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Space storms 1 are the dominant contributor to space weather. During storms, rearrangement of the solar wind and Earth’s magnetic field lines at the dayside enhances global plasma circulation in the magnetosphere 2 , 3 . As this circulation proceeds, energy is dissipated into heat in the ionosphere and near-Earth space. As Earth’s dayside magnetic flux is eroded during this process, magnetotail reconnection must occur to replenish it. However, whether dissipation is powered by magnetotail (nightside) reconnection, as in storms’ weaker but more commonplace relatives, substorms 4 , 5 , or by enhanced global plasma circulation driven by dayside reconnection is unknown. Here we show that magnetotail reconnection near geosynchronous orbit powered an intense storm. Near-Earth reconnection at geocentric distances of ~6.6–10 Earth radii—probably driven by the enhanced solar wind dynamic pressure and southward magnetic field—is observed from multi-satellite data. In this region, magnetic reconnection was expected to be suppressed by Earth’s strong dipole field. Revealing the physical processes that power storms and the solar wind conditions responsible for them opens a new window into our understanding of space storms. It encourages future exploration of the storm-time equatorial near-Earth magnetotail to refine storm driver models and accelerate progress towards space weather prediction. Magnetic reconnection in the near-Earth magnetotail is observed to power a space storm, although suppression of magnetic reconnection caused by the Earth’s magnetic dipole was expected close to Earth.

Energetic electron precipitation from Earth’s outer radiation belt heats the upper atmosphere and alters its chemical properties. The precipitating flux intensity, typically modelled using inputs from high-altitude, equatorial spacecraft, dictates the radiation belt’s energy contribution to the atmosphere and the strength of space-atmosphere coupling. The classical quasi-linear theory of electron precipitation through moderately fast diffusive interactions with plasma waves predicts that precipitating electron fluxes cannot exceed fluxes of electrons trapped in the radiation belt, setting an apparent upper limit for electron precipitation. Here we show from low-altitude satellite observations, that ~100 keV electron precipitation rates often exceed this apparent upper limit. We demonstrate that such superfast precipitation is caused by nonlinear electron interactions with intense plasma waves, which have not been previously incorporated in radiation belt models. The high occurrence rate of superfast precipitation suggests that it is important for modelling both radiation belt fluxes and space-atmosphere coupling. Energetic electron densities in the radiation belt increases during geomagnetic storms. Here, the authors show oblique whistler mode waves enhance electron losses and create strong fluxes of about 100 keV electrons precipitating into the atmosphere, that should be considered in radiation belt models.

Energetic electron precipitation (EEP) during substorms significantly affects ionospheric chemistry and lower‐ionosphere (<100 km) conductance. Two mechanisms have been proposed to explain what causes EEP: whistler‐mode wave scattering, which dominates at low latitudes (mapping to the inner magnetosphere), and magnetic field‐line curvature scattering, which dominates poleward. In this case study, we analyzed a substorm event demonstrating the dominance of curvature scattering. Using ELFIN, POES, and THEMIS observations, we show that 50–1,000 keV EEP was driven by curvature scattering, initiated by an intensification and subsequent earthward motion of the magnetotail current sheet. Using a combination of Swarm, total electron content, and ELFIN measurements, we directly show the location of EEP with energies up to ∼1 MeV, which extended from the plasmapause to the near‐Earth plasma sheet (PS). The impact of this strong substorm EEP on ionospheric ionization is also estimated and compared with precipitation of PS (<30 keV) electrons. Plain Language Summary During magnetospheric substorms, energetic electrons in the Earth's plasma sheet (PS), the night‐side magnetosphere region filled by hot plasma, precipitate to the ionosphere. Energetic electron precipitation (EEP) affects the density, temperature, and composition of the ionosphere. However, the exact process that causes such precipitation is not well understood due to observational constraints. The challenge lies in simultaneously measuring the EEP properties at the ionosphere and the plasma and wave properties in the PS. We analyze a fortuitous satellite conjunction during a substorm, during which EEP was simultaneously captured by ELFIN, Swarm, and POES at low altitudes, and THEMIS in the equatorial PS. EEP was observed to extend across a broad equatorial domain, projecting into a wide ionospheric region and encompassing the PS region and a significant portion of the inner magnetosphere. High‐energy‐resolution measurements from ELFIN reveal that the main driver of precipitation is the scattering of energetic electrons by strongly curved magnetic field lines in the PS, as opposed to the more commonly suggested scattering mechanisms associated with wave‐particle interactions. We also show that the EEP drastically altered the ionization profile of the ionosphere. Key Points We investigate the radial location of energetic (50–1,000 keV) electron precipitation (EEP) during a substorm We compare the impact of plasma sheet electron precipitation (<30 keV) and EEP (50–1,000 keV) on the altitudinal profile of ionization Our results underscore the key role of curvature scattering in energetic electron precipitation during substorms

Night‐side chorus waves are often observed during plasma sheet injections, typically confined around the equator and thus potentially responsible for precipitation of ≲100 keV electrons. However, recent low‐altitude observations have revealed the critical role of chorus waves in scattering relativistic electrons on the night‐side. This study presents a night‐side relativistic electron precipitation event induced by chorus waves at the strong diffusion regime, as observed by the ELFIN CubeSats. Through event‐based modeling of wave propagation under ducted or unducted regimes, we show that a density duct is essential for guiding chorus waves to high latitudes with minimal damping, thus enabling the strong night‐side relativistic electron precipitation. These findings underline both the existence and the important role of density ducts in facilitating night‐side relativistic electron precipitation. Plain Language Summary Chorus waves, an important mode of electromagnetic waves in Earth's magnetosphere, play a vital role in scattering energetic electrons (electron precipitation) in the radiation belts. It has been shown in observations that night‐side chorus waves usually remain confined near their equatorial source and thus do not significantly affect relativistic electron precipitation. However, recent observations challenge this notion, suggesting a viable connection between the night‐side relativistic electron precipitation and chorus waves. In this work, we present an event observed on the ELFIN CubeSats that reveals intense relativistic electron precipitation on the night‐side, where the ratio between precipitation and trapped fluxes reaches the theoretical maximum of 1. To investigate the physical mechanism responsible for this event, we used numerical modeling to simulate scenarios with and without a density‐enhancement duct along magnetic field lines. Our results show that such ducts can efficiently trap chorus waves and guide them to high latitudes without significant damping where they can efficiently interact with the relativistic electrons. By comparing the precipitation intensity in ducted and unducted cases, we affirm the crucial role of density ducts in driving strong night‐side relativistic electron precipitation. Key Points We present observations of night‐side relativistic electron precipitation induced by whistler‐mode waves We perform a comparison of observations with simulation results for different wave propagation regimes We show that only ducted whistler‐mode waves can effectively scatter relativistic electrons on the night‐side

In this paper we review recent spacecraft observations of oblique whistler-mode waves in the Earth’s inner magnetosphere as well as the various consequences of the presence of such waves for electron scattering and acceleration. In particular, we survey the statistics of occurrences and intensity of oblique chorus waves in the region of the outer radiation belt, comprised between the plasmapause and geostationary orbit, and discuss how their actual distribution may be explained by a combination of linear and non-linear generation, propagation, and damping processes. We further examine how such oblique wave populations can be included into both quasi-linear diffusion models and fully nonlinear models of wave-particle interaction. On this basis, we demonstrate that varying amounts of oblique waves can significantly change the rates of particle scattering, acceleration, and precipitation into the atmosphere during quiet times as well as in the course of a storm. Finally, we discuss possible generation mechanisms for such oblique waves in the radiation belts. We demonstrate that oblique whistler-mode chorus waves can be considered as an important ingredient of the radiation belt system and can play a key role in many aspects of wave-particle resonant interactions.

The solar wind is filled with various magnetic field fluctuations, and one of the most widespread types of such fluctuations is solar wind discontinuities. They are rapid high-amplitude magnetic field rotations sharing properties of nonlinear Alfvén waves and plane plasma slabs. They are believed to play an important role in the interaction of the solar wind with the Earth’s magnetosphere. Most studies of solar wind discontinuities are based on observations in pristine solar wind, often by solar wind monitors at L1. However, before interacting with the Earth’s magnetosphere, solar wind discontinuities cross the bow shock and can change their properties. In this study, we investigate the transformation of discontinuities due to the bow shock crossing. We compiled a set of 100 high-amplitude ( > 3 nT) discontinuities observed by ARTEMIS in the upstream of the bow shock and by THEMIS in the downstream from the bow shock crossing (in the Earth’s magnetosheath). Comparison of discontinuity properties in the solar wind and magnetosheath demonstrates discontinuity thinning and current density increase in the magnetosheath. Although all considered solar wind discontinuities mostly resemble rotational discontinuities, in the magnetosheath they start having properties of tangential discontinuities. We reveal a clear dawn–dusk asymmetry of discontinuity properties that are likely related to the asymmetry of the ion foreshock. We discuss how solar wind discontinuity transformation at the bow shock crossing can alter their interaction with the Earth’s magnetosphere.

Electron precipitation by chorus whistler‐mode waves generated by the same electron population is expected to play an important role in the dynamics of the outer radiation belt, potentially setting a hard upper limit on trapped energetic electron fluxes. Here, we statistically analyze the relationship between equatorial electron fluxes and the power of mid‐latitude cyclotron‐resonant chorus waves precipitating these electrons, both inferred from ELFIN low‐altitude energy and pitch‐angle resolved electron flux measurements in 2020–2022. We provide clear evidence of a flux limitation coinciding with an exponential increase of precipitation. We statistically demonstrate that the actual inferred resonant wave power gains are well correlated with theoretical linear gains, as in the classical Kennel‐Petschek model, for moderately high linear gains and high fluxes. However, we also find a finite occurrence of very high fluxes, corresponding to resonant waves of moderate average amplitude, implying a softer, more dynamical upper limit than traditionally envisioned. Plain Language Summary Using high‐precision spacecraft measurements of electron fluxes at low altitude, we experimentally revisit the classical Kennel‐Petschek paradigm of electron flux self limitation in the Earth's outer radiation belt. Our statistical analysis demonstrates that, above a threshold, higher electron fluxes are associated with the generation of more intense electromagnetic waves, which scatter electrons more efficiently toward the atmosphere, where they are lost. This leads to a lower occurrence of higher fluxes. Furthermore, we find that wave intensity increases as predicted by theory. While these results confirm the classical notion of an upper limit on electron fluxes, we discover that this limit is softer and more gradual than previously expected. This is probably due to an unforeseen saturation of the wave intensity that reduces the flux‐limiting effects of these waves during a sufficiently strong increase of electron fluxes. Key Points We statistically analyze the saturation of radiation belt electron flux, using low‐altitude pitch‐angle resolved electron flux measurements The inferred resonant chorus wave power gain increases proportionally to theoretical linear wave power gain for moderately high linear gain The average amplitudes of resonant chorus waves exhibit saturation, resulting in a soft limitation of electron fluxes

Solar wind discontinuities carry intense transient currents and significantly contribute to the turbulent spectrum and plasma heating. The most-investigated characteristics of these discontinuities are the magnetic field configuration and the current density, whereas plasma characteristics attract less attention. In this study, we utilize eight years of ARTEMIS spacecraft observations in the solar wind to investigate plasma density, velocity, and temperature variations across discontinuities. We also consider the role of discontinuities in the development of Earth’s magnetospheric perturbations. We show that observed discontinuities can be separated into two groups: (i) discontinuities with weak plasma density variations and a significant correlation between the solar wind and Alfvén velocities, (ii) discontinuities with significant variations of plasma density and temperature. For most discontinuities, observed density variations anti-correlate with temperature variations, but larger density/temperature variations correspond to stronger current densities. Time intervals characterized by increased occurrence rate of solar wind discontinuities correspond to enhanced geomagnetic activity in the Earth’s magnetosphere as characterized by geomagnetic indices.

Thin current sheets (TCS) in Earth's magnetotail are fundamental to magnetospheric dynamics. A key question concerning static magnetotail TCSs is the mechanism of radial force balance. Using the unprecedented measurements from the Magnetospheric Multiscale mission, we statistically analyze TCS crossing events from 2017 to 2020 to investigate this issue. Our analysis reveals a strong magnetic tension within TCSs, with the radial thermal pressure gradient accounts for only 10%–30% of the required balance. The off‐diagonal pressure components (Pi,xz and Pe,xz) are crucial for achieving force balance, contributing ∼55% of the required force in the further‐Earth region (−30 RE < X < −20 RE, where RE is Earth's radius), and ∼30% in the near‐Earth region (−20 RE < X < −10 RE). This work provides the first direct observational evidence demonstrating that particle kinetic effects (ion nongyrotropy and electron pressure anisotropy) play a significant role in the force balance of magnetotail TCSs.

NASA’s Magnetospheric Multi-Scale (MMS) mission is designed to explore the proton- and electron-gyroscale kinetics of plasma turbulence where the bulk of particle acceleration and heating takes place. Understanding the nature of cross-scale structures ubiquitous as magnetic cavities is important to assess the energy partition, cascade and conversion in the plasma universe. Here, we present theoretical insight into magnetic cavities by deriving a self-consistent, kinetic theory of these coherent structures. By taking advantage of the multipoint measurements from the MMS constellation, we demonstrate that our kinetic model can utilize magnetic cavity observations by one MMS spacecraft to predict measurements from a second/third spacecraft. The methodology of “observe and predict” validates the theory we have derived, and confirms that nested magnetic cavities are self-organized plasma structures supported by trapped proton and electron populations in analogous to the classical theta-pinches in laboratory plasmas. Magnetic cavities play important roles in the energy cascade, conversion and dissipation in turbulent plasmas. Here, the authors show a theoretical insight into magnetic cavities by deriving a self-consistent, kinetic theory of these coherent structures.