Andrei Runov

Using combined MHD/test particle simulations, we further explore characteristics of ion (proton) acceleration tailward of a near‐tail reconnection site related to tailward moving plasmoids. In this paper we focus on local features, addressing specifically energy‐time spectrograms, directional fluxes and phase space distributions, in comparison to some typical ion observations made by the “Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun” mission near the plasma sheet boundary layer (PSBL) and in the central plasma sheet (CPS). In agreement with the observations, the simulations show boundary populations consisting of a core and an accelerated tailward beam, which decreases in speed but increases in intensity. While the core is found to be of PSBL or lobe origin the beam ions tend to include also origins in the central plasma sheet. Farther inward from the boundary, similar core/beam populations can also be found, both predominantly originating from the outer CPS. The rise in energetic ion fluxes is found to result from acceleration at or near the near‐tail reconnection site. In contrast to the boundary populations, CPS distributions within the tailward moving plasmoid tend to be more isotropic, shifted by their bulk flow speed, again in agreement with observations.

High‐speed plasma flows in Earth's magnetotail are important for the global dynamics of the magnetosphere. We survey 11 yrs of high‐speed plasma flows observed by the ARTEMIS spacecraft in Earth's distant magnetotail between X _GSE = −52 R _E and X _GSE = −66 R _E to investigate their properties for a wide range of solar and geomagnetic activity levels. We find that tailward and earthward high‐speed flows at these distances exhibit notable asymmetries, with higher ion temperatures and larger dawn‐dusk magnetic field magnitudes in tailward flows compared to earthward flows. These asymmetries suggest that a significant portion of tailward high‐speed flows originate from near‐Earth magnetotail reconnection, while earthward high‐speed flows originate from a distant magnetotail reconnection site tailward of ARTEMIS. The occurrence rate of high‐speed flows follows the solar cycle progression and is about twice as high during solar maximum compared to solar minimum. Furthermore, both tailward and earthward high‐speed flows have higher median ion and electron temperatures, outflow speeds, and dawn‐dusk magnetic fields during solar maximum. In addition to the solar cycle dependence, the ion and electron temperatures in the high‐speed flows increase with increasing geomagnetic activity, for both the Auroral Electrojet (AE) and the Disturbance Storm Time (Dst) indices. Interestingly, during large substorms (AE > 1,100 nT) and geomagnetic storms (Dst < −90 nT), only tailward high‐speed flows are observed at lunar distances in this data set. In essence, our results indicate that the properties of both near‐Earth and distant tail reconnection are functions of the solar cycle and geomagnetic activity.

We present a statistical study of 272 proton isotropy boundary (IB) events, spanning 50 keV up to 2 MeV observed by ELFIN on the nightside magnetosphere, extending the energy range beyond the 120 keV population considered in prior works. Viewed from Low Earth Orbit, the IB is the magnetic latitude poleward of which persistently isotropic pitch‐angle distributions () are first detected. We characterize the IB distribution in local time, energy, geomagnetic activity, and 50 keV precipitation from isotropic protons. We find these IBs primarily exhibit negative energy‐latitude dispersion patterns consistent with equatorial magnetic field‐line curvature (FLC) scattering, with a 20%–30% chance of any particular energy channel exhibiting positive dispersion. The lowest latitude and most energetic IBs were detected in the pre‐midnight sector, consistent with the typical location of maximal cross‐tail current‐sheet thinning. We identify that proton IBs form the lower‐latitude boundary of a region with significant FLC‐caused proton precipitation between the outer ring current and inner edge of the plasma sheet (“PS2RC”), resulting in perpetual loss of protons exceeding typical plasma sheet energies. We show this 50 keV precipitation is often sufficiently intense and distributed to produce ionization enhancements over a range of altitudes at auroral/sub‐auroral latitudes. We also compare proton IB properties to those of electron IBs observed by ELFIN, finding similar trends across local time and activity. These results demonstrate that the IB and precipitation of isotropic particles in their poleward vicinity can be geophysically significant in connecting the magnetosphere and ionosphere.

Energetic electrons in Earth's magnetosphere play a critical role in space weather processes and pose risks to spacecraft electronics. Combining measurements from multiple missions, such as Cluster and Time History of Events and Macroscale Interactions during Substorms (THEMIS), enhances our understanding of these populations but requires cross‐calibration due to instrumental differences. This study presents a statistical comparison of 40–400 keV electron flux measurements from Cluster/Research with Adaptive Particle Imaging Detector (RAPID)/Imaging Electron Spectrometer (IES) and THEMIS/Solid State Telescope (SST) over 2007–2018. By analyzing the statistical distance between local flux distributions, we quantify discrepancies between the data sets and derive cross‐calibration factors, covering radial distances up to 16 . Our results show that SST fluxes consistently exceed IES observations, with the difference increasing with energy. The derived correction factors enable the joint use of the Cluster and THEMIS energetic electron data for multipoint space weather studies.

Energetic electrons in Earth's magnetosphere play a critical role in space weather processes and pose risks to spacecraft electronics. Combining measurements from multiple missions, such as Cluster and Time History of Events and Macroscale Interactions during Substorms (THEMIS), enhances our understanding of these populations but requires cross-calibration due to instrumental differences. This study presents a statistical comparison of 40–400 keV electron flux measurements from Cluster/Research with Adaptive Particle Imaging Detector (RAPID)/Imaging Electron Spectrometer (IES) and THEMIS/Solid State Telescope (SST) over 2007–2018. By analyzing the statistical distance between local flux distributions, we quantify discrepancies between the datasets and derive cross-calibration factors, covering radial distances up to 16 RE. Our results show that SST fluxes consistently exceed IES observations, with the difference increasing with energy. The derived correction factors enable the joint use of the Cluster and THEMIS energetic electron data for multipoint space weather studies.

We report on prolonged enhancements of electron fluxes at energies at or above 500 keV, observed in the magnetotail by the lunar‐orbiting Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) during the recovery phase of a magnetic storm with minimum −200 nT during periodic auroral electrojet () activations. The enhanced energetic electron fluxes were omnidirectional and observed near the magnetic equator. No solar energetic particle background was detected. Although ARTEMIS detected earthward magnetic flux transport impulses exceeding 2 mV/m, along with associated broadband electrostatic fluctuations, no correlation was evident between these phenomena and the relativistic electron flux enhancements. Spectra obtained during the relativistic electron flux enhancements are fit by the Kappa function, = 3.75, similar to that of the quiet‐time plasma sheet electron population at lunar distance. Multiple reconnection events at large distances are, most likely, responsible for the electron heating.

Dawnside auroral polarization streams (DAPS) are fast eastward flows in the dawn convection cell of Earth's ionosphere. With a steep flow gradient near the interface between Region 1 and 2 currents and a peak poleward of it, DAPS were suggested to be responsible for instabilities and dramatic events in the magnetosphere‐ionosphere (M‐I) system. To predict these events, it is important to investigate when and where DAPS prefer to occur and how they are related to other M‐I phenomena. We conduct this investigation statistically using 10 years of Swarm data and find that DAPS under sunlit and dark ionospheric conditions exhibit different dependencies on magnetic local times and geomagnetic activities, reflecting a complicated interplay between magnetotail dynamics and ionospheric conductance. The statistical results also reveal a strong correlation between DAPS and embedded Region 2 currents. These findings provide new insights into the DAPS generation mechanism.

The magnetotail current sheet plays a crucial role in substorm dynamics, affecting the entire magnetosphere. Formation and reconnection of thin (ion‐gyroscale) current sheets initiate magnetospheric substorms. Theoretical models suggest that a transient, demagnetized ion population is key element of the thin current sheet configuration. At lunar distances, the magnetotail provides a unique opportunity for in situ investigation of this population due to the high fraction of hot demagnetized ions. Using observations of thin current sheets by the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun mission, we show that the relative drift between demagnetized hot ions and magnetized cold ions, likely driven by a strong polarization electric field, reduces the ion current density to nearly zero in the spacecraft rest frame. Hot ions exhibit a crescent‐like velocity distribution, contributing to ion agyrotropy. We discuss this population in the context of advanced thin current sheet models, including transient ions performing Speiser‐like motion. These observations provide valuable insights for numerical and theoretical studies.

High‐intensity long‐duration continuous auroral electrojet (AE) activity (HILDCAA) events are associated with intensification of relativistic electron fluxes in the inner magnetosphere. The physical mechanisms of this intensification are not well established yet. We study observations by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the near earth plasma sheet at radial distances of 10 Earth radii, at the transition region between tail and dipole‐like magnetic configurations, referred to as the nightside transition region (NTR), during a HILDCAA event. The observations revealed recurrent dipolarizations accompanied by plasma flow vortices, impulsive electric field enhancements, and increases in electron fluxes at energies of 100 keV up to 1 MeV. Electron pitch angle (PA) distributions at THEMIS showed field‐aligned flux enhancements at energies of 100 keV. This indicates a Fermi‐type energization. Arguably, electrons gain energy up to MeV via repetitive bouncing through the acceleration region. Energization of ions was insignificant which led to 1. We suggest that the increased ratio leads to a local increase of the Hall conductivity in the conjugate ionosphere, which causes ionospheric current intensification and strong , consistent with observations.

Earth's magnetotail, a night‐side region characterized by stretched magnetic field lines and strong plasma currents, is the primary site for the release of magnetic field energy and its transformation into plasma heating and kinetic energy plus charged particle acceleration during magnetic reconnection. In this study, we demonstrate that the efficiency of this acceleration can be sufficiently high to produce populations of relativistic and ultra‐relativistic electrons, with energies up to several MeV, which exceeds all previous theoretical and simulation estimates. Using data from the low‐altitude ELFIN and CIRBE CubeSats, we show multiple events of relativistic electron bursts within the magnetotail, far poleward of the outer radiation belt. These bursts are characterized by power‐law energy spectra and can be detected during even moderate substorms.