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Spinning disk of plasma results in deformation of Jupiter's magnetic field from a dipole to a magnetodisc configuration with significant radial and azimuthal components. Polar orbit of the Juno spacecraft enables a comprehensive in situ observation of Jupiter's plasmadisc at different latitudes. Observations from magnetometer and plasma distribution instruments on Juno mission were used to examine interaction of magnetic field and heavy ions in Jupiter's nightside magnetosphere. Power spectrum analysis was used to evaluate magnetic fluctuations inside and outside of the plasmadisc. Extensive radial maps of magnetic activity in Jupiter's nightside magnetosphere are presented. Comprehensive examination of magnetic fluctuations enable to identify local plasma density variations that correlate with plasma distribution count rate fluctuations. Observations of magnetic signatures together with examination of plasma distribution are used to detect features that point to plasma transport inside and outside of Jupiter's plasmadisc. This work shows that plasma in Jupiter's magnetosphere is contained near centrifugal equator and is not a subject to curvature drift that could generate ring currents. Signatures described as current sheet crossings were investigated. On close inspection these signatures show attributes that belong to stand alone formations rather than a single current sheet structure. This work shows that current flow is not an adequate mechanism to explain deformation of Jupiter's magnetic field. Plain Language Summary: The goal of this paper is to use observations from Juno's magnetometer and plasma instruments to investigate structure of Jupiter's magnetosphere. Magnetometer instrument observations are used to construct detailed maps of magnetic activity. Identified magnetic signatures enable to detect structures that point to plasma transport inside and outside of the giant planet's plasmadisc. This study shows that plasma in Jupiter's magnetosphere is contained near centrifugal equator and is not a subject to curvature drift in dipole configuration, that could generate ring currents. Since ring currents are not generated then they cannot be responsible for distorting magnetosphere from dipole in to magnetodisc configuration. Close inspection of magnetic field signatures described as current sheet crossing show that they possess attributes that belong to stand alone individual structures and are not a part of one continuous current sheet formation. It is important to note that multiple publications have been reporting observations of nonexistent magnetosphere deforming "current sheet" since pioneer 10 encounter with Jupiter. Findings in this work warrants a further comprehensive examination of formation of Jupiter's magnetosphere and plasma transport mechanisms within it. Key Points: Shear waves propagate out the plasmadisc while compressive field fluctuations are associated with plasma distribution variationsMagnetic field signatures are used to identify structures that point to plasma transport mechanisms inside and outside of the plasmadiscCurrent flow is not an adequate mechanism to describe deformation of the magnetosphere of Jupiter from dipole to magnetodisc configuration [ABSTRACT FROM AUTHOR]

Coalescence of magnetic flux ropes (MFRs) is suggested as a crucial mechanism for electron acceleration in various astrophysical plasma systems. However, how electrons are being accelerated/heated via MFR coalescence is not fully understood. In this paper, we quantitatively analyze electron heating and acceleration during the coalescence of three MFRs at Earth's magnetopause using in‐situ Magnetospheric Multiscale (MMS) observations. We find that suprathermal electrons are enhanced in the coalescing MFRs than those in the ambient magnetosheath and non‐coalescing MFRs. Both first‐order Fermi and E|| acceleration were responsible for this electron acceleration, while the overall effect of betatron process cooled the electrons. The most intense Fermi acceleration was observed in the trailing part of the middle MFR, while E|| acceleration occurred primarily at the reconnection sites between the coalescing MFRs. For non‐coalescing MFRs, the dominant mechanism for energizing electrons is the E|| acceleration. Our results further consolidate the important role of MFR coalescence in electron heating/acceleration in space plasma. Plain Language Summary: Magnetic flux ropes are common magnetic structures in space environments and are believed to play a significant role in electron heating. Adjacent magnetic flux ropes can coalesce through magnetic reconnection, forming larger‐scale magnetic flux ropes. The significant efficiency of electron heating within coalescing magnetic flux ropes has been reported thoroughly by theoretical and numerical simulation studies, but has not been confirmed by in‐situ observation. Our research reports an event of three magnetic flux ropes coalescing in pairs and provides detailed quantitative analyses of associated mechanisms. Furthermore, we compare the electron mechanisms within these coalescing magnetic flux ropes with other non‐coalescing flux ropes. Our study contributes to a further understanding of the production mechanisms of high‐energy electrons in space plasmas. Key Points: Observation of two coalescing reconnections among three contiguous magnetic flux ropes (MFRs)Coalescing MFRs produced more suprathermal electrons than non‐coalescing MFRsFermi and E|| mechanisms were primarily responsible for electron heating and acceleration within coalescing MFRs [ABSTRACT FROM AUTHOR]

The ion foreshock is highly dynamic, disturbing the bow shock and the magnetosphere‐ionosphere system. To forecast foreshock‐driven space weather effects, it is necessary to model foreshock ions as a function of upstream shock parameters. Case studies in the accompanying paper show that magnetosheath ions sometimes exhibit strong field‐aligned asymmetry toward the upstream direction, which may be responsible for enhancing magnetosheath leakage and therefore foreshock ion density. To understand the conditions leading to such asymmetry and the potential for enhanced leakage, we perform case studies and a statistical study of magnetosheath and foreshock region data surrounding ∼500 Time History of Events and Macroscale Interactions during Substorms mission bow shock crossings. We quantify the asymmetry using the heat flux along the field‐aligned direction. We show that the strong field‐aligned heat flux persists across the entire magnetosheath from the magnetopause to the bow shock. Ion distribution functions reveal that the strong heat flux is caused by a secondary thermal population. We find that stronger asymmetry events exhibit heat flux preferentially toward the upstream direction near the bow shock and occur under larger IMF strength and larger solar wind dynamic pressure and/or energy flux. Additionally, we show that near the bow shock, magnetosheath leakage is a significant contributor to foreshock ions, and through enhancing the leakage the magnetosheath ion asymmetry can modulate the foreshock ion velocity and density. Our results imply that likely due to field line draping and compression against the magnetopause that leads to a directional mirror force, modeling the foreshock ions necessitates a more global accounting of downstream conditions. Key Points: Ion field‐aligned asymmetry persists across the magnetosheath, caused by a secondary thermal population of magnetosheath ionsLarger IMF strength and solar wind dynamic pressure and/or energy flux favor stronger field‐aligned asymmetryNear the bow shock, the foreshock ion velocity and density are modulated by the magnetosheath ion field‐aligned asymmetry [ABSTRACT FROM AUTHOR]

Terrestrial Gamma Ray Flashes (TGFs) are short bursts of gamma rays occurring during thunderstorms. They are believed to be produced by Relativistic Runaway Electron Avalanches (RREAs). In this paper, we present a new numerical model based on the Particle‐In‐Cell (PIC) method to simulate the interactions between the electromagnetic fields and the electron avalanche self‐consistently. The code uses a cylindrical Yee lattice to numerically solve the electromagnetic fields, a Monte Carlo approach to simulate collisions with air molecules, and a plasma fluid model to calculate the effects of low‐energy electrons and ions. The model is first tested through the reproduction of dispersion relations in a hot plasma. RREAs propagating under a homogeneous background electric field are then simulated. Owing to the self‐consistent nature of this description, we report here new physical properties such as saturation processes in the electron density and in the number of high‐energy electrons, detailed dynamical screening of the electric field in the ion trail of the avalanche, and the associated electric currents. We find that the saturation of RREAs is obtained when the numbers of high‐energy electrons and photons is consistent with those believed to be representative of TGF sources. Key Points: Development of a self‐consistent model coupling a Monte Carlo code with an electromagnetic particle‐in‐cell code to simulate Relativistic Runaway Electron Avalanches (RREAs)We identify a saturation phenomenon of the RREA, constraining the maximum number of high‐energy electrons and photons in terrestrial gamma‐ray flashesOnce saturation is reached, the electric field is partially screened, preventing the subsequently injected electrons from accelerating [ABSTRACT FROM AUTHOR]

This paper aims to quantify the magnetospheric magnetic flux contents under moderate to intense space weather conditions using global simulations. This study is a companion to Akhavan‐Tafti, Atilaw, et al. (2023, https://doi.org/10.1029/2023JA031832) where magnetic flux evolution is presented for a catalog of storm events, using Heliophysics System Observatory (HSO) observations. For this study, we used the Space Weather Modeling Framework (SWMF) in Geospace configuration to study magnetic flux dynamics for a subset of their storm events (15 events). Simulations reliably resolve the storm‐time magnetic flux Bz and current density |J| asymmetries across the different storm phases. It is revealed that: relative to the quiet period, flux content is enhanced during the storm sudden commencement (SSC) phase in the dayside by ΔBz/Bz, quiet = +17%, and reduced in the nightside magnetosphere (r[RE] < −6 RE) by −15%. At the same time, the cross‐tail current is found to enhance (|J| = 2 nA/m2), which suggests the storm impact in the nightside magnetosphere is much earlier in the storm cycle than previously shown. Concurring with previous studies, a significant depletion of magnetic flux by up to −40%, with day‐night and dawn‐dusk asymmetries, can be seen during the main and recovery phases. This corresponds to the enhanced current density (|J| = 5–8 nA/m2) at ∼6 RE further confirming the role of ring current in driving magnetospheric dynamics during the main and recovery phases. This is in contrast with the SSC phase wherein the Chapman‐Ferraro and cross‐tail currents are the dominant current systems. Key Points: The nightside magnetospheric flux is impacted by the storm event earlier in the storm cycle than expected, (i.e., the SSC phase)Decrease in flux content of up to −40% within the inner magnetosphere for the main and recovery phases is associated with the enhanced ring currentStorm‐time asymmetries in magnetic flux and current density are well‐resolved throughout the storm phases [ABSTRACT FROM AUTHOR]

The present paper studies the latitudinal and longitudinal variation of ionospheric semidiurnal lunar tide (M2) perturbations in 2013 with total electron content (TEC) data. Results show that the latitudinal distribution of M2 exhibits longitudinal difference. At 100 ∼ 120°E and 80 ∼ 100°W longitudes, the strongest M2 perturbations in the northern and southern hemispheres roundly appeared at about 10° ∼ 20°N and 15° ∼ 25°S magnetic latitude (MLAT), respectively, which supports previous suggestion that the M2 maximized in the EIA crest regions. Moreover, the latitude range of M2 at 80 ∼ 100°W longitudes was wider than that at 100 ∼ 120°E longitudes, especially for southern hemisphere (SH). The M2 maximized at 20° ∼ 30°N MLAT in the northern hemisphere (NH) and was absent in the SH at 50°W ∼ 20°E longitudes, which was distinctly different than those at other longitudes. This abnormal behavior of latitudinal distribution of M2 may be caused by the large magnetic declination angle at 50°W ∼ 20°E longitudes. Meanwhile, the longitudinal variation and hemispheric asymmetry of M2 amplitude are different between NH winter (SSW period) and other seasons. The M2 amplitudes were stronger disturbances in the NH and in the American longitudinal sector in NH winter. However, in other seasons, the M2 amplitude was largest in Asian sector, and the M2 amplitude in NH was larger than SH at 0° ∼ 130°E longitudes, which was contrast to these at 0° ∼ 110°W longitudes. These differences of M2 variation between NH winter and other seasons may be related to the differences of the combined effects of the geomagnetic configuration and background conditions of the atmosphere and ionosphere. Key Points: The latitudinal and longitudinal variation of the semidiurnal lunar tide (M2) in the ionosphere were studied with total electron contentThe latitudinal distribution of M2 exhibits longitudinal differenceThe longitudinal variation and hemispheric asymmetry of M2 amplitude are different between Northern Hemisphere winter and other seasons [ABSTRACT FROM AUTHOR]

The auroral arc is the typical track of the interaction between the solar wind and the Earth's magnetosphere. A sketch of skeletons for arc-like aurora is usually used to describe auroral structures, such as vortex, fold and curl structures, etc. With artificial intelligence technologies, sketching auroral skeleton structure (AuroSS) in all-sky images enables automatic detection and measurement of aurora arcs in very large amounts of ground-based auroral observation data. The skeleton is a highly characterizing topological structure that has been extensively studied in the field of computer vision. However, AuroSS is not the medial axis of auroral shapes and a large number of accurate AuroSS annotations are not available. It is difficult to detect AuroSS by using an unsupervised or fully-supervised method. In this paper, we formulate the automatic AuroSS extraction to learn a mapping from an all-sky auroral image to a ridge style AuroSS. Without accurate AuroSS annotations, emission ridge and coarse localization of aurora are incorporated to generate pseudo-labels of AuroSS. A series of functional weakly supervised models are trained and cascaded to achieve AuroSS detection. Experimental results on auroral images obtained from all-sky imagers at Yellow River Station (YRS) show that the detected AuroSS is consistent with that of human visual perception. Based on the obtained AuroSS, the orientations and lengths of auroral arcs can be estimated automatically. By browsing the temporal variation in arc orientation from dusk to dawn, we can acquire synoptic observations of auroral activities at YRS. [ABSTRACT FROM AUTHOR]

Both the bow shock and magnetopause move in response to varying solar wind and magnetospheric conditions. Tracking their locations can provide important clues to the state of the solar wind-magnetosphere interaction, but is difficult with single spacecraft observations. This paper employs multipoint THEMIS observations of velocity gradients in the subsolar magnetosheath to remotely sense boundary locations on a continual basis for various solar wind conditions. We present three cases: (a) continuous northward IMF and no magnetopause motion; (b) southward IMF and no magnetopause motion with evidence of nightside activity; and (c) southward IMF and pressure increase with inward motion. When observing spacecraft are located near the Sun-Earth line, inferred boundary locations agree well with the predictions of the BATS-R-US global magnetohydrodynamic model, confirming the utility of both the new method and the models. Results show that boundaries often lie nearly at rest with amplitudes less than 0.5 RE. They provide evidence indicating that nightside reconnection and a strong sunward convection in the outer magnetosphere can counteract the magnetopause erosion expected when a southward interplanetary magnetic field (IMF) initiates reconnection on the dayside magnetopause. [ABSTRACT FROM AUTHOR]

Much of the longitude/local time dependence of the thermosphere is controlled by non-migrating tides. Observations of semidiurnal (12-hr) tides between 120 and 200 km altitude, that is, the middle thermosphere, are rare owing to the lack of systematic measurements in this region. Since late 2018, the Global-scale Observations of the Limb and Disk (GOLD) Mission has provided unique measurements of thermospheric disk temperature and the column density ratio of atomic oxygen to molecular nitrogen ratio (ΣO/N2) from geostationary orbit. In this paper, we present an approach to deduce the strongest semidiurnal non-migrating tides in the middle thermosphere by adapting the method of Krier et al. (2021, https://doi.org/10.1029/2021ja029563) that deduces diurnal non-migrating tides in simultaneous observations of temperature and ΣO/N2 made by GOLD. Testing of this approach suggests that the principal sources of uncertainties in the derived semidiurnal non-migrating tides are the limitation on the longitudes sampled, such that uncertainties are higher for tides with longer horizontal wavelength, and contamination of the local time sums by stationary planetary waves, which causes amplitudes to be overestimated. Our approach is applied to GOLD data during solstice conditions in 2019-2021. Comparison to models yield disagreements which are likely due to uncertainties intrinsic to the method and/or misrepresentation of tidal dynamics in the models. These results are the first observations of semidiurnal non-migrating tides in the middle thermosphere from a geostationary observational platform. [ABSTRACT FROM AUTHOR]

The sporadic E (Es) layer virtual height h'Es and the ordinary wave critical frequency f oEs are routinely measured ionogram parameters, used to characterize the Es altitude occurrence and intensity. It has become common practice to take the real height hEs to be about equal to h'Es by assuming that signal propagation delays in the E region plasma below the layer are small and can be neglected. Although this applies for nighttime, during daytime it may overestimate hEs significantly. The present paper relies on true height analysis theory to devise a simplified method and propose an algorithm that can estimate Es real heights reasonably well. The method relies on h'Es and f oEs ionosonde measurements and E region electron density profiles obtained from the International Reference Ionosphere model. The algorithm is applied to a typical set of Digisonde observations to compute hEs and examine real height variations and functional dependencies. Whereas hEs ≃ h'Es at nighttime, during daytime there are notable h'Es - hEs differences taking values less than 10 km for most of the observed layers. During the early morning and early afternoon hours, however, when weak layers appear at upper heights, the virtual to real height differences become larger reaching 20-25 km. The method proposed here for the estimation of hEs can be easily applied to improve the accuracy of the results of sporadic E layer studies. [ABSTRACT FROM AUTHOR]

We use a 20 years database of Super Dual Auroral Radar Network (SuperDARN) observations to investigate the two component model of ionospheric convection. A convection pattern is included in the database if it is derived from at least 250 radar vectors and has a distribution of electric potential consistent with Dungey‐cycle twin vortex flow (a negative potential peak in the dusk cell and a positive potential peak in the dawn cell). We extract the locations of the foci of the convection cells from the SuperDARN convection patterns, and compare their dependencies on the north‐south component of the interplanetary magnetic field, IMF BZ, and the SuperMAG auroral electrojet index, SML. We use these parameters to define intervals of expected dayside or nightside dominated reconnection. Our results show that, under conditions favorable for dominant dayside reconnection, the dawn and dusk foci are shifted toward the dayside and that, under conditions favorable for dominant nightside reconnection, the dawn and dusk foci are shifted toward the nightside. Plain Language Summary: The Earth's upper atmosphere is coupled to the near‐Earth space environment—the magnetosphere—via the planet's magnetic field. This magnetic coupling drives a circulation of plasma—the electrically charged component of the atmosphere, called the ionosphere—from day to night across the poles and back again at lower latitudes. This circulation of plasma is a key component of the energy transport in the magnetosphere‐ionosphere system. The circulation is not steady, instead changing in strength whilst expanding and contracting due to the time‐dependence of the driving mechanisms. To understand these mechanisms we can model the ionospheric circulation and test the models with observations. In this paper we use a 20 years database of ionospheric radar observations of the plasma flow to test one such model—the expanding‐contracting polar cap model—and find evidence to support its predictions of separate dayside and nightside components of the flow. Key Points: The separation of the dawn and dusk ionospheric convection cell foci is found to vary from 4 to 22 hr of magnetic local timeWhen the interplanetary magnetic field (IMF) is southwards and the auroral westward electrojet weak, the foci move to the daysideWhen the IMF is strongly northwards and the auroral electrojet modestly active, the foci move toward the nightside [ABSTRACT FROM AUTHOR]

Measuring the plasma motion vector field accurately is beneficial to understanding the physical processes in the ionosphere. As a newly built advanced modular phase array Incoherent Scatter Radar in Sanya (SYISR), SYISR can measure the line‐of‐sight (LOS) velocity of ion drift in multiple directions, potentially yielding the spatial distribution of ionospheric plasma drift at low latitudes in East Asia. Compared to the traditional ISR, it can operate continuously for a long‐time and perform with fast and flexible scanning. This paper evaluates different beam configurations and inversion methods for vector ion velocity detection by comparing the vector ion velocities input in the forward modeling and derived by inversion. For the mono‐static SYISR, the all‐sky scan mode is suitable for observing the distribution of vector ion velocity with height, and the fine scan mode is suitable for observing the spatial distribution of two components of vector ion drift perpendicular to the magnetic field in the northern region of SYISR's non‐grating lobe. For the future tri‐static SYISR, it could simultaneously observe the spatial distribution of all three components of the vector ion velocity. Besides, the inversion method based on Bayesian estimation shows good accuracy and stability in deriving the vector ion velocity, especially when the LOS ion velocity has a large error. These results will benefit the design and optimization of the actual experiments for the vector ion velocity measurement and inversion using the SYISR system. Key Points: A procedure is constructed to guide and evaluate the detection of the vector ion velocity by the Incoherent Scatter Radar in Sanya (SYISR) systemThe ion velocity inversion method based on Bayesian estimation shows good accuracy and stability in deriving the vector ion velocityMono‐static and tri‐static SYISR can be used to detect the height profile of vector ion velocity and spatial distribution, respectively [ABSTRACT FROM AUTHOR]

The polar cap hot patch is an enhanced density structure which is associated with particle precipitation, ion upflow and flow shears. Based on combined observations from DMSP satellites, EISCAT radar, and all‐sky imager at the Chinese Yellow River Station, the plasma characteristics and evolution of hot dense plasma and classical polar cap patch are investigated. Both of them show enhanced F region density, but the hot dense plasma is associated with higher electron temperature and enhanced E region electron density caused by particle precipitation. Compared to the cold patch, the hot dense plasma is closer to the cusp region and is associated with strong auroral emissions. Based on the joint observations, the evolution of the hot/cold dense plasma is discussed. In the initial phase the dense plasma comes from the sunlit lower latitude region, or from particle precipitation in the duskside auroral oval, and is heated locally by poleward moving auroral structures in the dayside auroral oval. Next the patches move poleward into the polar cap where a decaying electron temperature (i.e., less precipitation) gradually transform them into a cold patch. Plain Language Summary: The polar cap hot patch is a high‐density ionospheric irregularity with enhanced electron temperature and particle precipitation. The density source and associated auroral emission of the hot patch are not clear. The relationship between the hot dense plasma (hot patch) and classical polar cap patch (cold patch) is a lingering question. In this paper, we show two hot dense plasma and cold patch events using multi‐instrument observations. Two density sources are suggested: dense plasma in the lower latitude sunlit region, and plasma ionized by particle precipitation in the duskside auroral oval. Our study shows the hot dense plasma is associated with stronger aurora emissions and higher E region density in the cusp region, followed by no auroral emission (colder electron temperature) as the hot dense plasma drifts poleward and turns into a cold patch in the polar cap. It suggests that the hot dense plasma and cold patch may be different states of the same irregularity structure. Key Points: The different plasma characteristics of hot dense plasma and cold patch in the polar cap are investigated using multi‐instrument observationsBoth hot dense plasma and cold patches are associated with auroral forms, which are located further equatorward in the cusp regionThe hot dense plasma and cold patch may represent different energy states of the same plasma structure as it traverses the polar ionosphere [ABSTRACT FROM AUTHOR]

In this paper, two‐dimensional hybrid simulations are used to study wave excitation and evolution throughout a low‐Mach number quasi‐parallel shock. Simulation results show that quasi‐parallel fast magnetosonic waves, ion Bernstein waves with harmonics and possible Alfven/ion cyclotron waves can be excited in the upstream region, and their small phase velocities compared to injected flow velocity results in the convection to the shock front where they are mode converted into several groups of downstream waves, including Alfven waves along the directions parallel to downstream average magnetic fields and perpendicular to the shock normal, the quasi‐perpendicular kinetic slow waves and possible kinetic Alfven waves. We suggest that downstream Alfven waves originate from the mode conversion of upstream quasi‐parallel fast magnetosonic waves with left‐hand polarization in the downstream rest frame under helicity conservation, while the downstream left‐hand polarized kinetic slow waves and right‐hand polarized kinetic Alfven waves can be from the upstream quasi‐perpendicular ion Bernstein waves. Key Points: Upstream and downstream waves at a low‐Mach number quasi‐parallel shock are investigated by 2‐D hybrid simulationsUpstream quasi‐parallel and quasi‐perpendicular waves are excited by the interaction of injected flow and backstreaming ionsDownstream waves are generated by the mode conversion of upstream waves [ABSTRACT FROM AUTHOR]

Jupiter's satellite auroral footprints are a consequence of the interaction between the Jovian magnetic field with co‐rotating iogenic plasma and the Galilean moons. The disturbances created near the moons propagate as Alfvén waves along the magnetic field lines. The position of the moons is therefore "Alfvénically" connected to their respective auroral footprint. The angular separation from the instantaneous magnetic footprint can be estimated by the so‐called lead angle. That lead angle varies periodically as a function of orbital longitude, since the time for the Alfvén waves to reach the Jovian ionosphere varies accordingly. Using spectral images of the Main Alfvén Wing auroral spots collected by Juno‐UVS during the first 43 orbits, this work provides the first empirical model of the Io, Europa, and Ganymede equatorial lead angles for the northern and southern hemispheres. Alfvén travel times between the three innermost Galilean moons to Jupiter's northern and southern hemispheres are estimated from the lead angle measurements. We also demonstrate the accuracy of the mapping from the Juno magnetic field reference model (JRM33) at the completion of the prime mission for M‐shells extending to at least 15 RJ. Finally, we shows how the added knowledge of the lead angle can improve the interpretation of the moon‐induced decametric emissions. Plain Language Summary: The interaction between the Jovian magnetospheric plasma and the Galilean moons gives rise to a complex set of phenomena, including the generation of auroral spots magnetically related to the moons and the generation of radio emissions. The magnetic perturbations local to the moons propagate at a finite speed along the magnetic field lines, and reach the northern and southern Jovian hemispheres where they produce the auroral spots. Studying the position of these auroral spots and how they vary over a complete Jovian rotation provides information about the magnetic mapping, as they map directly to the actual physical positions of the moons. The magnetic field model derived from Juno's prime mission is in good agreement with the observation of the satellite footprints. This paper provides information about how the electromagnetic perturbation resulting from the interaction propagates at a finite speed to create auroral spots, leading to an angular shift between the instantaneously magnetically‐mapped position of the moon and the auroral footprint, a quantity also known as the "equatorial lead angle". The present work provides an empirical fit of the equatorial lead angle for Io, Europa, and Ganymede derived from Juno data. Key Points: Over 1,600 ultraviolet spectral images of the Io, Europa, and Ganymede footprints from Juno are analyzedEmpirical formulae for the Io, Europa, and Ganymede equatorial lead angles derived from Juno data are providedAlfvén travel time estimates are derived, constraining the Alfvénic interaction at the three innermost Galilean moons [ABSTRACT FROM AUTHOR]

The solar wind ion transport across the magnetopause is one of the main sources of plasma for the Earth's magnetotail. Such a transport is supported by various dynamic processes at the flank magnetopause, where wave‐particle interactions play a crucial role in ion flow thermalization and diffusion across magnetic field surfaces of the magnetopause tangential discontinuity. In this paper we numerically model such ion thermalization by the most intense electromagnetic waves observed in the magnetosheath, kinetic Alfven waves. We aim to develop an approach for long‐term simulations of ion scattering by waves and ion dynamics around realistic magnetopause magnetic fields. This approach is based on a combination of test particle simulations and stochastic differential equations modeling ion diffusion in velocity space. We demonstrate that for realistic magnetopause configuration and wave characteristics, the magnetosheath ion flow can be substantially thermalized around the magnetopause. This result explains observations of ion energy conservation across the flank magnetopause: kinetic and thermal energies of flowing magnetosheath ions approximately equal to the thermal energy of stagnant magnetospheric ions. Key Points: We model the ion flow thermalization by kinetic Alfven waves around the flank magnetopauseThe model uses stochastic differential equations for particle trajectories to include effects of wave‐particle interactionsScattering and thermalization may explain ion energy conservation across the magnetopause [ABSTRACT FROM AUTHOR]

Tracking the location and motion of the bow shock and magnetopause can reveal many aspects concerning the nature of the solar wind‐magnetosphere interaction. In the absence of missions that image the magnetopause, we propose to detect and track the magnetopause and bow shock motion using multipoint in situ observations of the magnetosheath. Making theoretical assumptions corroborated by numerical simulations, Sibeck et al. (2022, https://doi.org/10.1029/2022JA030704) proposed a method to predict the subsolar magnetopause and the bow shock from the linear variation in magnetosheath plasma velocity expected from global magnetohydrodynamic simulations along the Sun‐Earth line. This paper presents a case study that employs the velocity gradient method using multi‐point THEMIS plasma observations from 21 to 22 June 2007. Using the five THEMIS spacecraft we demonstrate that the radial gradient in plasma velocity remains nearly linear across the subsolar magnetosheath, except when the magnetosheath lies behind the quasi‐parallel bow shock and conditions are very turbulent. The bow shock and magnetopause move outward in response to a gradual decrease in solar wind dynamic pressure. We show that results from the velocity gradient method agree well with predictions of global magnetohydrodynamic and empirical models for this time interval. The velocity gradient method can therefore be used to remotely determine the location of the magnetopause and bow shock, solely based on two‐spacecraft measurements at the subsolar magnetosheath when not downstream of the quasi‐parallel shock. Key Points: Plasma velocities diminish linearly from ∼0.25 VSW inside the quasi‐perpendicular subsolar bow shock to 0 outside the subsolar magnetopauseSimultaneous two‐point measurements of stagnation streamline velocity gradients define the positions of the subsolar magnetopause and bow shockTurbulent magnetosheath flows preclude the use of the method behind the quasi‐parallel bow shock [ABSTRACT FROM AUTHOR]

Particle injection by magnetotail reconnection plays an important role in the magnetospheric physics, since these particles may be accelerated to high energy and constituent energetic particles in the inner magnetosphere. In this paper, we trace such ion particles from a near‐Earth reconnection region to the inner magnetosphere self‐consistently in a 3‐D global hybrid simulation using ANGIE3D, and demonstrate the important roles of the earthward fast flows in the ion acceleration process in the tail plasma sheet. Although the particles can gain several times of their initial energy from the reconnection X‐line region, a dramatic increase of the ion energy (from a few keV up to a few tens of keV) occurs in a very short period of time due to their encounter with the earthward fast flows. A large portion (≳70%) of the earthward moving particles around the focused reconnection site encounter fast flows and are significantly accelerated. Our results indicate that fast flow electric fields play major roles, as opposed to other common mechanisms such as the adiabatic Betatron and Fermi processes, in ion acceleration from the near‐tail region to the inner magnetosphere. In addition, dipolarization fronts associated with magnetic reconnection also affect the particle acceleration. On the global scale, it is found that particles can encounter the reconnection region first and then return to the fast flows multiple times, and thus the acceleration is through multiple local acceleration processes, leading to particle energy ∼50 keV by reconnection/fast flows in the tail. The global simulation shows that particle acceleration involves multiple regions during their injection into the ring current. Key Points: Earthward moving ion particles originating near the magnetotail reconnection region are traced self‐consistently in a global hybrid simulationThe most intense acceleration in a short period is due to particles' encounter with a fast‐flow electric fieldA great portion of the energetic particles is found to transport to the inner magnetosphere [ABSTRACT FROM AUTHOR]

We associate new data from icebear, a coherent scatter radar located in Saskatchewan, Canada, with scale‐dependent physics in the ionosphere. We subject the large‐scale icebear 3D echo patterns (treated as 2D point clouds) to a data analysis technique hitherto never applied to the ionosphere, a technique that is widely applied in cosmological red‐shift surveys to characterize the spatial clustering of galaxies. The technique results in a novel method to calculate the spatial power spectral density of the greater ionospheric irregularity field. We compare results from this method to in‐situ plasma density and magnetic field observations from the Swarm mission. We show that there is a remarkable similarity between echo clustering spectra in the E‐region and the field‐aligned current structuring spectrum observed in the F‐region: a clear and characteristic preferred scale (5 km) both in the E‐ and F‐region spectra. We discuss the possibility that this represents evidence of an energy injection into the ionospheric irregularity field via energetic particle precipitation, but offer alternative interpretations with wider connotations for the ionosphere‐magnetosphere system. These findings open new and promising avenues of research for the study of the location of ionospheric scatter echoes with 3D information. It constitutes a novel way to consider the pattern of ionospheric irregularities over wide fields of view when there is an abundance of radar echoes, which allows for the analysis of radar data as point clouds. Plain Language Summary: With the advent of precision radar data from the aurora containing detailed information about radar echo locations, we are in a position to apply statistical methods from cosmology to extract information about plasma turbulence in the ionosphere. This interdisciplinary paper presents some birds‐eye view discussions about the use of correlation functions in physics, showing how it can be used to understand physics on virtually all spatial scales in the observable universe. We present results pertinent to the ionosphere, shedding light on plasma turbulence and its relation to energetic particles in the aurora. Here, we use observations both from space, by the orbiting satellites in the European Space Agency's Swarm mission, and using an experimental auroral radar in Saskatchewan, Canada. Key Points: Clustering Statistics are used to extract large scale turbulence information from 3‐m structures in the E‐regionE‐region clustering spectra are remarkably similar to in‐situ spectra from field‐aligned currents in the F‐regionA preferential 5‐km scale is simultaneously observed in the E‐region and in the F‐region field‐aligned currents [ABSTRACT FROM AUTHOR]

Midlatitude ionosonde observations show that there is a sporadic E (Es) diurnal cycle that starts in higher altitudes at sunrise. This property is labeled as the "Sporadic E sunrise effect." Given that sporadic E layers are composed of metal ions, the sunrise effect implies that metal atom solar photoionization could play a role in the diurnal variability of sporadic E occurrence. This possibility is endorsed by Arecibo's incoherent scatter radar observations, showing that weak ion layers at lowest E and uppermost D region heights appear at sunrise to live during the daytime, apparently coming out of nighttime metal atom mesospheric layers. The solar photoionization of metal atoms increases the abundance of metal ions available for Es layer generation during the daytime, whereas this effect is absent at nighttime. This can explain why sporadic E layers start or intensify at sunrise all year round and why Es activity maximizes during sunlit hours, as has been reported in many ionosonde and satellite radio occultation studies. The significance of metal atom solar photoionization on the regular diurnal variation of Es went unnoticed, despite existing evidence for a long time. The present paper provides a base for a better physical understanding of the pronounced 24-hr periodicity in Es layer intensity and places a step toward the improvement of simulation models for the predictions of sporadic E layer characteristics. [ABSTRACT FROM AUTHOR]

Previously, suggesting a scenario of "injected and drifting electron cloud" to describe the enhancements and precipitations of energetic electrons during substorms we used the linear prediction filter (LPF) method to build the empirical dynamical model of auroral absorption in the middle of auroral zone, driven by the midlatitude positive bay (MPB) index time series. In this paper, to understand better the relationship between magnetic dipolarization, injection, and energetic electron precipitation, we quantitatively explore correlations between dipolarization proxies (MPB and SML indices, substorm current wedge (SCW) intensity, location, and size) and precipitation‐related auroral absorption in the morning maximum region for 148 isolated substorms. We confirm good correlation of precipitation peak values with dipolarization proxies and show that the absorption amplitude is most strongly controlled by total SCW current and its azimuthal size, and is also influenced by solar wind‐dependent background energetic electron flux in the conjugate plasma sheet. This result confirms the adequacy of our starting assumption that there is a close relationship between the intensities and size of substorm dipolarizations and energetic particle injections. To extend the latitudinal coverage, we computed the LPF response functions for individual riometers of NORSTAR array. Finding similar shapes and amplitudes of their response at latitudes 63°–69° in the regions where maximal precipitation is statistically observed, we suggest, test, and approve that the auroral absorption in the belt 63°–69° may be predicted using a set of LPFs determined empirically in the center of auroral zone for various magnetic local times. Key Points: Intensity and azimuthal width of the dipolarization together with available plasma sheet electron flux control the peak auroral absorptionSubstorm variations in the auroral zone can be predicted using statistical LPF response functions based on MPB driver [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study of the intense current structures (ICSs) at the dayside magnetopause, by using the high‐resolution data from the Magnetospheric Multiscale (MMS) mission. About 3,600 ICSs with a current density exceeding 1.2 μA/m2 have been detected within the magnetopause boundary layer (MBL). We find that most ICSs crossed the spacecraft in less than 1 s, corresponding to an average thickness of less than one ion inertial length (di). The number of ICSs decreases with the thickness increasing from the electron‐scale to the ion‐scale. Boundary layers closer to Earth have more ICS than boundary layers further away from Earth, probably caused by the large solar wind dynamic pressure. The occurrence rate of the ICS is higher in the dusk sector near the meridian. For most ICSs, the current is carried by electrons. The perpendicular current (90° to the magnetic field) is larger than the parallel current (0° or 180° to the magnetic field) for more ICSs. The energy conversion between the magnetic field and plasma as measured by J·E $\boldsymbol{J}{\dot{}}\boldsymbol{E}$ is primarily through the perpendicular current and electric field, while the energy dissipation J·E′=J·(E+Ve×B) $\boldsymbol{J}{\dot{}}{\boldsymbol{E}}^{\prime }=\boldsymbol{J}{\dot{}}(\boldsymbol{E}+{\boldsymbol{V}}_{\boldsymbol{e}}\times \boldsymbol{B})$ is mainly dominated by the parallel component. ICSs provide much stronger energy conversion and dissipation compared to the ambient plasma in the MBL. This study improves our understanding of the characteristics of the ICS and its role in solar wind‐magnetosphere coupling. Plain Language Summary: Current sheet is the hotbed for various instabilities and an important site for energy exchange between electromagnetic fields and plasmas. Current structures with electric current density exceeding 1 μA/m2 have frequently observed by the Magnetospheric Multiscale mission at the dayside magnetopause boundary layer. Important features of these intense current structures, such as the occurrence rate, current carrier, contribution to energy conversion, are not clear. This paper presents a comprehensive analysis of more than three thousand intense current structures at the magnetopause. We show that most of these current structures are extremely short in time domain and have thicknesses less than the ion inertial length. Moreover, they play important roles in energy dissipation within the boundary layer. These results are of great help in understanding the characteristics of the intense current structures and their role in the solar wind‐magnetosphere coupling. Key Points: Most intense current structures crossed the spacecraft in less than 1 s, and their thickness are mostly less than one ion inertial lengthThe intense current structure has a higher occurrence rate closer to Earth and in the dusk sector near the meridianThe intense current structure is important for energy dissipation in the magnetopause boundary layer [ABSTRACT FROM AUTHOR]

The paper underlines the view that the appearance of beading and its nonlinear growth in the onset arc occurs independently from the onset of reconnection in the tail at about 20 RE. Both events follow from an extreme thinning of the central current sheet of the tail at the end of the growth phase. Subsequently, we concentrate on the processes connected with the onset arc breakup. Its origin lies in the instability of a high‐beta plasma layer building up at the outer boundary of the dipolar magnetosphere during the substorm growth phase, the growth phase arc (GPA) being the ionospheric trace. The observation of auroral streamers triggering the onset arc instability lets us analyze what is known about auroral streamers with strong support from high‐resolution videos of two substorm onsets. We conclude that they may be low‐entropy content bubbles with a balanced field‐aligned current system, framing a flow channel. However, there are unresolved questions. The visible streamer is identified as an Alfvénic arc. In searching for a mechanism by which a streamer bubble lining up along the GPA can trigger the instability, we are led to the recognition that an entirely new non‐MHD process must be at work. Taking also into account the surprising fact that the beads are moving oppositely to the convection in GPA and auroral streamer, we postulate the appearance of a new current system in the gap between the two. What happens can be described as the mating of two current sheets, which were completely separated before. It breaks the stability of the high‐beta plasma layer and channels the release and conversion of free internal energy. For this reason, we name the process mating instability. A physical analysis of this process shows consistency with detailed features exhibited by the two videos Plain Language Summary: The substorm begins with two independent events of common origin. The first one is normally the brightening of the growth phase arc, which is the trace of a hot plasma layer forming at the inner border of the tail. The second one is due to flow bursts, emerging from reconnection in the tail, arriving at the inner edge of the tail, the very reason for the substorm. The common origin is the thinning of the tail current sheet due to the stretching by the solar wind. The paper is devoted to the understanding of the first event. As observed about 10 years ago, the brightening of the growth phase arc with beadlike structures and growth of instability is often triggered by a weak arc, an auroral streamer, arriving from high latitudes and lining up with the growth phase arc. The paper proposes that the trigger process involves the formation of a new current circuit between the two arcs by the mating of the neighboring current sheets, which involves a non‐MHD process. It generates the sudden appearance and motion of the beads and constitutes a channel for the outflow of internal energy of the high‐energy plasma. Key Points: Growth phase arc brightening and instability and reconnection in the near‐Earth tail are completely separate processesAuroral streamers may be low‐entropy content bubbles with a flow channel attached and manifested by an Alfvénic arcMating of two unconnected current sheets by a non‐MHD process creates a channel for outflow of energy from the high‐beta plasma layer [ABSTRACT FROM AUTHOR]

The global‐scale observations of the limb and disk (GOLD) Mission images middle thermosphere temperature and the vertical column density ratio of oxygen to molecular nitrogen (ΣO/N2) using its far ultraviolet imaging spectrographs in geostationary orbit. Since GOLD only measures these quantities during daylight, and only over the ∼140° of longitude visible from geostationary orbit, previously developed tidal analysis techniques cannot be applied to the GOLD data set. This paper presents a novel approach that deduces two specified non‐migrating diurnal tides using simultaneous measurements of temperature and ΣO/N2. DE3 (diurnal eastward propagating wave 3) and DE2 (diurnal eastward propagating wave 2) during October 2018 and January 2020 are the focus of this paper. Sensitivity analyses using TIE‐GCM simulations reveal that our approach reliably retrieves the true phases, whereas a combination of residual contributions from secondary tides, the restriction in longitude, and random uncertainty can lead to ∼50% error in the retrieved amplitudes. Application of our approach to GOLD data during these time periods provides the first observations of non‐migrating diurnal tides in measurements taken from geostationary orbit. We identify discrepancies between GOLD observations and TIE‐GCM modeling. Retrieved tidal amplitudes from GOLD observations exceed their respective TIE‐GCM amplitudes by a factor of two in some cases. Plain Language Summary: The uppermost region of the Earth's atmosphere, known as the thermosphere (∼80–600 km altitude), is connected to the lowermost region by planetary‐scale atmospheric waves, called non‐migrating tides, which are thermally driven and do not follow the apparent motion of the Sun across the sky. Understanding non‐migrating tides is essential to describing the global dynamics of the Earth's upper atmosphere. There is a gap in observations of these waves in the middle thermosphere temperature, around 150 km altitude. The NASA/GOLD instrument, in geostationary orbit above the mouth of the Amazon River, images the temperature and composition of the middle thermosphere. Conventional tidal analysis techniques cannot be applied to the GOLD data set, so we have designed a novel technique that infers important tides using simultaneous measurements of temperature and composition. For two separate time periods, we apply our technique to simulated observations and actual GOLD data. We find that our technique generally infers the most important tides in simulated data with high accuracy. The GOLD data reveal valuable observations of tides in the middle thermosphere as well as discrepancies with the simulated data. Key Points: First estimates of non‐migrating diurnal tides from an observational platform in geostationary orbit using GOLDDeduction of non‐migrating tides via known phase relationships between temperature and compositionRetrieved tidal amplitudes from GOLD observations are generally stronger than their predicted model counterparts [ABSTRACT FROM AUTHOR]

The terrestrial magnetopause forms the boundary between the solar wind plasma with its embedded interplanetary magnetic field on one side, and the terrestrial magnetosphere, dominated by Earth's dipole field, on the other side. It is therefore a key region for the transfer of mass, momentum, and energy from the solar wind to the magnetosphere. The Cluster mission, comprising a constellation of four spacecraft flying in formation was launched more than 20 years ago to study boundaries in space. During its lifetime, Cluster has provided a wealth of new knowledge about the magnetopause. In this paper, we give an overview of Cluster‐based studies of this boundary, and highlight a selection of interesting results. Key Points: The magnetopause is a key region for transfer of mass, momentum and energy from the solar wind into the magnetosphereCluster has probed the terrestrial magnetopause for more than 20 yearsAn overview of key magnetopause results from the Cluster mission is given [ABSTRACT FROM AUTHOR]

In this paper, the ionospheric quiet‐time disturbances otherwise known as Pre‐Magnetic Storm Signatures (PMS) have been studied using the F2‐layer peak electron density (NmF2) data obtained from 12 Digisonde/ionosonde stations distributed across equatorial, low and mid‐latitudes. The datasets used spans the years 2010–2012. Results from this study reveals strong PMS in NmF2 with percentage deviations (ΔNmF2) ranging from −91% to 500% at the equatorial, low and mid‐latitudes, with maxima occurring at the equatorial region. Significant effects on the peak height of the F2‐layer (hmF2) were also observed, and they are correlated with the variations in NmF2 particularly at the equatorial station during the PMS. The duration of a PMS is found to be 12–48 h. Although, it was difficult to state clearly the connection between the PMS and the geomagnetic storm that usually follows within 24–48 h; but the NmF2 and hmF2 responses during the PMS were quite similar to those observed during geomagnetic storms. A slight increase in the Solar‐wind‐plasma speed (>20 km/s) was also observed during PMS. The PMS occur under a southward IMF‐Bz, moderate aurora activity (AE ranging from 114 to 560 nT) and quiet ring current (Dst >−10 nT). Therefore, it is pertinent to consider a certain threshold of the aurora indices (AE, AL, and AU) in addition to the Dst, ap, and Kp in the definition of a geomagnetically quiet day. This may eliminate the ambiguity in explaining the ionospheric variability that occurs few days before Sudden Storm Commencement (SSC). Key Points: Pre‐Magnetic Storm Signatures (PMS) on NmF2 were observed to be maximum at the equatorial ionosphereIonospheric Responses during PMS were observed to be similar to the effect of geomagnetic stormsThere is a need to consider a certain threshold of the aurora indices (AE, AL, and AU) in the definition of a geomagnetic quiet day [ABSTRACT FROM AUTHOR]

The Cluster mission was the first constellation using four identical spacecraft to study Sun‐Earth connection plasma processes. Using four spacecraft in a tetrahedron shape, it could measure, for the first time, 3D quantities such as electrical currents, plasma gradients or divergence of the electron pressure tensor and 3D structures such as boundaries, surface waves or vortices. Launched in pairs in July and August 2000, on two Soyuz rockets from Baikonur, the four spacecraft have been collecting data continuously for more than 20 years. The mission faced many challenges during the years of operations as some spacecraft subsystems had a lifetime of a few years beyond the initial two‐year mission. The major one was to operate without functioning batteries and to successfully pass short and long eclipses, up to 3 h long, without damaging the on‐board computers and transmitters and without freezing the fuel. More than 1,000 eclipses have been successfully passed since 2010 using a specially made procedure which switches off the complete spacecraft before entering into eclipse and switches it on when the Sun is again illuminating the solar panels. During 20 years, many discoveries and science results have been published in more than 2,700 scientific papers. A few highlights are presented here, focusing on how varying the spacecraft separation was essential to achieve the science goals of the mission. The Cluster Science Data System and the Cluster archive allows public access to all science data as well as spacecraft ancillary data. Key Points: Selected science results achieved by Cluster are presentedOperation of the mission for 20 years, challenges and technical difficulties are reportedScience is enhanced by combining with other constellations and by making all Cluster data available to the public [ABSTRACT FROM AUTHOR]

Current models of Titan's ionosphere have difficulties in explaining the observed electron density and/or temperature. In order to get new insights, we re‐analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science (RPWS) instrument. This is the first of two papers that present the new analysis method (current paper) and statistics on the whole data set. We suggest that between two and four electron populations are necessary to fit the data. Each population is defined by a potential, an electron density and an electron temperature and is easily visualized by a distinct peak in the second derivative of the electron current, which is physically related to the electron energy distribution function (Druyvesteyn method). The detected populations vary with solar illumination and altitude. We suggest that the four electron populations are due to photo‐ionization, magnetospheric particles, dusty plasma and electron emission from the probe boom, respectively. Key Points: The Cassini Langmuir probe dataset in Titan's ionosphere is re‐analyzed with a specific interest on the electron density and temperature2 to 4 cold electron populations with distinct potentials are observedElectron populations vary with altitude and solar illumination suggesting origins linked to solar photons, magnetospheric particles and dust [ABSTRACT FROM AUTHOR]

Inside the magnetosheath, the IBEX‐Hi energetic neutral atom (ENA) imager measures a distinct background count rate that is more than 10 times the typical heliospheric ENA emissions observed when IBEX is outside the magnetosheath. The source of this enhancement is magnetosheath ions of solar wind (SW) origin that deflect around the Earth's magnetopause (MP), scatter and neutralize from the anti‐sunward part of the IBEX‐Hi sunshade, and continue into the instrument as neutral atoms, behaving indistinguishably from ENAs emitted from distant plasma sources. While this background pollutes observations of outer heliospheric ENAs, it provides a clear signature of IBEX crossings over the magnetospheric boundaries. In this study, we investigate IBEX encounters with the magnetosheath boundaries using ∼8 yr of orbital data, and we determine the MP and bow shock (BS) locations derived from this background signal. We find 280 BS crossings from XGSE ∼ 11 Re to XGSE ∼ −36 Re and 241 MP crossings from XGSE ∼ 6 Re to XGSE ∼ −48 Re. We compare IBEX BS and MP crossing locations to those from IMP‐8, Geotail, Cluster, Magion‐4, ISEE, and Magnetospheric Multiscale Mission, and we find that IBEX crossing locations overlap with the BS and MP locations inferred from these other data sets. In this paper, we demonstrate how IBEX can be used to identify magnetosheath crossings, and extend boundary observations well past the terminator, thus further constraining future models of magnetosheath boundaries. Furthermore, we use the IBEX data set to show observational evidence of near‐Earth magnetotail squeezing during periods of strong interplanetary magnetic field By. Plain Language Summary: The Interstellar Boundary Explorer (IBEX) spacecraft is designed to use remote‐sensing to examine the outer reaches of the solar system, yet it has been shown to have many magnetospheric applications as well. The primary interaction between the SW and the Earth's magnetosphere is the creation of a comet‐tail like structure known as the magnetosheath. The magnetosheath is highly dynamic, and its size and shape changes in response to changing SW conditions, with the exact details presently unknown. In this paper, we demonstrate how the IBEX's highly eccentric orbit allows for hundreds of encounters with both the inner and outer magnetosheath boundaries, and we use these crossings to create a new magnetosheath boundary data set. We then compare our new data set with data sets from previous magnetospheric missions. Finally, we use our new data set to provide observational evidence for a poorly understood phenomenon known as magnetotail squeezing. Key Points: We demonstrate a technique for producing both Bow Shock and Magnetopause data sets using the Interstellar Boundary ExplorerWe introduce new boundary crossings further tailward than previously used data sets into underexplored magnetosheath regionsWe provide observational evidence of magnetotail squeezing during periods when the interplanetary magnetic field has a strong y‐component [ABSTRACT FROM AUTHOR]

We present a new application of a multispacecraft method which provides estimates of the full electric current density vector in the ionospheric low‐ and mid‐latitude F region. The method uses the three satellites of the Swarm constellation in configurations when they were close to each other. The current density is calculated inside triangular prisms defined by the satellite positions. The technique is inspired by similar approaches such as the curlometer. Here, we propose an alternative mathematical treatment, which involves the use of the well‐known curl‐B technique and a least squares minimization. The proposed formalism provides a rigorous way to propagate the errors through the problem and special care is taken to validate the algorithm and assess the relevance of the derived currents. Such techniques are powerful tools to study the F‐region ionosphere currents as they naturally exclude all the other contributions outside the considered volumes. The F region is known to host currents with a complex spatiotemporal variability. Besides the intrinsic interest of this system itself, understanding the dynamics of these currents is also critical for global geomagnetic field modeling as they are an important source of error in the models. In this paper, we present the full algorithm and test the method on both synthetic and real Swarm satellite data on February 15, 2014. Several sources of error are investigated. Additionally, the results reveal expected characteristics of F‐region interhemispheric field‐aligned currents. Some new characteristics of F‐region currents, potentially associated with dynamo and pressure currents, are also observed. Key Points: A new application of a multispacecraft method to derive the three components of the current density in the F‐region ionosphere is presentedAn assessment of the effect of biases and errors on the current density estimates is providedApplication of the method on February 15, 2014 reveals both expected and new characteristics of F‐region currents [ABSTRACT FROM AUTHOR]

The cause of the solar wind is sought on a large‐scale (J × B) force resulting from the solar unipolar induction current, rather than coronal heating and small‐scale processes in the corona. In this paper, we consider the unipolar induction system considered by Alfven (1981, https://doi.org/10.1007/978-94-009-8374-8) the polar current (1.5 × 109 A) flowing out from the polar region of the sun spreads down longitudinally, which produces azimuthal magnetic fields. We considered two models, A and B. In Model A, the current spreads down fairly uniform in five divided regions of the heliosphere (rather than flowing only along the outer surface of the heliosphere). The resulting (J × B) force is mainly directed outward, causing an outward flow of the heliospheric plasmas. However, in this case, the speed at a distance of 1 AU is only 1 km/s, although the acceleration in each divided region is much greater than the solar gravity. In Model B, one third of the dynamo current is assumed to be concentrated in the area between 9.5 R☉ and 10.5 R☉ (where R☉denotes the solar radius) and B = 10−6 T (103 nT) at 10 R☉. The acceleration is estimated to be 1.4 × 102 m/s2 and the resulting speed at 1 AU is 200 km/s. Therefore, there is a possibility that the solar unipolar induction provides partially a new possibility for the cause of a slow, bulk flow of the solar wind. Plain Language Summary: Based on Alfven's solar unipolar induction system, it is shown that the resulting (J × B) force could generate solar wind speed of 200 km/s at 1 AU under several reasonable assumptions. Key Points: The cause of the solar wind is sought on the basis of Alfven's solar unipolar induction current system, rather than an initial heatingThe resulting (J × B) force is applied to the corona between 9.5 and 10.5 solar radii, accelerating the solar wind to 200 km/sThe result is applicable for slow and uniform solar wind with polar angle of 0–70° during the solar cycle minimum phase [ABSTRACT FROM AUTHOR]

This paper presents the magnetospheric multiscale (MMS) observations of three flux ropes (FRs) sequentially in a reconnection exhaust at Earth's magnetopause. Three active X‐lines, which are responsible for the formation of these FRs, were also detected by MMS, providing strong evidence that the FRs were generated by multiple X‐line reconnection. We find that the core field inside the FRs considerably affects the Hall current in the adjacent X‐lines. The intense core field of the FRs deflected the electron outflow jet, forming the electron‐scale Hall magnetic field on the magnetosheath side. Additionally, an active secondary reconnection was observed at the center of one FR. This secondary reconnection split the FR into two smaller FRs and produced sharp density gradients and intense fluctuations of electric field and current within the FR. These observations advance the understanding of the formation and evolution of FRs during multiple X‐line reconnection at magnetopause. Plain Language Summary: Flux transfer event (FTE) is generally explained as the flux rope (FR) generated by multiple X‐line reconnection at the dayside magnetopause. Although the theory and simulations have shown that the multiple active X‐lines and FRs can be produced simultaneously by the tearing instability in the magnetopause current sheet, there is no in‐situ evidence to directly support this scenario yet. Using high‐resolution measurements from the magnetospheric multiscale mission, we present a case study that all active X‐lines expected at the edges of three successive FRs were observed. These observations confirm that reconnection could simultaneously occur at multiple sites to produce the FRs at Earth's magnetopause. Interestingly, the electron‐scale Hall magnetic field was observed at the magnetosheath side in this multiple X‐line reconnection. This is due to the electron outflow jet deflected into the magnetosheath side by the strong core field of FR. Additionally, an ongoing secondary reconnection was observed at the center of one FR. Intense waves and current perturbations were observed in a smaller FR which was produced by this secondary reconnection. Key Points: Ongoing multiple X‐line reconnection generates a series of flux ropes at the magnetopauseElectron‐scale Hall magnetic fields are formed due to the high deflection of electron jets caused by the core field of the flux ropeSecondary reconnection within the flux rope split the flux rope into smaller flux ropes and produces intense electromagnetic fluctuations [ABSTRACT FROM AUTHOR]

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

Medium Energy Electron (MEE) precipitation (≳30 keV) ionizes the mesosphere and initiates chemical reactions, which ultimately can reduce mesospheric and stratospheric ozone. Currently, there are considerable differences in how existing parameterizations represent flux response, timing, and duration of MEE precipitation, especially considering its high‐energy tail (≳300 keV). This study compares the nature of ≳300 to ≳30 keV electron fluxes to better understand differences within MEE precipitation. The MEE fluxes are estimated from measurements by the Medium Energy Proton and Electron Detector (MEPED) onboard the Polar Orbiting Environmental Satellite (POES) from 2004 to 2014. The fluxes are explored in the context of solar wind drivers: corotating high‐speed solar wind streams (HSSs) and coronal mass ejections (CMEs) alongside their associated solar wind properties. Three key aspects of ≳300 keV electron fluxes are investigated: maximum response, peak timing, and duration. The results reveal a structure‐dependent correlation (0.89) between the peak fluxes of ≳30 and ≳300 keV electrons. The epsilon coupling function correlates well (0.84) with the ≳300 keV peak flux, independent of solar wind structure. The ≳300 keV flux peaks 0–3 days after the ≳30 keV flux peaks. The highest probability (∼42%) occurs for a 1‐day delay, while predictive capabilities increase when accounting for solar wind speed. The ≳300 keV flux response has the highest probability of lasting 4 days for both CMEs and HSSs. The results form a base for a stochastic MEE parameterization that goes beyond the average picture, enabling realistic flux variability on both daily and decadal scales. Plain Language Summary: Electrons with energies >30 keV precipitating into the Earth's atmosphere is known as medium energy electron (MEE) precipitation. Solar wind properties drive the rate and energy of MEE. MEE precipitation is a relevant solar forcing as it produces ozone‐depleting substances. Variations in ozone concentration can modify the atmospheric temperature profile and lead to changes in atmospheric circulation that can map down onto surface climate. The community's capability of parameterizing MEE precipitation is an active field of research. This study aims to build a foundation for an MEE parameterization that represents realistic variability on daily and decadal scales by exploring the variability in the context of different solar wind properties. The study focuses on three key aspects regarding the differences within the energy spectrum of MEE precipitation: maximum response, timing, and duration. The key aspects are evaluated regarding the probability of a specific response. This study looks at an entire solar cycle (11 yr). It is found that solar wind properties, such as solar wind speed, can help determine the probability of a particular MEE response. The results from this paper will be key elements for constructing an MEE precipitation parameterization to be applied in a chemistry‐climate model. Key Points: The solar wind parameter ε best predicts the peak flux of the >292 keV electron precipitation independent of solar wind structureThe delay in the peak flux of >292 keV electron precipitation relative to that of >43 keV electrons is influenced by solar wind speedOn average, electron precipitation >292 keV persists 2–3 days longer than electron precipitation >43 keV [ABSTRACT FROM AUTHOR]

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

Recent studies have reported electromagnetic ion cyclotron (EMIC) waves with nonlinear second harmonics (SHs) whose frequency and wave number are twice those of the fundamental waves (FWs). In this paper, we provide detailed analyses of the cross‐band SH generation of EMIC waves in a multi‐ion plasma. In the presence of heavy ions, EMIC wave modes can be divided into distinct wave bands according to heavy ion gyrofrequencies, and the SH can exist in a wave band different from that of the FW. The SH can be generated around or even exactly on the normal modes of the plasma system, leading to a more efficient energy transfer than that in H+‐only plasmas. The detailed parametric analysis also manifests the vital role of heavy ions in promoting SH's amplitude. Further, multiple 1‐D hybrid simulations are performed to investigate the cross‐band SH generation and validate the theoretical results. The SHs are well reproduced in simulations and the quantitative relation of frequency and wave number between the FW and the SH is well satisfied. The variations of the amplitude ratios (the SH to the FW) with fundamental frequencies are well consistent with those obtained from theoretical calculations in magnitude. Therefore, our results demonstrate the significant roles of heavy ions in the SH generation. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves have recently been observed to have nonlinear second harmonics (SHs) with frequencies twice those of the fundamental waves (FWs). With the existence of heavy ions (He+ and O+), EMIC waves are categorized into three distinct wave bands in terms of heavy ion gyrofrequencies, i.e., H+ band, He+ band, and O+ band. In this situation, the FW in the He+ band could generate the SH in the H+ band. The cross‐band energy transfer would be expected to be more efficient than that in a singular H+ plasma, leading to larger amplitudes of the SH. But such effects of heavy ions on the SH generation have not been studied and verified in detail yet. In this letter, we provide theoretical analyses of the EMIC wave cross‐band energy transfer and utilize hybrid simulations to reproduce the SH generation and verify the theoretical results. Heavy ions are found to play significant roles in the SH excitation. Key Points: The theoretical amplitude ratio of the fundamental to the second harmonic (SH) for electromagnetic ion cyclotron (EMIC) waves is obtained in cold multi‐ion plasmasThe cross‐band SHs of EMIC waves in multi‐ion plasmas are successfully reproduced by 1‐D hybrid simulationsHeavy ions can promote the cross‐band energy transfer efficiency from the fundamental waves to the SHs [ABSTRACT FROM AUTHOR]

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

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

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

Warm (several eV to 100 eV) ions are an important, and still poorly understood, component of the magnetospheric plasma environment. We present the first comprehensive statistical analysis of several distinct populations of warm ions in the dayside middle and outer magnetosphere. We analyze 7 months (1 September 2015–31 March 2016) of Magnetospheric Multiscale Hot Plasma Composition Analyzer data comprising 734,200 moments (density, temperature) and energy‐dependent pitch angle distributions (PADs) of three major ion species (H+, He+, and O+) with energy ≤100 eV. PADs are represented by an energy‐averaged index that characterizes the shape of the PAD as field‐aligned (FA), pancake, or isotropic. We use filtering by temperature, pitch angle, and concentration to distinguish different populations, and obtain new and more complete information about average density, temperature, PADs, and composition. Our analysis explores two known populations of warm ions: the warm plasmasphere (WPS) and the warm cloak/trough (C/T). The WPS is a higher‐temperature, higher‐L extension of the duskside plasmaspheric bulge, containing mostly trapped (pancake/isotropic) ions with an H+:He+:O+ order of ion dominance. The C/T contains mostly FA warm ions with a dawnward (duskward) temperature gradient for H+ (He+ and O+), lower densities, and an H+:O+:He+ order of ion dominance. The WPS‐C/T overlap contains a mixture of the two populations (e.g., FA He+ in WPS, trapped O+ in C/T). Pancake (FA) PADs are correlated with higher (lower) density/temperature. Our analysis also identifies warm ions in the low‐energy plasma sheet. Our work consolidates and systematically extends the characterizations of warm ions reported in previous studies. Plain Language Summary: The plasmasphere is often defined as the cold and dense innermost region of the magnetosphere. The ring current may be defined as the torus of hot ions encircling the planet at several Earth radii of geocentric distance. Warm plasma (several eV to 100 eV) is an intermediate‐energy population. In this paper, we present the first comprehensive statistical analysis of several distinct populations of warm ions in the dayside middle and outer magnetosphere. We analyze 7 months (1 September 2015–31 March 2016) of data from the Magnetospheric Multiscale mission. Several‐eV to 100‐eV ions constitute a large reservoir of mass and momentum that can influence numerous basic, system‐level aspects of the magnetosphere. Warm ions are also a "bridge population" whose intermediate‐energy dynamics reveal heating and energization processes that trap ions to enable plasmaspheric refilling and transform very cold, ionospheric‐origin plasma into an integral component of the stormtime plasma sheet and ring current. Key Points: Statistical analysis of dayside warm (≤100 eV) ions uses 734,200‐point database of density, temperature, and pitch angle (PA) distributionsFiltering by temperature, PA, and concentration helps distinguish and separate different warm ion populationsThis analysis consolidates and systematically extends the characterizations of the warm plasmasphere and the warm cloak/trough (C/T) [ABSTRACT FROM AUTHOR]

Recent studies have found that magnetic dips resulting from the diamagnetic motion of injected plasmas can provide a favorable environment for electromagnetic ion cyclotron (EMIC) wave excitation. However, another critical question how magnetic dips affect the propagation of EMIC waves remains uncovered. In this paper, with plasma environment as well as wave properties observed in a typical event, theoretical analysis about this topic has been provided. Our results show that in this event both H+ band and He+ band EMIC waves can be constrained in the magnetic dip, which is consistent with the wave emissions observed inside the magnetic dip. The energetic protons have also been found to fall around the saturated states, suggesting a stable balance built among EMIC waves, energetic protons, and magnetic structures. Our results provide one new insight into the EMIC wave propagation inside a magnetic dip and their important roles in the dynamics of magnetic dips. Plain Language Summary: Magnetic structures and electromagnetic waves are two of the most common phenomena in space plasmas. One key scientific topic is to study the relation between them. Recent studies have found that plasma waves can be excited inside magnetic structures. Specifically, electromagnetic ion cyclotron (EMIC) waves have been reported to be excited in a large‐scale magnetic dip resulting from the diamagnetic drift of energetic protons. Here, we focus on the effect of the magnetic dip on the propagation of EMIC waves. If EMIC waves can be ducted inside the magnetic dip, they will be involved into the total balance of the magnetic structure since these waves would influence the dynamics of energetic protons through long‐time scattering effects. Thus, our result will be of importance to understanding the dynamics of magnetic dips. Key Points: Theoretical analysis about the effects of magnetic dips on electromagnetic ion cyclotron (EMIC) wave propagations has been givenOur theoretical result shows that both H+ and He+ band EMIC waves can be ducted inside the magnetic dipEnergetic protons inside the magnetic dip are verified near the saturated states, suggesting a stable force balance in the magnetic dip [ABSTRACT FROM AUTHOR]

Upper atmospheric long‐term trends have been characterized through the analysis of the ionospheric ion temperature (Ti). Previous studies used Ti observations from various incoherent scatter radar (ISR) facilities located at different latitudes. In this paper, we analyze Arecibo Observatory's (AO) ISR (18°20'N, 66°45'W) data sets from 1985 to 2019 to detect Ti long‐term trends as a function of altitude from ∼140 to ∼677 km. We empirically modeled the responses of Ti to the known forcings of solar activity, geomagnetic activity, and the annual and semi‐annual oscillations. The Ti trend is determined through least squares fitting to the residuals of the Ti, which were estimated by removing the empirically modeled Ti from the observed Ti. Since the ions and neutrals are closely coupled, our results indicate that the upper atmosphere/ionosphere over Arecibo has been cooling over the 35 years studied. Above 350 km, a latitudinal dependency is seen by comparison of all ISR estimated Ti trends, which agrees with the earlier reported results. These observed cooling trends exceed the magnitude expected by the modeling studies from increased greenhouse gas (GHG) concentrations. These excess coolings are as high as −1.2 K/year below 320 km altitude, where an increase in GHG dominates. Nighttime cooling trends in the altitude of ∼320–400 km might be caused by the increasing GHG concentrations and magnetic field variations since the trends of AO‐ISR match with the Whole Atmosphere Community Climate Model eXtension simulations. Key Points: Long‐term trends of ion temperature at Arecibo show the upper atmosphere is coolingIon temperature cooling trends at Arecibo are significantly larger than the anticipated effects of the global greenhouse gas (GHG) increaseNighttime cooling trends in the altitude of ∼320–400 km might be caused by the GHG and magnetic field variations [ABSTRACT FROM AUTHOR]

This paper investigates concentric traveling ionospheric disturbances (CTIDs) associated with the Tonga volcanic eruption. Results show that: (a) two types of CTIDs (CTID #1 and CTID #2) were identified that traveled radially from Tonga at the speed of 610–880 m/s (acoustic‐mode) and 300–380 m/s (Lamb‐mode), respectively. CTID #1 reached 3,800 and 5,000 km away from the eruption location toward the directions of New Zealand and Australia, respectively. CTID #2 propagated persistently for ∼9 hr over New Zealand and Australia. (b) The CTID #2 wavefront changed after 08:35 UT over New Zealand, possibly due to a combination of factors including the anisotropic propagation of CTID #2, the regional geomagnetic declination, and westward‐moving Lamb waves. (c) Topside total electron content (TEC) enhancement with a magnitude over two TECu was observed from COSMIC‐2 measurements. The enhancement agrees with CTID #1 peak from nearby ground‐based TEC observations and could be related to the upward propagation of the F layer's CTID #1 signatures. Plain Language Summary: The Tonga volcanic eruption on 15 January triggered various atmospheric waves that propagate from the Earth's surface and throughout the atmosphere and ionosphere. In this study, we discuss two types of concentric traveling ionospheric disturbances (CTIDs, #1 and #2) propagating outward from the Tonga site based on measurements collected by ∼1,000 ground‐based global navigation satellite system receivers. Our analysis based on the CTID propagation speed showed that CTIDs #1 and #2 traveled at acoustic and Lamb wave modes, respectively. We also analyzed COSMIC‐2 satellite radio occultation observations and showed that CTID #1‐related enhancement signatures were observed at the topside ionosphere near the eruption site. Moreover, it is interesting to note that CTID #2 wavefront changed over New Zealand after 08:35 UT on 15 January likely followed the regional geomagnetic declination and westward‐moving Lamb waves. Key Points: Two distinctive types of concentric traveling ionospheric disturbances (CTIDs #1 and #2) were identified, and they propagated radially outward from Tonga at the speed of 610–880 m/s (acoustic‐mode) and 300–380 m/s (Lamb‐mode), respectivelyThe wavefront of the long‐lasting CTID #2 changed after 08:35 UT over New Zealand, possibly due to the regional geomagnetic declination and westward‐moving Lamb wavesDistinctive total electron content (TEC) enhancement of over 2 TECu magnitude observed above 530 km near the eruption site could be associated with the upward propagation of the acoustic‐mode CTID #1 signatures in the F layer [ABSTRACT FROM AUTHOR]

This paper reports a large‐scale magnetospheric line radiation (MLR) event during a moderate geomagnetic storm on 11 September 2018, which was well recorded by the China‐Seismo‐Electromagnetic Satellite (CSES) in the upper ionosphere. The event shows a symmetrical propagation feature at the conjugated locations between the two hemispheres, exhibiting a large spatial extension roughly from the latitudes 54°N to 53°S. The parallel structures are visible both in the electric and magnetic fields at a frequency band ranging from the local proton cyclotron frequency to ∼1.6 kHz. The wave intensity of parallel spectral lines was primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings between neighboring spectral lines roughly vary from ∼80 to 110 Hz at the high latitudes and ∼80–130 Hz at the low latitudes, suggesting a slight variation feature with latitude. The parallel spectral structures of MLR drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. The wave vector analysis shows that the MLR waves are right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, where the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. The other large‐scale MLR events all exhibit similar parallel structures and polarization characteristics, suggesting the universality of such a phenomenon. However, the azimuthal angles differ among different events, showing complex features. Plain Language Summary: Magnetospheric line radiation (MLR) is a unique electromagnetic wave distinguished by parallel spectral lines. This study reports a large‐scale MLR event that occurred in the dayside ionosphere. The event shows a symmetrical propagation feature, with a large spatial extension between latitudes 54°N and 53°S. The parallel structures are visible both in the electric and magnetic spectrogram, ranging from the local proton cyclotron frequency to ∼1.6 kHz. The MLR structures were primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings of MLR roughly vary from ∼80 to 110 Hz in the high latitudes and from ∼80 to 130 Hz in the mid‐low latitude region, slightly varying with latitude. The MLR structures drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. This MLR event is right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, and the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. However, the azimuthal angles differ among different events, indicating the complexity of the wave propagation feature. Key Points: A large‐scale magnetospheric line radiation (MLR) event shows a symmetrical propagation feature in two hemispheres, presenting a distinct V‐shaped structureBoth the frequencies of the parallel spectral lines and their frequency spacings slightly drift with latitudesThe MLR waves are right‐hand polarized, obliquely propagating toward the Earth and azimuthal direction [ABSTRACT FROM AUTHOR]

In this paper, we detect a magnetic null inside the MMS space‐craft (SC) tetrahedron using a linear interpolation that might be nearly impossible for very small inter‐SC separation like MMS (previously implemented on Cluster mission which is larger separation SC). We also report, successive or identical magnetic nulls that enter/reenter and cross the MMS spacecraft tetrahedron, and the "bifurcations" (change of the null type) of magnetic nulls are captured when it crosses one of four MMS tetrahedral surfaces during the 19 September 2015 event. During this event, we observe the eigenvalues of the radial null that enters and crosses the MMS tetrahedron changes from all real to one conjugate complex pair and one real eigenvalues, when the null exits through one of the tetrahedral boundary cells. This bifurcation is also evidenced by measuring the topological distance between two nulls before and after "bifurcation" using the Earth Mover's Distance. Key Points: First, we find a magnetic null and track it using a linear interpolation method in MMSWe identify successive magnetic nulls that enter/reenter, inside, and cross the MMS tetrahedron during a reconnection event on 19 September 2015 at 07:43:30We measure the topological distance between each successive magnetic null and try to capture the "bifurcations" or the change of magnetic topology when null crosses one of the boundary cells of the MMS tetrahedron [ABSTRACT FROM AUTHOR]

With Doppler Interferometer instruments on thermosphere‐ionosphere‐mesosphere energetics and dynamics satellite (TIMED/TIDI) observations, the 1‐day resolution, non‐migrating DE3 tide in the zonal wind component can be extracted using the method from Li et al. (2015, https://doi.org/10.1002/2015JA021577). This paper presents the spatiotemporal variability of the DE3 tide in the zonal wind component between 85 and 105 km altitude and within ±30° latitude. The global structure and long‐term variation (intra‐annual and >1 year variability) are similar to the previous results. It is found that (a) it is dominant in the Southern Hemisphere between 95 and 105 km altitude and in the Northern Hemisphere between 90 and 100 km altitude; (b) the maximum amplitudes are in August and September around the equator, and the phases are stable and hardly vary with their latitudes; and (c) the influence of the quasi‐biennial oscillation (QBO) phenomenon can be noticeably detected. Additionally, the short‐term variability (after filtering out long‐term variability) can also be presented: (a) it increases with altitudes gradually between 85 and 105 km altitude, and the peak value is above 100 km; and (b) the short‐term variability does not demonstrate significant long‐term variation. Furthermore, through comparative analysis, we found that (a) the long‐term variations in DE3 in temperature and zonal wind are consistent with each other; however, the short‐term variabilities in DE3 are not similar. This indicates that the dominant dynamic processes of DE3 in the temperature and zonal wind components are different. (b) The symmetric DE3 tide in the zonal wind component is dominant at 0°–15° latitudes, and the asymmetric tide is dominant at 15°–30° latitudes. This indicates that the DE3 tide at different latitudes is dominated by wave modes with different symmetries. Key Points: Spatial distribution and temporal variation of the diurnal non‐migrating tide with zonal wavenumber 3 (DE3) in the zonal wind componentThe Quasi‐Biennial Oscillation (QBO) signals are observed in the long‐term variability of the DE3 tideShort‐term variability (after filtering out long‐term) of the DE3 tide in the zonal wind component is different from that in temperature [ABSTRACT FROM AUTHOR]

This paper presents the first numerical study on a new concept for the direct measurement of D‐region absorption in the high‐frequency (HF) band. Numerical simulations based on the Appleton‐Hartree and Garrett equations of refractive index are presented. Electron temperature as a result of HF radio pumping of the ionosphere is included in the calculations using proper numerical formulation. Both O‐ and X‐mode radio wave polarizations are taken into consideration. A global map of HF absorption in the northern hemisphere is calculated. Detailed calculations of HF radio wave absorption as it propagates through the lower atmosphere are presented. The effect of several parameters on the amount of absorption is calculated. The best frequencies to be used for the purpose of this study are discussed. A machine learning model is developed and the capability of the model in estimation of D and E‐region constituents includes N2, O, O2, as well as T and Ne is examined. Such a technique can also lead to global mapping of HF absorption and improve OTHR (over‐the‐horizon‐radar) performance. Key Points: First study on a new concept of a multi‐frequency high‐frequency (HF) beacon for the direct measurement of D‐region absorption is presentedFull physics‐based model of HF radio absorption in the upper atmosphere is developedA machine learning model is developed and the capability of the model in estimation of D and E‐region constituents is examined [ABSTRACT FROM AUTHOR]

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