Anton V. Artemyev

Ion pickup at the outer planets’ active moons is a fundamental plasma process in which newly ionized particles from moon exospheres interact with the ambient corotating plasma and are accelerated to match the background flow. Spacecraft observations have revealed intense electromagnetic wave activity commonly attributed to this pickup process. Here, we investigate ion pickup using hybrid‐kinetic simulations in which ions are treated kinetically whereas electrons are modeled as a massless fluid. In the moon's rest frame, ambient ions initially stream perpendicular to the background magnetic field at the corotation velocity, creating a nongyrotropic velocity distribution with two ion populations clustered at opposite gyrophases. Within a few ion gyroperiods, this configuration simultaneously excites transverse magnetic perturbations associated with electromagnetic ion cyclotron waves and compressional perturbations associated with mirror‐mode and ion Bernstein waves, reaching amplitudes of several percent of the background field strength. Using field‐particle correlation analysis, we quantify the energy transfer between waves and particles and demonstrate how these perturbations scatter ions in velocity space, efficiently incorporating newly created ions into the background plasma and leading to isotropization in both gyrophase and pitch angle. These results provide a kinetic framework for understanding pickup‐driven wave‐particle interactions and offer guidance for interpreting in situ measurements at active moons throughout the outer solar system.

Pitch‐angle diffusion of trapped electrons driven by whistler‐mode and EMIC waves and by field‐line curvature scattering (FLCS) plays an important role in energetic and relativistic electron loss from the inner magnetosphere. The strong diffusion regime, characterized by a nearly full loss cone, rarely occurs, but is expected to account for a significant part of electron precipitation, particularly during disturbed periods. Using the full set of ELFIN low‐altitude measurements of 60 keV to 1.1 MeV electron fluxes at adiabatic ‐shells , we estimate net precipitating‐to‐trapped flux ratios corrected for atmospheric backscatter and use them to identify strong diffusion events. We classify these events into three categories: FLCS‐driven, EMIC‐driven, and whistler‐mode‐driven. We provide comprehensive statistics of strong diffusion occurrence rates in the outer radiation belt, and determine the respective contributions of these drivers as a function of electron energy, , magnetic local time, auroral electrojet (AE) index, and solar‐wind dynamic pressure. Time‐averaged statistics and a superposed epoch analysis both show a clear link between elevated strong diffusion occurrence rates and high AE and solar‐wind dynamic pressure. After strong substorms, the total occurrence rate of strong diffusion at reaches 9.4 % at 300 keV and 5.8 % at 1.1 MeV. FLCS‐driven events occur closer to the last closed drift shell than EMIC‐driven events. Wave‐driven events prevail up to 300–1,100 keV, depending on AE and . Two drivers are often simultaneously important. Therefore, our statistics should be useful for evaluating electron loss in the outer radiation belt more accurately.

On the nightside, the low‐altitude projection of the plasmapause is commonly associated with the ionospheric mid‐latitude trough, which is typically identified from ionospheric density measurements and is clearly manifested in total electron content (TEC) data. In the equatorial magnetosphere, the plasmapause, marks the outer boundary of the plasmasphere, which, in turn, is associated with intense whistler‐mode hiss waves. These waves efficiently scatter energetic electrons, producing a characteristic energy–‐shell structure in energetic electron fluxes. In this study, we identify this energy‐latitude structure in low‐altitude energetic electron measurements from the ELFIN CubeSats. We compare its latitudinal location with the position of the mid‐latitude trough inferred from TEC data and find a strong agreement between the two. Based on this relationship, we propose a new method for identifying the mid‐latitude trough, and thus the proxy of the low‐altitude projection of the plasmapause, using energetic electron measurements alone.

Particle precipitation is the ultimate driver of aurorae. Monoenergetic (∼keV) electrons accelerated above the ionosphere produce discrete auroral forms and are associated with field‐aligned currents that govern magnetosphere–ionosphere (MI) coupling. These precipitating electrons exhibit complex angular distributions and are often collocated with energetic ( keV) electron precipitation, affecting ionospheric conductance and current closure. Here, we present observations from the recently launched NASA TRACERS mission, resolving the fine energy and angular structure of monoenergetic precipitation. Coordinated observations with MetOp spacecraft and equatorial measurements from the THEMIS mission reveal the associated energetic electron precipitation. The impact of this energetic electron precipitation on current closure is quantified and discussed. We further develop an empirical model for the angular distributions of monoenergetic precipitation, enabling their realistic incorporation into ionospheric models.

The transport, confinement, and accessibility of energetic charged particles in magnetized plasmas are fundamental problems spanning laboratory, space, and astrophysical environments. Earth's radiation belts provide a natural laboratory for investigating these processes, hosting relativistic ("killer") electrons that pose significant hazards to spacecraft systems and human activities in space. Plasma injections in the nightside magnetosphere transport substantial electromagnetic energy and are widely recognized as a major source of seed populations for radiation-belt acceleration. However, it remains unclear whether relativistic electrons accelerated during these injections can directly access and replenish the radiation belts. Determining whether energetic electrons can be accelerated outside the radiation belts and subsequently penetrate into the inner magnetosphere is therefore central to understanding radiation-belt variability and the broader question of how magnetic topology regulates energetic-particle transport. Here we combine multi-platform observations with advanced magnetic field modeling to show that relativistic electrons can form outside the radiation belts yet remain excluded from them by a magnetic transport barrier. Our results reveal a previously unrecognized constraint on magnetosphere-radiation belt coupling, demonstrating that particle acceleration and particle accessibility are distinct physical processes.

Electron-mediated transport is fundamental to the thermal and dynamical evolution of plasmas across a vast range of physical environments, from the solar wind and solar atmosphere to astrophysical systems such as galaxy clusters and accretion flows, to laboratory and fusion plasmas. This article reviews the current state of understanding of electron transport, drawing together observations, simulations, and theory to contextualise key challenges and open questions. We characterise plasma environments using the Knudsen number and electron beta, and highlight that some systems fall in an intermediate regime between the collisional and collisionless limits. In the collisional limit, transport is well described by the Chapman–Enskog–Braginskii framework, yielding anisotropic heat fluxes and momentum transfer rates governed by the ratio of the electron mean free path to the electron gyroradius. In the collisionless limit, Landau-fluid closures and quasilinear wave–particle interaction theory describe heat flux regulation through resonant instabilities, particularly whistler-mode waves, which suppress parallel electron heat flux below Spitzer–Härm predictions in high-beta plasmas. We review approaches to bridging collisional and collisionless regimes, including velocity-space-dependent collision operators, moment-based interpolating closures, and data-driven methods trained on fully kinetic simulations. Comparison with solar wind observations from the Parker Solar Probe and Wind missions reveals that the current attempts at bridging collisional and collisionless closures account for only limited regions of parameter space, underscoring the need for improved models. Four central challenges are identified for future transport theory: capturing the velocity-space dependence of collisional and collisionless scattering, describing spatial transitions between transport regimes, incorporating turbulence, and developing computationally tractable effective collision operators for large-scale simulations.

Relativistic electron precipitation from the radiation belts is one of the most important channels of radiation belt loss and an essential element of magnetosphere‐ionosphere coupling. While precipitation patterns associated with different mechanisms can be identified using pitch‐angle‐resolved measurements at low‐Earth orbit (LEO), many potentially valuable data sets contain only observations of locally trapped electrons. It therefore remains unclear whether distinct signatures of these precipitation mechanisms can be reliably identified using such measurements. Here we analyze conjunction measurements of relativistic electron precipitation using the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment CubeSat, together with the Monitor of All‐sky X‐ray Image (MAXI) and the CALorimetric Electron Telescope aboard the International Space Station (ISS). We demonstrate that three primary precipitation mechanisms, magnetic field‐line curvature scattering, whistler‐mode wave scattering, and electromagnetic ion cyclotron wave scattering, produce distinct and quantifiable signatures in the high‐energy‐resolution spectra of locally trapped electrons measured by REPTile‐2. Furthermore, cross‐comparison of REPTile‐2 and MAXI measurements enables the conversion of MAXI count rates to integral fluxes of keV electrons, extending the capability to interpret long‐term ISS observations in physically meaningful flux units. These results indicate that locally trapped electron measurements in LEO can contribute to the identification of precipitation mechanisms, thereby expanding the observational basis for studies of relativistic electron precipitation.

Energetic electron precipitation is a primary driver of ionization and heating in the ionospheric E and D regions and is a key component of magnetosphere--ionosphere--thermosphere coupling. During magnetospheric substorms, electron precipitation intensifies across a broad energy range, from thermal to relativistic, driven by multiple wave-particle interaction and transport processes. However, an empirical model that describes substorm-associated precipitation across this broad energy range with realistic spatial dependence has been lacking. Here we develop the first empirical model of substorm-associated electron precipitation spanning auroral to relativistic energies (1 keV--1 MeV) from conjugate ELFIN and DMSP observations. The model parameterizes precipitating electron fluxes as functions of substorm activity, magnetic latitude (MLAT), and magnetic local time (MLT). The resulting spectra show higher fluxes at auroral than at sub-auroral latitudes, stronger precipitation from midnight through dawn than toward dusk, and marked enhancements from a few keV to several hundred keV near the SME-peak reference time. We further demonstrate its application by quantifying substorm-driven ionospheric energy deposition and ionization. This model provides a practical framework for incorporating realistic precipitation inputs into ionospheric and thermospheric simulations, enabling improved assessment of substorm energy deposition and its geospace consequences.

Electron and proton precipitation into the nightside ionosphere is a key process in magnetosphere–ionosphere coupling. Although auroral electron precipitation at –10 keV is traditionally considered the main driver of auroral emissions, recent studies show that keV electrons and protons can significantly contribute to ionospheric energy deposition, especially during storms. While energetic electron precipitation has been studied extensively, the role of energetic proton precipitation remains poorly quantified. Here, we use coordinated ELFIN and DMSP nightside observations during two substorm events, resolving electron and proton precipitation spectra from eV to ∼MeV. We show that keV proton precipitation, consistent with curvature scattering, can have an impact on E‐region ionization comparable to that of energetic electrons. These analyses are further supported by magnetospheric observations from the Arase and THEMIS spacecraft and ground‐based incoherent scatter radar measurements from PFISR.