quiet daily curve (QDC), taken as reference level for counting the PC index value, and incorrect QDC amplitudes, incorrect yearly average values of PC index (and EKL field), incorrect methods of the analysis. In our answer we argue the correctness of our analysis and resulted conclusions. Plain Language Summary: The quality of the IAGA endorsed PC index is supplied by choice of proper quiet daily variation (QDC), which makes it possible to account the effect of irregular solar UV irradiation related to solar flares. This special features of the PC index derivation made it possible to reveal the correlation between the QDC amplitude and the solar flares intensity in course of solar 23/24 cycle. The value of the yearly PC index may be estimated with use of median or mean PC quantities, resulting in essential discrepancy of the deduced yearly PC index values. The relative values of median quantities were used in the analysis. The preliminary PCS indices turned out to be incorrectly evaluated in case of 4 months in 2011, but this circumstance did not affect the general regularity of PCN/PCS changes over period 1997–2019. Divergence between values of the corresponding PCN and PCS indices, typical of solstice seasons in periods of high solar activity, is conditioned by different conductivity of ionosphere in summer and winter polar caps, not by quality of the PCN or PCS indices. Definitive PC indices have demonstrated the perfect agreement with preliminary PC indices validating the conclusions made in (Troshichev et al., 2021, https://doi.org/10.1029/2020JA028491). Key Points: Claims (Stauning, 2022) of using the unapproved polar cap indices in study (Troshichev et al., 2021) are baselessYearly QDC‐X and QDC_Y values demonstrated agreed alterations at stations Thule and Vostok in 1998–2019The relative yearly mean values derived via hourly median quantities used in analysis provide the same result as hourly mean quantities [ABSTRACT FROM AUTHOR]

The theta aurora is reproduced by global simulation. First, we construct a solution for the stationary northward interplanetary magnetic field (IMF) forming the separatrices, the separators, the nulls, and the stemlines. From the drawing of last‐closed field lines, the overall structure under this condition is summarized as the northern lobe is generated by a separatrix emanating from the southern null. In this paper, all variations are antisymmetric in the southern hemisphere. Afterward, the IMF By is switched to reproduce the theta aurora. The ionospheric theta aurora is reproduced as closed magnetic field regions. The polar cap is divided to old and new parts, by the theta bar. In the magnetosphere, two dayside nulls occur corresponding to the new IMF and two nulls corresponding to the old IMF retreat tailward. The four nulls form a structure connected by four separators, constructing the magnetospheric topology corresponding to the theta aurora. In this topology, old and new nulls in the southern hemisphere generate old and new lobes in the northern hemisphere. Each lobe is projected onto northern old and new polar caps. The origin of the theta bar is the stagnating closed magnetic field region that occurs between old and new lobes. Separator reconnection occurs between the old lobe in the southern and the new lobe in the northern hemispheres, reducing southern old polar caps. This is the cause of the movement of the theta bar. The theta aurora is the phenomenon that demonstrates the existence of the null‐separator structure. Key Points: The theta aurora is an image of the magnetospheric null‐separator structure projected to the virtual mirror in the ionosphereNorthern new and old lobes are respectively generated from new dayside and old retreating nulls in the southern hemisphereThe theta bar is the projection of stagnating closed magnetic field regions accumulating at the interface between old and new lobes [ABSTRACT FROM AUTHOR]

This paper represents the second part of an investigation of the acceleration of energetic oxygen ions from encounters with a dipolarization front (DF), based on test particle tracing in the fields of an MHD simulation. In this paper, we focus on distributions in the plasma sheet boundary layer (PSBL). O+ beams close to the plasma sheet boundary are found to be less pronounced and/or delayed against the H+ beams. The reason is that these particles are accelerated by nonadiabatic motion in the duskward electric field such that O+ ions gain the same amount of energy, but only 1/4 of the speed of protons. This causes a delay and larger equatorward displacement by the E × B drift. In contrast, the O+ beams somewhat deeper inside the plasma sheet, where previously multiple proton beams were found, are accelerated at an earthward propagating DF just like H+, forming a field‐aligned beam at a similar speed as the lowest‐energy H+ beam. We found that the source location depends on the adiabaticity of the orbit. For larger adiabaticity, the beam ions originate initially from the outer plasma sheet, but later from the opposite PSBL or lobe, but for low adiabaticity, sources are well inside the plasma sheet. The energy gained from a single encounter of a DF is comparable to the kinetic energy associated with the front speed. Assuming maximum speeds of 500–1,000 km/s, this yields a mass dependent acceleration of about 1–5 keV for protons and 20–80 keV for oxygen ions, independent of their charge state. Key Points: A propagating dipolarization front can accelerate O+ ions to form a field‐aligned plasma sheet boundary layer (PSBL) beam just as with protonsBeam particles originate from the opposite lobes or PSBL at a higher adiabaticity, but from the central plasma sheet at a low adiabaticityClose to the boundary, O+ beams are less pronounced than H+ beams due to a slower speed and an equatorward drift after acceleration [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]

Previous studies have shown that Strong Thermal Emission Velocity Enhancement (STEVE) events occur at the end of a prolonged substorm expansion phase. However, the connection between STEVE occurrence and substorms and the global high-latitude ionospheric electrodynamics associated with the development of STEVE and non-STEVE substorms are not yet well understood. The focus of this paper is to identify electrodynamics features that are unique to STEVE events through a comprehensive analysis of ionospheric convection patterns estimated from SuperDARN plasma drift and ground-based magnetometer data using the Assimilative Mapping of Geospace Observations (AMGeO) procedure. Results from AMGeO are further analyzed using principal component analysis and superposed epoch analysis for 32 STEVE and 32 non-STEVE substorm events. The analysis shows that the magnitude of cross-polar cap potential drop is generally greater for STEVE events. In contrast to non-STEVE substorms, the majority of STEVE events investigated are accompanied by with a pronounced extension of the dawn-cell into the pre-midnight subauroral latitudes, reminiscent of the Harang reversal convection feature where the eastward electrojet overlaps with the westward electrojet, which tends to prolong over substorm expansion and recovery phases. This is consistent with the presence of an enhanced subauroral electric field confirmed by previous STEVE studies. The global and localized features of high-latitude ionospheric convection associated with optical STEVE events characterized in this paper provide important insights into cross-scale magnetosphere-ionosphere coupling mechanisms that differentiate STEVE events from non-STEVE substorm events. [ABSTRACT FROM AUTHOR]

The magnetopause standoff distance characterizes global magnetospheric compression and deformation in response to changes in the solar wind dynamic pressure and interplanetary magnetic field orientation. We cannot derive this parameter from in situ spacecraft measurements. However, time series of the magnetopause standoff distance can be obtained in the near future using observations by soft X‐ray imagers. In two companion papers, we describe methods of finding the standoff distance from X‐ray images. In Part 1, we present the results of MHD simulations which we use for the calculation of the X‐ray emissivity in the magnetosheath and cusps. Some MHD models predict relatively high density in the magnetosphere, larger than observed in the data. Correcting this, we develop magnetospheric masking methods to separate the magnetosphere from the magnetosheath and cusps. We simulate the X‐ray emissivity in the magnetosheath for different solar wind conditions and dipole tilts. Plain Language Summary: The highly dynamic solar wind continuously bombards the Earth's magnetosphere, which changes shape in response. The magnetopause is the outer boundary of the magnetosphere, which is known to move. We are limited in our knowledge of the overall shape of the magnetopause, as current in situ measurements can only tell us about any change in the magnetopause at one specific location. Spacecraft carrying soft X‐ray imagers, however, will soon revolutionize our understanding by monitoring large areas of the magnetopause as the solar wind varies. In this first of a series of two papers, we simulate X‐ray emissions in the vicinity of the Earth using two magnetohydrodynamic models, and for two case studies with vastly different incoming solar wind conditions. In a subsequent paper, we examine methods how to extract the magnetopause shape from the simulated X‐ray images. Key Points: The SMILE mission will carry soft X‐ray imager onboardMagnetospheric masking methods separate the magnetosphere from the magnetosheath and cuspsWe make magnetospheric masks using threshold conditions and flowlines [ABSTRACT FROM AUTHOR]

Ultralow frequency (ULF) waves in the Pc5 band are ubiquitous in the inner magnetosphere and can impact radiation belt dynamics by interacting with electrons through drift or drift‐bounce resonance. In this paper, based on ∼ 19 years of Cluster measurements, we perform a comprehensive study of the three‐dimensional distribution of poloidal, toroidal, and compressional ULF waves from L = 4 to 10, for magnetic latitudes (MLAT) up to Ultralow frequency ±50°, and in all magnetic local times (MLT). The distribution of the Pc5 ULF wave power is found to vary greatly as a function of L and MLT. For all L and MLT sectors, wave power of the poloidal and toroidal modes of the magnetic field increase with increasing |MLAT|, while the compressional mode decreases with increasing |MLAT|. The dawn–dusk asymmetries of wave power in poloidal and toroidal modes are more pronounced at higher |MLAT|. Furthermore, the wave power for Kp > 2 is approximately 2.93, 3.21, and 3.42 times greater than the wave power for Kp ≤ 2, respectively for compressional, poloidal and toroidal components. The information on the latitudinal distributions of ULF waves presented in this paper is important for future investigations on the radial diffusion process of radiation belt electrons with non‐90° pitch angles while they bounce away from the magnetic equator. Key Points: About 19 years of Cluster measurements are used to study the latitudinal distribution of magnetic Pc5 pulsationsWave power of the poloidal and toroidal modes increases with increasing |MLAT|, while compressional mode decreases with increasing |MLAT|The statistical results can be used for the study of radial diffusion of radiation belt electrons with non‐90° pitch angles [ABSTRACT FROM AUTHOR]

The Mid‐latitude All‐sky‐imaging Network for Geophysical Observations (MANGO) employs a combination of two powerful optical techniques used to observe the dynamics of Earth's upper atmosphere: wide‐field imaging and high‐resolution spectral interferometry. Both techniques observe the naturally occurring airglow emissions produced in the upper atmosphere at 630.0‐ and 557.7‐nm wavelengths. Instruments are deployed to sites across the continental United States, providing the capability to make measurements spanning mid to sub‐auroral latitudes. The current instrument suite in MANGO has six all‐sky imagers (ASIs) observing the 630.0‐nm emission (integrated between ∼200 and 400 km altitude), six ASIs observing the 557.7‐nm emission (integrated between ∼90 and 100 km altitude), and four Fabry‐Perot interferometers measuring neutral winds and temperature at these wavelengths. The deployment of additional imagers is planned. The network makes unprecedented observations of the nighttime thermosphere‐ionosphere dynamics with the expanded field‐of‐view provided by the distributed network of instruments. This paper describes the network, the instruments, the data products, and first results from this effort. Plain Language Summary: A network of cameras and optical sensing instruments is deployed across the continental United States and is used to study the processes in the earth's upper atmosphere. These processes range from response to explosive events on the sun, as well as, response to strong events in the troposphere like thunderstorms or hurricanes. This paper describes this network, how it works, how the data are being produced, and initial scientific results. Key Points: A network of red and green line cameras and Fabry‐Perot interferometers is being developed in mid‐latitude USAThe network observes dynamics in two altitude regions of the upper atmosphereThe data are used to understand the upper atmospheric response to energetic events generated in the lower atmosphere or the sun [ABSTRACT FROM AUTHOR]

During its polar orbits around Jupiter, Juno often crosses the boundaries of the Jovian magnetosphere (namely the magnetopause and bow shock). From the boundary locations, the upstream solar wind dynamic pressure can be inferred, which in turn illustrates the state of compression or relaxation of the system. The aim of this study is to examine Jovian radio emissions during magnetospheric compressions, in order to determine the relationship between the solar wind and Jovian radio emissions. In this paper, we give a complete list of bow shock and magnetopause crossings (from June 2016 to August 2022), and the associated solar wind dynamic pressure and standoff distances inferred from Joy et al. (2002, https://doi.org/10.1029/2001JA009146). We then select two sets of magnetopause crossings with moderate to strong compression of the magnetosphere for two case studies of the response of the Jovian radio emissions. We confirm that magnetospheric compressions lead to the activation of new radio sources. Newly activated broadband kilometric emissions are observed almost simultaneously with compression of the magnetosphere, with sources covering a large range of longitudes. Decametric emission sources are seen to be activated more than one rotation later only at specific longitudes and dusk local times. Finally, the activation of narrowband kilometric radiation is not observed until the magnetosphere is in its expansion phase. Key Points: This paper provides a list of the Jovian magnetosphere boundary crossings by the Juno spacecraft from June 2016 to August 2022Jovian magnetospheric compressions lead to increased bKOM radio emissions (immediately) and DAM on the dusk sector (more than one rotation later)nKOM radio emission appears later during relaxation phase of the compression [ABSTRACT FROM AUTHOR]

Magnetosonic (MS) waves are common plasma waves in the Earth's magnetosphere. The self‐consistent excitation of MS waves has been studied by 2D particle‐in‐cell simulations in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Therefore, the 3D simulation of MS waves needs to be investigated. In this paper, we investigate the excitation and evolution of MS waves in the Earth's dipole magnetic field based on a 3D general curvilinear particle‐in‐cell simulation. We find that the MS waves are excited primarily within 3° of the equator when the thermal velocity of the ring distribution is much less than the ring velocity of the ring distribution. These waves propagate along both the radial and azimuthal directions nearly perpendicular to the background magnetic field. In the linear stage, the growth rates of MS waves are almost equal in the radial and azimuthal directions. Compared with the waves propagating along the radial direction, the waves propagating along the azimuthal direction can grow for a longer time, resulting in a larger wave amplification in this direction after saturation. The simulation results provide a valuable insight to understand the self‐consistent evolution of MS waves in the Earth's dipole magnetic field, and the findings are useful for understanding the plasma wave‐particle interaction in the Earth's radiation belts. Plain Language Summary: The Earth's magnetosphere is a natural plasma laboratory. Since the plasma in the magnetosphere is collisionless, electromagnetic waves are significant agents for the transport of energy and momentum among different particles. The MS waves are important electromagnetic waves in the magnetosphere, which have a frequency range from several Hertz to several hundred Hertz. Recently, these waves have been receiving an increasing interest due to their potential importance in accelerating radiation belt electrons and heating plasmaspheric ions. The 2D particle‐in‐cell simulations have been used to investigate MS waves in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Thus, the 3D simulation of MS waves still needs to be studied. In this paper, we perform a 3D particle‐in‐cell simulation to investigate the excitation of the MS waves in a dipole magnetic field. A larger wave amplification is found in the azimuthal direction because they can grow for a longer time in this direction. Our results can contribute to the understanding of the spatial distribution of MS waves in the Earth's magnetosphere. Key Points: Three‐dimensional particle‐in‐cell simulation of magnetosonic (MS) wave excitation in a dipole magnetic field has been studied for the first timeThe linear growth rates of MS waves are almost equal in the radial and azimuthal directionsA larger wave amplification is observed in the azimuthal direction since the waves can grow for a longer time in this direction [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]

The ionosphere of Titan hosts a complex ion chemistry leading to the formation of organic dust below 1,200 km. Current models cannot fully explain the observed electron temperature in this dusty environment. To achieve new insight, we have re‐analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science package. A first paper (Chatain et al., 2021) introduces the new analysis method and discusses the identification of four electron populations produced by different ionization mechanisms. In this second paper, we present a statistical study of the whole LP dataset below 1,200 km which gives clues on the origin of the four populations. One small population is attributed to photo‐ or secondary electrons emitted from the surface of the probe boom. A second population is systematically observed, at a constant density (∼500 cm−3), and is attributed to background thermalized electrons from the ionization process of precipitating particles from the surrounding magnetosphere. The two last populations increase in density with pressure, solar illumination and Extreme ultraviolet flux. The third population is observed with varying densities at all altitudes and solar zenith angles (SZA) except on the far nightside (SZA > ∼140°), with a maximum density of 2,700 cm−3. It is therefore certainly related to the photo‐ionization of the atmospheric molecules. Finally, a fourth population detected only on the dayside and below 1,200 km reaching up to 2000 cm−3 could be photo‐ or thermo‐emitted from dust grains. Key Points: Four cold electron populations are detected by the Cassini Langmuir probe in Titan's ionosphere below 1,200 km altitudeThree populations are likely produced by particle precipitation ionization and photo‐ionization in the atmosphere and on the probe boomThe fourth electron population, observed on dayside below 1,200 km, could be photo‐ or thermo‐emitted from dust grains [ABSTRACT FROM AUTHOR]

This paper investigates the influence of Earth's dipole tilt angle on the reverse convection cells (sometimes referred to as lobe cells) in the Northern Hemisphere ionosphere during northward IMF, which we relate to high‐latitude reconnection. Super Dual Auroral Radar Network plasma drift observations in 2010–2016 are used to quantify the ionospheric convection. A novel technique based on Spherical Elementary Convection Systems (SECS) that was presented in our companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026634) is used to isolate and quantify the reverse convection cells. We find that the dipole tilt angle has a linear influence on the reverse cell potential. In the Northern Hemisphere the reverse cell potential is typically two times higher in summer than in winter. This change is interpreted as the change in interplanetary magnetic field‐lobe reconnection rate due to the orientation of the dipole tilt. Hence, the dipole tilt influence on reverse ionospheric convection can be a significant modification of the more known influence from vswBz. These results could be adopted by the scientific community as key input parameters for lobe reconnection coupling functions. Key Points: For purely northward IMF the reverse convection potential difference is typically two times higher in summer than in winterThe reverse convection potential difference has a linear dependence on the Earth's dipole tilt angleThe Earth's dipole tilt angle is a secondary important controlling parameter of the lobe reconnection rate [ABSTRACT FROM AUTHOR]

A new incoherent scatter radar (ISR) has recently been dedicated at Sanya (SYISR), China (geographic 18.34°N and 109.62°E) to detect the low‐latitude ionospheric plasma by conducting continuous operation and electronic beam steering on a pulse‐to‐pulse basis. This paper provides an overview of the processing procedure of SYISR ion line data and presents some preliminary observational results under various transmitting signal schemes and different working modes, such as the zenith‐directed mode and meridional and zonal scanning modes, with comparison to the international reference ionosphere model (IRI‐2016), ionosonde and Ionospheric Connections Explorer (ICON) satellite measurements. The diurnal and altitude versus latitude variation characteristics of electron density (Ne), electron temperature (Te), and ion temperature (Ti) are in accordance with those of other ISR measurements in mid‐low latitudes, and IRI shows some discrepancies with SYISR observations, including the sunrise and sunset enhancement in SYISR_Ti and the overestimated IRI_Te. The comparison between the daytime SYISR_Ti and ICON_Ti shows good consistency based on 31 observational samples. The measurement of line‐of‐sight velocity (Vi) for long pulse can reveal the properties of the medium scale traveling ionospheric disturbances manifesting the radar's potential to study the mesoscale ionospheric variation in the Sanya area. The derived vector velocities with a better measurement accuracy during the day are generally in line with ionospheric plasma physics. The results show that SYISR can obtain continuous ionospheric parameters through multi‐beam scanning measurements at ∼700 km horizontal scale, which provides important information for studying the atmosphere‐ionosphere‐magnetosphere coupling and ionospheric scintillation at low latitudes in East Asia. Plain Language Summary: China's first phased‐array incoherent scatter radar was completed in 2021 in Sanya, which is called SYISR. SYISR aims to investigate fundamental scientific problems in the ionosphere, including atmosphere‐ionosphere‐magnetosphere coupling, regional ionospheric properties and ionospheric irregularities in southern China. SYISR can directly measure electron density, electron and ion temperature, and drift velocity of the ionospheric plasma and can be used to indirectly obtain the temperature of the neutral atmosphere, wind field and irregularity information. This paper describes in detail the data processing from the IQ signal to the derived ionospheric parameters for SYISR. Ionospheric experiments in various scanning modes have been performed, mainly including a single beam in zenith‐directed mode and multiple beams in meridian and zonal scanning modes, and the obtained results are evaluated. These experimental results are evaluated through comparison with the IRI‐2016 model, digital ionosonde and Ionospheric Connections Explorer satellite. The comparison results indicate that SYISR is able to provide reliable, unique, and important ionospheric information for the study of ionospheric science at low latitudes in East Asia. Key Points: Sanya incoherent scatter radar (SYISR) is a new phased array system dedicated to the ionospheric observations at low latitudes in Asia sectorSYISR is operational, providing high‐quality ionospheric measurements in multiple radar scanning modes and radio pulse schemes modesData processing procedures and initial ion‐line results using typical experiment modes are reported [ABSTRACT FROM AUTHOR]

In this study, we used observations from multiple instruments (all‐sky imager [ASI], digisonde, and Beidou geostationary satellite global positioning system receivers) to show the occurrence and evolution of two plasma density enhancement structures (PDESs) during the night of 8 February and 4 November 2018. The PDESs were elongated from northwest to southeast. Their zonal scales were about 200–400 km and longitudinal ranges were about 800–1,400 km. Within the PDES, the plasma density can be enhanced by about two times over the background density. The PDESs appeared before midnight and gradually dissipated after midnight. Plasma density depletion structures appeared within the PDES at the later stage of PDES. Each PDES event lasted for over 4 hours. The morphology and evolution of the PDESs presented in this paper are different from those reported in previous studies. In addition, based on the thermosphere‐ionosphere‐electrodynamics general circulation model simulations and digisonde observations, we proposed a possible cause for the PDESs in the events, in which the occurrence and evolution of PDES are related to the variations of thermospheric meridional winds in the equatorial region. Plain Language Summary: Ionospheric plasma irregularities or disturbances are frequently observed in the equatorial region and have been the focus of ionospheric research. These irregularities include plasma bubbles, traveling ionospheric disturbances, and plasma density enhancements. They have a great impact on the trans‐ionospheric radio wave signals. Most of the previous studies show that the plasma density enhancements occur over large scales and the occurrence of plasma density enhancements is usually accompanied by plasma density depletions in the adjacent areas, and hence, the formation of the density enhancement is suggested to be related to the plasma density depletion. In this study, two unique plasma density enhancement structures (PDESs) were observed using multiple instruments. The observed plasma density enhancements were elongated from northwest to southeast, which is different from previous studies. This paper presents the full evolution of the plasma density enhancement from its appearance to disappearance. In addition, we proposed a possible formation process for the equatorial plasma density enhancement in which the occurrence and evolution of PDES are related to the variations of thermospheric meridional winds in the equatorial region. Our results provide new insight into the dynamics and variability of the coupled thermosphere and ionosphere system in the equatorial region. Key Points: Evolution of equatorial ionospheric plasma density enhancements from occurrence to disappearance was illustrated with multiple data setsThe plasma density enhancement structures (PDESs) were elongated from northwest to southeast, with their sizes changing with time, and they produced Spread F in ionogramsVariations in neutral winds may contribute to the occurrence of the observed PDES [ABSTRACT FROM AUTHOR]

A Very Low Frequency (VLF) wave detection system has been designed at Wuhan University (WHU) and recently deployed by the Polar Research Institute of China at the Chinese Great Wall station (GWS, 62.22°S, 58.96°W) in Antarctica. With a dynamic range of ∼110 dB and timing accuracy of ∼100 ns, this detection system can provide observational data with a resolution that can facilitate space physics and space weather studies. This paper presents the first results of the wave measurements by the WHU VLF wave detection system at GWS to verify the performance of the system. With the routine operation for 3 months, the system can acquire the dynamic changes of the wave amplitudes and phases of various ground‐based VLF transmitter signals emitted in both North America and Europe. A preliminary analysis indicates that the properties of the VLF transmitter signals observed at GWS during the X‐class solar flare events are consistent with previous studies. As the HWU‐GWS path crosses the South Atlantic Anomaly region, the observations also imply a good connection in space and time between the VLF wave disturbances and the lower ionosphere variation potentially caused by magnetospheric electron precipitation during the geomagnetic storm period. It is therefore well expected that the acquisition of VLF wave data at GWS, in combination with datasets from other instruments, can be beneficial for space weather studies related to the radiation belt dynamics, terrestrial lightning discharge, whistler wave propagation, and the lower ionosphere disturbance, etc., in the polar region. Plain Language Summary: Considering the good coverage and quiet electromagnetic environment, Antarctica is an ideal place for plasma wave measurements. Various stations have been established in Antarctica, of which Palmer station is particularly noteworthy and has historically provided valuable VLF data for atmospheric, ionospheric, and magnetospheric studies. An Extremely Low Frequency/Very Low Frequency (VLF) wave detection system has been designed by Wuhan University and recently set up at Great Wall station (GWS) in Antarctica. This device can effectively record VLF signals with frequencies of 1–50 kHz, including artificial transmitter signals and natural emissions. This paper gives the broadband spectrum of 1–50 kHz signals in the north‐south and east‐west directions recorded at GWS for the first time. VLF signatures from lightning discharges, environmental disturbances, and navy transmitters in North America and Europe can be clearly identified. The overall trend of amplitude and phase of transmitter signals is consistent with the X‐ray fluxes measured by the Geostationary Operational Environment Satellite satellite. Based on the GWS observations, it is expected to reveal some significant new phenomena reflected in VLF signals and further study the distribution and propagation characteristics of VLF waves. These are of great significance for physical research and application, especially in the unique geographical location of the South Pole. Key Points: A high‐sensitivity Very Low Frequency (VLF) wave detection system with a dynamic range of ∼110 dB and timing accuracy of ∼100 ns has been designedThis system has been recently deployed by the Polar Research Institute of China in Antarctica and operated routinely for 3 monthsThe VLF wave data collected by this system can be widely used to monitor and study space weather events in the polar region [ABSTRACT FROM AUTHOR]

The cusp is a region in which the solar wind plasma can directly penetrate into the ionosphere. Most previous studies mainly focused on the low‐ and high‐latitude boundaries of the cusps, the identification of the dawnside and duskside boundaries of them appears to be questionable. The auroras are the ionospheric manifestations of solar‐terrestrial energy coupling. This paper presents a new approach to diagnose the duskside boundary of the cusp with the observation of a particular polar cap arc (PCA) located near the ∼1500 magnetic local time sector (15MLT‐PCA). Using measurements from Defense Meteorological Satellite Program satellite, we selected several 15MLT‐PCA examples where the satellite flew over different regions and analyze the particle characteristics. We identified the precipitating particles in dawnside and duskside regions next to 15MLT‐PCA are from the low latitude boundary layer/cusp and boundary plasma sheet/central plasma sheet respectively. The statistical result further confirms our findings. Therefore, we proposed that 15MLT‐PCA can be used as an identifiable optical signature of the duskside boundary of the cusp. This paper provides a valuable approach to identify the cusp boundaries, and the results are critical for understanding the generation mechanism of 15MLT‐PCA. Plain Language Summary: The polar cusps are important regions in which the particles originating from the solar wind can directly enter the upper atmosphere. Those precipitating particles can produce auroras in the ionosphere. The polar cusps have been studied in great detail for several decades and their properties have been characterized under different interplanetary magnetic field conditions. Until now, how to determine the cusp boundaries is still a topic of investigation. This study emphasizes one special auroral phenomenon called 1500 magnetic local time‐polar cap arc (15MLT‐PCA), and we check the particle distributions around 15MLT‐PCA using the observations provided by the Defence Meteorological Satellite Program satellite. We found that the 15MLT‐PCA occurs at the duskside boundary of the cusp, thus allowing the possibility that 15MLT‐PCA can be regarded as an optical signature of duskside boundary of the cusp. Key Points: The 1500 magnetic local time‐polar cap arc (15MLT‐PCA) is suggested to be an auroral indicator of the duskside boundary of the cuspAt the dawnside (duskside) auroral oval of 15MLT‐PCA, the precipitating particles appear as low latitude boundary layer (LLBL)/cusp (boundary plasma sheet/central plasma sheet [BPS/CPS]) characteristicsThe 15MLT‐PCA can be viewed as a separator between dayside LLBL/cusp and duskside CPS/BPS auroras [ABSTRACT FROM AUTHOR]

Intense, midnight‐to‐dawn sector, near‐equatorial, chorus rising tones which cross frequencies of ∼0.5fce $\sim 0.5{f}_{\text{ce}}$ have been analyzed to determine their structures and possible substructures. Upper band (f≥0.5fce $f\ge 0.5{f}_{\text{ce}}$) chorus and "gap" (f∼0.5fce $f\sim 0.5{f}_{\text{ce}}$) chorus are examined in detail for the first time. It is found that upper band chorus and gap chorus are composed of the same structure as lower band (f≤0.5fce $f\le 0.5{f}_{\text{ce}}$) chorus: they are composed of short‐duration subelements, which are monochromatic with σ≤1% $\sigma \le 1\%$. These findings have strong implications for the chorus element generation mechanism. Following Kennel and Petschek (1966, https://doi.org/10.1029/JZ071i001p00001) the overall chorus riser is most likely generated by anisotropic (T⊥/T∥ ${T}_{\perp }/{T}_{{\Vert} }$ > 1) ∼10–100 keV substorm‐injected electrons. Assuming cyclotron resonance, the upper band chorus is generated by the low energy portion of the electron spectrum. The often‐present gap at ∼0.5fce $\sim 0.5{f}_{\text{ce}}$ is related to Landau/cyclotron damping. This however is not the end of the story. There is another type of two‐frequency chorus (called Type 2) for which the lower band is not well connected to the upper band. A Type 2 chorus reported previously by Fu et al. (2014, https://doi.org/10.1002/2014JA020364) has also been studied in detail. Both the lower band and upper band are composed of subelements which are monochromatic. Such a similar fine structure for the different type of chorus may imply a similar generation mechanism, for which the difference between them is just the energy range of resonant energetic electrons. One mechanism discussed here, generation by phase bunched electrons, will be tested in the near future. Plain Language Summary: Understanding chorus structure and microstructure is essential toward understanding the wave generation mechanisms and wave‐particle interaction consequences. In this paper we show that upper band (f≥0.5fce $f\ge 0.5{f}_{\text{ce}}$) chorus and gap chorus (f∼0.5fce $f\sim 0.5{f}_{\text{ce}}$) are composed of substructures (subelements) which are monochromatic with σ≤1% $\sigma \le 1\%$. These are the same features of lower band (f≤0.5fce $f\le 0.5{f}_{\text{ce}}$) chorus. The Kennel‐Petschek theory therefore needs to be enlarged such that phase‐bunching of ∼10–100 keV substorm injected anisotropic electrons occur, which then "lase" to yield the monochromatic wave subelements. Coherent and monochromatic chorus can explain the rapid burstiness of ionospheric microburst X‐ray structures. There is another type of upper band chorus, called Type 2 upper band chorus, where the lower band chorus elements are not clearly connected to the upper band chorus. A Type 2 chorus reported previously by Fu et al. (2014, https://doi.org/10.1002/2014JA020364) has been examined in this paper. The apparently unrelated upper band has been found to be composed of subelements which are monochromatic in nature. Thus the different type of chorus may be excited by a similar generation mechanism, for which the difference between them is only the energy interval of the resonant energetic electrons. Key Points: Type 1 upper band chorus and gap chorus are found to be composed of short duration subelements which are monochromatic (σ≤1% $\sigma \le 1\%$)Since all parts of chorus risers have similar constructions, it is most probable that they are generated by anisotropic narrow energy‐range phase‐bunched electronsA Type 2 chorus (no connection between the lower band and upper band) was studied. The upper band was composed of monochromatic subelements [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 study, analytical approximation is used to solve the wave equations near the plasmapause boundary layer in order to examine the validity of ray tracing approach for fast magnetosonic (MS) wave propagations, and then analytical solutions for electromagnetic fields of MS waves through the plasmapause boundary layer are provided for the first time. Both theoretical calculations from the analytical expressions and observations of Van Allen Probes have indicated that electric fields of MS waves decrease rapidly but magnetic fields increase rapidly when propagating across the plasmapause boundary layer from the outside. Considering effects of the plasmapause width and wave frequency, parameter analysis has shown that when the width of the plasmapause boundary layer is narrow in comparison with the wavelength of MS waves, a significant part of waves will be reflected. In these circumstances, the WKB approximation and then ray tracing method might become invalid, and analytical approach provided in this paper could be utilized to solve the wave equation. [ABSTRACT FROM AUTHOR]

The Floating Potential Measurement Unit onboard the International Space Station includes a Wide sweeping Langmuir Probe that has been operating in the F‐region of the ionosphere at ∼400 km since 2006. While traditional Langmuir probe estimates include critical plasma parameters like electron density and temperature, we have also extracted the O+ percentage from the total ion constituents. This O+ composition data set from the recent minimum in the Solar Cycle 24 reveals orbits with dropouts in O+ to below 80% of the total background ion density at International Space Station orbital altitudes. The observed O+ percentages during these dropouts are much lower than the values predicted by the International Reference Ionosphere 2016 (IRI2016) empirical model. In this paper, we present the climatology of these O+ dropouts with their dependency on season, local time and geographical location. The results show that the lowered O+ percentages are more significant in the winter hemispheres and are routinely observed for orbits in the pre‐sunrise periods. The patterns in O+ dropouts can be explained in part from the lowering of the O+/H+ transition height during solar minimum along with patterns in neutral wind variation. Plain Language Summary: The plasma environment experienced by the International Space Station (ISS) is of key importance to the space physics community. A suite of instruments on the ISS has been operating since 2006 that consist of a spherical Langmuir probe, called the Wide sweeping Langmuir Probe, which is a sensor that periodically collects information about the density and energy of the ambient electrons. Recently, we presented a technique to also infer the composition of O+ ions relative to the total ion density which is an important parameter in understanding the interaction between the terrestrial atmosphere with the exosphere. Results from the most recent period of solar minimum reveal dropouts in O+ densities compared to predictions from empirical models. We show global climatology plots from 2018 to 2019 and observe that these large dropouts in O+ are more prevalent in the post‐midnight region of the winter hemisphere. Past studies have discussed increased presence of lighter H+ ions at higher orbital altitudes, but the fact that O+ dropouts are observed even at ISS altitudes is striking and one of the prominent features of solar minimum. Key Points: O+ percentages often drop below 80% at International Space Station altitudes during solar minimumThe dropouts in O+ abundance are common in post‐midnight sector in winter hemisphereThe climatology of the dropouts is governed by neutral wind variations and lowered O+/H+ transition height [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]

The scale size of the plasma boundary region between the sheath and ionosphere in the Martian system is often similar to the gyro‐radii of sheath protons, ∼200 km. As a result, ion energization via kinetic structures may play an important role in modifying the ion trajectories and thus be important when evaluating the large‐scale dynamics of the Martian system. In this paper, we report observations made with the MAVEN Langmuir Probe and Waves instrument of solitary bipolar electric field structures, and assess their potential role in ion energization in the Martian system. The observed structures appear as short duration (∼0.5 ms) bipolar electric field pulses of ∼1–25 mV/m, and are frequently observed in the upstream solar wind and inside the sheath. The study presented in this paper suggests that the bipolar electric field structures observed at Mars have an average electrostatic potential drop of ∼0.07 V. The estimated upper rate at which these structures could further energize the protons is estimated, assuming the protons gain the full 0.07 eV, to be ∼0.13 eV per gyration, or a change in proton energy of ∼0.3%, and a corresponding change in the gyroradius of ∼0.3 km. These numbers imply that to first order the bipolar structures are not a significant source of ion energization in the Martian magnetosheath. Key Points: The direct observation of bipolar electric field structures in Mars' sheath and solar wind, with durations of ∼0.5 ms and amplitudes ∼1–25 mV/mThe bipolar structures have associated electric potentials of ∼0.002–2 V, ∼0.07 V on averageThe bipolar structures do not significantly impact sheath plasma through proton energization [ABSTRACT FROM AUTHOR]

Normally, auroral kilometric radiation (AKR) which is emitted in the auroral zones escapes from the Earth. But since a few decades very similar radiations are observed by ground‐based receivers and by satellites at altitudes below the AKR generation area. They are called leaked AKR or AKR‐like emissions because it is expected that there are linked to AKR. This paper deals with observations of such AKR‐like emissions observed in the auroral zones (in the North and in the South) by the low‐altitude satellite DEMETER. In total, 2,526 events have been recorded during 6.5 years. These events are not very rare as they occur at least 2% of the time. Although this data set has a severe flaw due to a latitudinal constraint, it was possible to draw interesting properties of these emissions. In fact they are very similar to usual AKR observed at much higher altitudes during auroral activities (the same frequency range, magnetic local time (MLT) sector, and invariant latitude). The main difference concerns a strong asymmetry between the Northern and the Southern hemispheres: (a) the number of AKR‐like emissions in the Northern hemisphere is 32% larger than in the Southern hemisphere but this percentage decreases when the auroral activity increases, and (b) there is an important seasonal effect because the number of events decreases during the winter season both in the North and in the South. Plain Language Summary: Auroral kilometric radiation (AKR) is the strongest terrestrial radio emission generated between 30 and 800 kHz along the auroral field lines and associated with a discrete auroral arc. This emission is generated by superthermal electrons (several keV) which are injected from the magnetotail when the solar activity increases. Typical location of the source is ∼22 hr of magnetic local time (MLT), ∼70° of invariant latitude, and (2–10)103 km of altitude. AKR propagates from the Earth and should not be able to reach low altitudes because of the ionospheric frequency cutoff. But a few decades ago, AKR‐like emissions have been recorded by low orbiting satellites and even by ground‐based receivers. It was explained by a mode conversion of the wave propagating in inhomogeneous plasma. Up to now, only studies of some AKR‐like events have been reported, and this paper presents the statistical properties of such emissions recorded by the low altitude satellite DEMETER during 6.5 years. AKR‐like emissions display similar characteristics of AKR in terms of frequency, MLT, and invariant latitude. It is shown that more events are observed in the North than in the South, and that there is a seasonal effect (the number of events decreases in winter hemispheres). Key Points: Numerous auroral kilometric radiation‐like events have been recorded by the low‐altitude satellite DEMETER during moderate and high magnetic activityThere are more events in the Northern than in the Southern hemisphere but this difference decreases when the auroral activity increasesThe number of events decreases in winter in each hemisphere [ABSTRACT FROM AUTHOR]

Due to Venus not having a substantial planetary magnetic field the fast‐flowing solar wind plasma can propagate to regions close to the planet. Therefore, thermal atomic oxygen in the thermosphere, hot oxygen in the corona, and the resulting pickup oxygen ions are essential for determining the overall interaction of the planet with plasma of the ambient solar wind. To investigate this complex system, we have initiated a project where a combination of Venus Thermosphere General Circulation Model (VTGCM) and Adaptive Mesh Particle Simulator (AMPS) codes are used to determine the variability of the "hot" O corona depending on the solar conditions. Here we present the results of modeling Venus' oxygen corona using the VTGCM ionosphere/thermosphere and AMPS kinetic particle models. VTGCM produces a self‐consistent calculation of the thermosphere/ionosphere, providing the spatial distributions of the dominant species. That is further used in AMPS' modeling of Venus' exosphere (a) to specify the source of the newly created hot O atoms produced by dissociative recombination of O2+ ${\mathrm{O}}_{2}^{+}$ ions and (b) to account for thermalization of these energetic oxygen atoms as they propagate in the upper thermosphere. The altitude distribution of hot O calculated for the solar maximum conditions agree well with Pioneer Venus Orbiter observations of the oxygen corona. The modeling that we have performed for the solar minimum conditions indicates a decrease of the oxygen density in the corona by almost a factor of six compared to that at solar maximum. That is consistent with the non‐detection of the oxygen corona from Venus Express. As expected, the solar moderate case is between the solar maximum and minimum cases. Plain Language Summary: Here we present an investigation of the variability of Venus' extended oxygen corona. For that, we employ a combination of fluid modeling for simulating Venus' ionosphere and thermosphere and kinetic modeling of the source and transport of energetic hot O atoms in the thermosphere. We have found a good agreement of the model results with the Pioneer Venus Orbiter observations of the "hot" O corona. We also found that the oxygen density strongly depends on solar conditions and varies by a factor of six over a solar cycle. That explains why the extended oxygen corona was observed only at the solar maximum. The result presented in this paper will be a part of a later study of the planet's interaction with the ambient solar wind, where the corona model would be used to calculate the mass loading coefficient. Key Points: The density of Venus' extended oxygen corona varies almost by a factor of six of magnitude during a solar cycleKinetic modeling reproduces PVO observations of Venus' "hot" oxygen corona when forward scattering of the energetic oxygen atoms is employedThe strong dependence of the oxygen density in the corona from solar conditions suggested by results of our modeling is consistent with the non‐detection of the oxygen corona from Venus Express conducted at solar minimum conditions [ABSTRACT FROM AUTHOR]

In this paper, by performing two‐dimensional (2‐D) particle‐in‐cell simulations of collisionless magnetic reconnection in a Harris current sheet, we analyze the formation of electron inflow along the separatrices toward the X‐line from the perspective of fluid. The results show that both the parallel electric field and mirror force can drive the electron inflow to stream toward the X‐line. The speed of the electron inflow can reach about 4VA(where VA is the Alfvén speed based on the upstream asymptotic magnetic field magnitude and peak density in the current sheet). Although the contribution of the parallel electric field is much larger than that of the mirror force, the mirror force cannot be ignored. When the electron inflow streams toward the X‐line, it is heated in the parallel direction. The resulted gradient of the parallel electron pressure leads to the attenuation of the electron inflow. We also investigate the effects of the density and temperature of the background plasma on the formation of the electron inflow. With the decrease of the background plasma density, the contribution of the parallel electric field becomes larger while that of the mirror force almost remains unchanged. With the decrease of the background plasma temperature, the contribution of the parallel electric field becomes larger while that of the mirror force becomes smaller. Plain Language Summary: Magnetic reconnection is a universal process in which the topology of the magnetic field changes, accompanied by plasma heating and particle acceleration. During reconnection, electrons moving along the separatrices toward the X‐line form the electron inflow. The electron inflow has been confirmed in simulations and observations, but its generation mechanism remains unclear. This paper focuses on the dynamics of inflow electrons along the separatrices during magnetic reconnection, and quantitatively analyzes the formation of electron inflow. We found that the formation of electron inflow is contributed by three effects: accelerated by the parallel electric field, accelerated by the magnetic mirror force, and decelerated by the gradient of parallel electron pressure. The acceleration contribution of the parallel electric field is dominant, but that of mirror force cannot be ignored, especially when the background plasma density and temperature are sufficiently high. Key Points: Electron inflow is accelerated by parallel electric field and mirror force and decelerated by the gradient of parallel electron pressureParallel electric field plays a major role in the formation of electron inflow, while the contribution of mirror force cannot be ignoredThe contributions of parallel electric field and mirror force change with the background plasma density and temperature [ABSTRACT FROM AUTHOR]

Equatorial Vortex Experiment (EVEX), the EVEX mission of May 2013, carried out from Roi‐Namur, Kwajalein Atoll, explored the initiation physics of equatorial spread‐F with a pair of rockets launched simultaneously during local sunset to apogees of 182 and 331 km, respectively. EVEX measurements presented in this paper establish the existence of a differential zonal flow between plasma and neutrals at bottom‐side F‐region heights during the sunset time in support of a post‐sunset spread‐F initiation mechanism described in Kudeki et al. (2007) that is characterized by large growth rates for oblique‐propagating 10's‐of‐km‐scale electron density perturbations. F‐region plasma drifts were measured insitu using an electric field double‐probe instrument while neutral winds were measured by triangulating drifting vapor clouds. The state of the ionosphere and spread‐F irregularities were monitored by ground based ARPA Long‐Range Tracking and Instrumentation Radar and Illinois Radar Interferometer System radars using incoherent and coherent scatter techniques, respectively, as well as an ionosonde. Plain Language Summary: Understanding how the upper atmosphere and ionosphere vary with altitude at low latitudes represent a long standing goal of the space physics community. Importantly, how the motions of atmospheric and ionospheric gases change with altitude and local time at sunset at low latitudes is believed to be the key to understanding why the subsequent night‐time low‐latitude ionosphere becomes unstable. Using two simultaneous sounding rockets, a satellite pass, ground based radars, and an ionosonde, we show evidence for a vortex‐like pattern of the ionospheric motion at sunset, as well as shears in upper atmosphere motions or winds. We also provide key evidence from the measurements for the generation of post‐sunset disturbances and plasma depletions in the equatorial ionosphere caused by differential flows of the neutral and ionized gases during the sunset period. Key Points: Radar backscatter observations monitor the ionospheric density irregularities developing in pre‐ and post‐sunset equatorial E and F regionsTwo rockets launched into the equatorial ionosphere at sunset collect measurements of E‐ and F‐region plasma drifts and neutral windsLinks between the generation of post‐sunset F‐region turbulence and sunset time F‐region winds within the plasma drift vortex are examined [ABSTRACT FROM AUTHOR]

Auroral Kilometric Radiation (AKR) is the strongest terrestrial radio emission, and emanates from the same electron acceleration regions from which particles precipitate into the ionosphere, exciting the aurorae and other phenomena. As such, AKR is a barometer for the state of solar wind ‐ magnetosphere ‐ ionosphere coupling. AKR is anisotropically beamed in a hollow cone from a source region generally found at nightside local times, meaning that a single source region cannot be viewed from all local times in the magnetosphere. In radio data such as dynamic spectra, AKR is frequently observed simultaneously to other radio emissions which can have a similar intensity and frequency range, making it difficult to automatically detect. Building on a previously published pipeline to extract AKR emissions from Wind/WAVES data, in this paper a novel automated AKR burst detection technique is presented and applied to Wind/WAVES data. Over a five year interval, about 5000 AKR bursts are detected with median burst length ranging from about 30 to 60 min. During detected burst windows, higher solar wind velocity is observed, and the interplanetary magnetic field clock angle is observed to tend toward BZ < 0, BY < 0, when compared with the entire statistical interval. Additionally, higher geomagnetic activity is observed during burst windows at polar, high and equatorial latitudes. Plain Language Summary: Auroral Kilometric Radiation (AKR) is a terrestrial radio emission which is excited by the same electrons which enhance the aurorae. Due to a combination of complex beaming, and the statistical position of the source region, an AKR event cannot be observed at all positions in the Earth's magnetosphere. A combination of different radio emissions are simultaneously observed in the radio data, including both AKR and non‐AKR sources. Building on previous work, in this paper individual AKR burst events are automatically detected from Wind/WAVES data over a five year interval. About 5000 events are detected over the interval, during which the observed geomagnetic activity was higher. Higher solar wind velocity and differences in the morphology of the interplanetary magnetic field are also observed during burst windows, both of which are known to excite magnetospheric dynamics. Key Points: A novel technique has been developed to detect individual Auroral Kilometric Radiation bursts in Wind/WAVES dataWhen the technique is applied to 2000–2004 data, about 5000 bursts are detected with median duration 30–60 minDuring burst windows, higher solar wind velocity, more negative IMF BZ and greater geomagnetic activity is observed [ABSTRACT FROM AUTHOR]

At Mercury, several processes can release ions and neutrals out of the planet's surface. Here we present enhancements of planetary ions (Na+‐group ions) in Mercury's northern magnetospheric cusp during flux transfer event (FTE) "showers." FTE showers are intervals of intense dayside magnetopause reconnection, during which FTEs are observed in quick succession, that is, only separated by a few seconds. This study identifies 1953 FTE shower intervals and 1795 Non‐FTE shower intervals. During the shower intervals, this study shows that the FTEs form a solar wind entry layer equatorward of the northern magnetospheric cusp. In this entry layer, solar wind ions are accelerated and move downward (i.e., planetward) toward the cusp, which sputter upward‐moving planetary ions with a particle flux of 1 × 1011 m−2 s−1 within 1 min. The precipitation rate is estimated to increase by an order of magnitude during FTE showers, to 2 × 1025 s−1, and the neutral density of the exosphere could vary by >10% in response to this FTE‐driven sputtering. Such rapid large‐scale variations driven by dayside reconnection may explain the minute‐to‐minute changes in Mercury's exosphere, especially on the high latitudes, observed by ground‐based telescopes on Earth. Our MESSENGER in situ observation of enhanced planetary ions in the entry layer likely corresponds to an escape channel for Mercury's planetary ions. Comprehensive, future multipoint measurements made by BepiColombo will greatly enhance our understanding of the processes contributing to Mercury's dynamic exosphere and magnetosphere. Plain Language Summary: For the airless objects in the solar system, energetic ion sputtering is an important process that can release particles out of the planet's surface. In Mercury's magnetosphere, the solar wind energetic ions have been suggested to be able to release neutrals out of the surface in a short period by models and simulations. This paper has examined the 4 years of magnetic field and plasma measurements collected by NASA's MESSENGER mission orbiting Mercury and has led to the discovery of how reconnection between the interplanetary and planetary magnetic fields leads to the formation of solar wind entry layers around Mercury's magnetospheric cusps. These entry layers act as magnetic channels that cause the solar wind energetic ions to move downward toward the planet and precipitate onto the surface beneath Mercury's northern and southern magnetospheric cusps where they sputter neutral sodium and sodium ions. This paper concludes that the solar wind sputtering could account for 10% or greater changes in the density of neutral sodium in Mercury's exosphere, and the sputtered sodium ions likely form an escape channel of Mercury's planetary species. The ion escape depended on the solar wind driving magnetopause reconnection at Mercury is different from the ion escape at Venus and Mars, where escape ions are ionized by the solar ultraviolet (UV). Key Points: Solar wind entry layer is observed to form equatorward of cusp during flux transfer event showers which enhances precipitation rate by an order of magnitudeSolar wind sputtering can release planetary ions and neutrals efficiently within 1 min of the onset of magnetopause reconnectionA new escape channel for planetary ions is formed by solar wind sputtering with an escape rate of ∼1024 s−1 for Na+‐group ions [ABSTRACT FROM AUTHOR]

In this paper we present an algorithm which identifies ion energy dispersion events in low altitude cusp observations. Such an algorithm is suitable for implementation to prioritize data for downlink, or to be run on the ground for data mining. The energy dispersion events, sometimes known as dispersed particle precipitation, have been interpreted as due to time‐of‐flight effects of energized ions from magnetopause reconnection sites. Our algorithm utilizes a novel method of integrating a scoring function across moving time windows to capture the dispersion structure. The algorithm is implemented and prototyped with Defense Meteorological Satellite Program data but specified in terms that apply to other missions for both ions and electrons. Two new types of dispersion events are uncovered with the algorithm, consisting of a cold inverse double dispersion event where one population dominates, and a V‐type dispersion event during northward interplanetary magnetic field. Plain Language Summary: Ion energy dispersion events occur in the low altitude cusp region, originating from reconnection at the magnetopause. This paper presents an algorithm for automatically detecting these events in data from the Defense Meteorological Satellite Program (DMSP) satellites. Though the algorithm is demonstrated with DMSP, the algorithm can work with other satellites. The algorithm is also used to search for new types of dispersion, and two new types of dispersion events are presented in this paper. Key Points: A novel method for capturing the energy dispersion structure is developed and applied to the DMSP dataThe algorithm is applied and validated against a G2‐class Moderate StormNew types of ion dispersion events are uncovered with the algorithm and their relevance is stated [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]

The possible relation between pre‐earthquake processes and ionospheric fluctuations is widely investigated and debated. Here, we conduct a numerical experiment to examine two studies that reported preseismic anomalies in the ionospheric total electron content (TEC) and argue for the significance of their respective analyses based on statistical evaluations. The first study is that by Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01), who statistically investigated the relationship between 62 M ≥ 6 earthquakes occurred in China over an 18‐year period and the TEC, which was deduced from the Global Ionospheric Map. According to this study, the TEC showed anomalies with specific polarities at certain days and times that preceded the earthquakes. Earthquake alarms they defined based on these anomalies showed a good performance. We conduct this same analysis using random synthetic earthquakes and obtain results that closely match those by Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01), thus indicating that the good performance of the Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01) alarm is an artifact. The second study is that by Le et al. (2011, https://doi.org/10.1029/2010ja015781), who classified the TEC time series into anomalous and non‐anomalous days based on the TEC perturbation. They found that the anomalous day rate increased as the nucleation time of the earthquakes was approached, especially for larger and shallower earthquakes. We conduct the same analysis using random synthetic earthquakes. The anomalous day rate that is comparable to their result occurs in ∼40% of the 1,000 random trials, thereby suggesting that their result may also be an artifact. Plain Language Summary: Fluctuations in the ionospheric electron have been considered as a possible precursor of earthquakes. Papers by Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01) and Le et al. (2011, https://doi.org/10.1029/2010ja015781) are important as positive reports evaluating the precursory behavior from a statistical point of view. Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01) reported that the electrons increases/decreases at specific leading times before 62 large earthquakes occurred in China. Earthquake alarms based on these anomalies showed a good performance. Le et al. (2011, https://doi.org/10.1029/2010ja015781) reported that rate of days with large fluctuation of electron contents increases before shallower and larger earthquakes, based on global 736 large earthquakes over 8 years. We test their methods by numerical experiments generating random earthquakes. Even for the random earthquakes never related to ionospheric processes, the method by Liu et al. (2018, https://doi.org/10.3319/tao.2018.03.11.01) achieves a high alarm performance comparable to their original paper. Regarding Le et al. (2011, https://doi.org/10.1029/2010ja015781), the rate of days with large electron fluctuation calculated by their equation increases before shallower and larger earthquakes even though the given rate is constant for all the earthquakes. These results show that the relation between ionospheric processes and earthquakes reported by the both studies should be artifact derived from the methodology. Key Points: Pre‐seismic ionospheric electron anomalies detected based on global navigation satellite system and Global Ionospheric Map are tested from a statistical point of viewNumerical experiments based on randomly generated earthquakes reproduced the statistical features of the "significant precursor"The statistical significance inferred by previous studies may have been caused by false predictions due to arbitrarily defined "precursor" conditions [ABSTRACT FROM AUTHOR]

Far ultraviolet (FUV) imaging of the aurora from space provides great insight into the dynamic coupling of the atmosphere, ionosphere, and magnetosphere on global scales. To gain a quantitative understanding of these coupling processes, the global distribution of auroral energy flux is required, but the inversion of FUV emission to derive precipitating auroral particles' energy flux is not straightforward. Furthermore, the spatial coverage of FUV imaging from Low Earth Orbit (LEO) altitudes is often insufficient to achieve global mapping of this important parameter. This study seeks to fill these gaps left by the current geospace observing system using a combination of data assimilation and machine learning techniques. Specifically, this paper presents a new data‐driven modeling approach to create instantaneous, global assimilative mappings of auroral electron total energy flux from Lyman‐Birge‐Hopfield (LBH) emission data from the Defense Meteorological System Program (DMSP) Special Sensor Ultraviolet Spectrographic Imager (SSUSI). We take a two‐step approach; the creation of assimilative maps of LBH emission using optimal interpolation, followed by the conversion to energy flux using a neural network model trained with conjunction observations of in‐situ auroral particles and LBH emission from the DMSP Special Sensor J and SSUSI instruments. The paper demonstrates the feasibility of this approach with a model prototype built with DMSP data from 17 February 2014 to 23 February 2014. This study serves as a blueprint for a future comprehensive data‐driven model of auroral energy flux that is complementary to traditional inversion techniques to take advantage of FUV imaging from LEO platforms for global assimilative mapping of auroral energy flux. Plain Language Summary: When energetic protons and electrons interact with the nitrogen gas molecules of the Earth's atmosphere at high latitudes, light emissions including ultraviolet emissions in the Lyman‐Birge‐Hopfield (LBH) band are created. Our goal is to make global maps of the energy flux of these particles using images of these LBH emissions observed by the Defense Meteorological System Program (DMSP) Low Earth Orbiting (LEO) satellites. This is a challenging task because ultraviolet imagers onboard LEO satellites can only provide partial coverage of the global high‐latitude region and emissions are indirect measurements of the energy flux. To address these problems, we first determine a small set of global patterns from lots of DMSP data that efficiently explain how these LBH emissions vary over time. By using these global patterns, we then make global maps of LBH emissions for a particular time from instantaneous LBH emission observations. We finally relate global LBH emission maps to the energy flux using a neural network model trained with data from another DMSP instrument that measures the energy flux of precipitating particles as well as LBH emission data. Our study serves as a blueprint for a future comprehensive data‐driven modeling of auroral energy flux from ultraviolet imagers onboard LEO satellites. Key Points: An end‐to‐end data‐driven modeling approach for assimilative mapping of auroral electron energy flux using Special Sensor Ultraviolet Spectrographic Imager (SSUSI) Lyman‐Birge‐Hopfield (LBH) emissions is developedA neural network model to predict electron flux from LBH emissions is learned using 1 week of Defense Meteorological System Program F16, F17, and F18 SSUSI and Special Sensor J dataA blueprint is presented of a novel approach that combines multiple machine learning methods and is scalable to larger Low Earth Orbit Far ultraviolet imaging data [ABSTRACT FROM AUTHOR]

This paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3‐D time‐dependent magnetohydrodynamic model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform eight simulation cases under solar minimum quiet‐solar‐wind conditions (four equinox/solstice seasons, each with two interplanetary magnetic field polarities) and in an actual interplanetary coronal mass ejection (ICME) case to assess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that (a) the ionosphere is magnetically connected with high altitudes through open field lines and (b) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small‐scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is ∼0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. Modeling shows the occurrence probability dramatically increases during the ICME event, particularly by more than an order of magnitude in weak crustal field regions. Our model results reasonably agree with NASA Mars Atmosphere and Volatile EvolutioN and Mars Express observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp‐like process but due to the recycling of nightside magnetospheric electrons. Plain Language Summary: This paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora at Mars on a planetary scale. Discrete aurora is a patchy and sporadic phenomenon, posing a daunting challenge for understanding how and where auroral electron precipitation takes place in the ionosphere. To address this challenge, we apply a state‐of‐the‐art global model to characterize the spatial and temporal dynamics of near‐Mars plasma and magnetic field environments. We simulate the penetration of energetic electrons from high altitudes to the nightside ionosphere through open magnetic field lines, a process responsible for auroral emissions. Our results show that during quiet solar activity, the overall chance of observing a discrete auroral event on the nightside is only ∼0.77% across the globe, but the odds are significantly higher over strong crustal field regions. The occurrence probability is enhanced globally during an extreme space weather event, especially in weak crustal field regions. This study reveals a different source mechanism from the prevailing hypothesis that discrete aurora may be excited by the direct entry of magnetosheath plasma in a cusp‐like process. Instead, we propose that it is caused by the recycling of nightside magnetospheric electrons that return to the ionosphere. Key Points: Modeling shows that discrete aurora occurs more likely on small‐scale patches embedded inside strong crustal field regionsAuroral occurrence probability greatly increases during space weather events, particularly in weak crustal field regionsDiscrete aurora is not caused by the direct entry of magnetosheath plasma but due to the recycling of nightside magnetospheric electrons [ABSTRACT FROM AUTHOR]

In this paper, we study the onset conditions and features of equatorial F region irregularities linked with equatorial plasma bubbles (EPBs) using observations made simultaneously by using a DPS‐4D digisonde and the Gadanki Ionospheric Radar Interferometer (GIRI), both collocated at Gadanki. Importantly, we have employed specific analysis tools on DPS‐4D observations, providing range‐time displays of radio frequency and signal‐to‐noise ratio (SNR) of the reflected/backscattered echoes and the angle‐of‐arrival of the echoes, to characterize the echoes and to study the ambient and disturbed states of the ionosphere. Observations clearly show noticeably different background conditions, in terms of the height of the F layer base, electron density gradient and vertical drift, for the freshly generated and drifting EPBs. The zonal wavelengths of the pre‐sunset large‐scale wave structures (LSWS) observed using the DPS‐4D, however, are found to show a close connection with EPB spacings for freshly generated and drifting EPBs, consistent with earlier findings. The satellite traces were observed just prior to the equatorial spread F (ESF) and were found to be associated with the bottomside upwellings. Comparison of the range‐time displays of radio frequency and SNR of the DPS‐4D observations and the height‐time variations of SNR of the GIRI observations demonstrates that the relatively weak echoes in the DPS‐4D observations, which represent the ESF echoes, correlate fairly well temporally with the plume structures observed by the GIRI. The GIRI observations also reproduce the bottomside upwellings observed by the DPS‐4D. We propose that the bottomside upwellings are due to the growth of the LSWS and these bottomside upwellings are instrumental for the growth of the Rayleigh‐Taylor instability generating EPBs. Finally, the new aspects of the digisonde observations are discussed with regard to their usefulness in understanding and forecasting EPBs/ESF. Plain Language Summary: Equatorial plasma bubbles (EPBs) and associated irregularities are the important constituents of the near‐earth space weather. They are highly detrimental for satellite‐based communication/navigation applications. In this paper, we investigate the onset conditions and features of equatorial F region irregularities linked with EPBs using observations made simultaneously by a DPS‐4D digisonde and the Gadanki Ionospheric Radar Interferometer (GIRI), both collocated at Gadanki. We have employed specific analysis tools on DPS‐4D observations, providing range‐time displays of radio frequency and signal‐to‐noise ratio (SNR) of the reflected/backscattered echoes and the angle‐of‐arrival of the echoes, to characterize the echoes and to study the ambient and disturbed states of the ionosphere. Observations clearly show noticeably different background conditions for the freshly generated and drifting EPBs. The zonal wavelengths of the pre‐sunset large‐scale wave structures (LSWS) observed by the DPS‐4D, however, are found to show a close connection with EPB spacings for freshly generated and drifting EPBs, consistent with earlier findings. The satellite traces were observed just prior to the ESF and were found to be associated with the bottomside upwellings. Comparison of the range‐time displays of radio frequency and SNR of the DPS‐4D observations and the height‐time variations of SNR of the GIRI observations demonstrates that the relatively weak echoes in the DPS‐4D observations, which represent the ESF echoes, correlate fairly well temporally with the plume structures observed by the GIRI. The GIRI observations also reproduce the bottomside upwellings observed by the DPS‐4D. We propose that the bottomside upwellings are due to the growth of the LSWS and these bottomside upwellings are instrumental for the growth of the Rayleigh‐Taylor (RT) instability generating EPBs. The usefulness of the new aspects of the digisonde observations are discussed in understanding and forecasting EPBs/ESF. Key Points: Untapped potentials of a digisonde in studying the ambient and disturbed states of the equatorial ionosphere are demonstratedUse of digisonde observations for predicting overhead equatorial plasma bubbles (EPBs) has been demonstratedResults indicate that bottomside upwellings are due to growth of LSWS and are responsible for the satellite trace and subsequent growth of the Rayleigh‐Taylor instability generating EPBs [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]