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The weakly ionized plasma in the Earth's ionosphere is controlled by a complex interplay between solar and magnetospheric inputs from above, atmospheric processes from below, and plasma electrodynamics from within. This interaction results in ionosphere structuring and variability that pose major challenges for accurate ionosphere prediction for global navigation satellite system (GNSS) related applications and space weather research. The ionospheric structuring and variability are often probed using the total electron content (TEC) and its relative perturbations (dTEC). Among dTEC variations observed at high latitudes, a unique modulation pattern has been linked to magnetospheric ultra low frequency (ULF) waves, yet its underlying mechanisms remain unclear. Here using magnetically-conjugate observations from the THEMIS spacecraft and a ground-based GPS receiver at Fairbanks, Alaska, we provide direct evidence that these dTEC modulations are driven by magnetospheric electron precipitation induced by ULF-modulated whistler-mode waves. We observed peak-to-peak dTEC amplitudes reaching ~0.5 TECU (1 TECU is equal to 10$^6$ electrons/m$^2$) with modulations spanning scales of ~5--100 km. The cross-correlation between our modeled and observed dTEC reached ~0.8 during the conjugacy period but decreased outside of it. The spectra of whistler-mode waves and dTEC also matched closely at ULF frequencies during the conjugacy period but diverged outside of it. Our findings elucidate the high-latitude dTEC generation from magnetospheric wave-induced precipitation, addressing a significant gap in current physics-based dTEC modeling. Theses results thus improve ionospheric dTEC prediction and enhance our understanding of magnetosphere-ionosphere coupling via ULF waves., Comment: 14 pages, 5 figures, manuscript under review in AGU Advances

Using particle data from the ELFIN satellites, we present a statistical study of 284 proton isotropy boundary events on the nightside magnetosphere, characterizing their occurrence and distribution in local time, latitude (L-shell), energy, and precipitating energy flux, as a function of geomagnetic activity. For a given charged particle species and energy, its isotropy boundary (IB) is the magnetic latitude poleward of which persistently isotropic pitch-angle distributions ($J_{prec}/J_{perp}\sim 1$) are first observed to occur. This isotropization is interpreted as resulting from magnetic field-line curvature (FLC) scattering in the equatorial magnetosphere. We find that proton IBs are observed under all observed activity levels, spanning 16 to 05 MLT with $\sim$100% occurrence between 19 and 03 MLT, trending toward 60% at dawn/dusk. These results are also compared with electron IB properties observed using ELFIN, where we find similar trends across local time and activity, with the onset in $\geq$50 keV proton IB occurring on average 2 L-shells lower, and providing between 3 and 10 times as much precipitating power. Proton IBs typically span $64^\circ$-$66^\circ$ in magnetic latitude (5-6 in L-shell), corresponding to the outer edge of the ring current, tending toward lower IGRF latitudes as geomagnetic activity increases. The IBs were found to commonly occur 0.3-2.1 Re beyond the plasmapause. Proton IBs typically span $<$50 keV to $\sim$1 MeV in energy, maximizing near 22 MLT, and decreasing to a typical upper limit of 300-400 keV toward dawn and dusk, with peak observed isotropic energy increasing by $\sim$500 keV during active intervals. These results suggest that FLC in the vicinity of IBs can provide a substantial depletion mechanism for energetic protons, with the total nightside precipitating power from FLC-scattering found to be on the order of 100 MW, at times $\geq$10 GW.

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.

In space plasmas, large-amplitude Alfv\'en waves can drive compressive perturbations, accelerate ion beams, and lead to plasma heating and the excitation of ion acoustic waves at kinetic scales. This energy channelling from fluid to kinetic scales represents a complementary path to the classical turbulent cascade. Here, we present observational and computational evidence to validate this hypothesis by simultaneously resolving the fluid-scale Alfv\'en waves, kinetic-scale ion acoustic waves, and their imprints on ion velocity distributions in the Earth's magnetopause boundary layer. We show that two coexisting compressive modes, driven by the magnetic pressure gradients of Alfv\'en waves, not only accelerate the ion tail population to the Alfv\'en velocity, but also heat the ion core population near the ion acoustic velocity and generate Debye-scale ion acoustic waves. Thus, Alfv\'en-acoustic energy channeling emerges as a viable mechanism for plasma heating near plasma boundaries where large-amplitude Alfv\'en waves are present.

A global reconfiguration of the magnetotail characterizes substorms. Current sheet thinning, intensification, and magnetic field stretching are defining features of the substorm growth phase and their spatial distributions control the timing and location of substorm onset. Presently, sparse in-situ observations cannot resolve these distributions. A promising approach is to use new substorm magnetic field reconstruction methods based on data mining, termed SST19. Here we compare the SST19 reconstructions to low-altitude ELFIN measurements of energetic particle precipitations to probe the radial profile of the equatorial magnetic field curvature during a 19~August 2022 substorm. ELFIN and SST19 yield a consistent dynamical picture of the magnetotail during the growth phase and capture expected features such as the formation of a thin current sheet and its earthward motion. Furthermore, they resolve a V-like pattern of isotropic electron precipitation boundaries in the time-energy plane, consistent with earlier observations but now over a broad energy range.

The strong variations of energetic electron fluxes in the Earth's inner magnetosphere are notoriously hard to forecast. Developing accurate empirical models of electron fluxes from low to high altitudes at all latitudes is therefore useful to improve our understanding of flux variations and to assess radiation hazards for spacecraft systems. In the present work, energy- and pitch-angle-resolved precipitating, trapped, and backscattered electron fluxes measured at low altitude by Electron Loss and Fields Investigation (ELFIN) CubeSats are used to infer omnidirectional fluxes at altitudes below and above the spacecraft, from 150 km to 20,000 km, making use of adiabatic transport theory and quasi-linear diffusion theory. The inferred fluxes are fitted as a function of selected parameters using a stepwise multivariate optimization procedure, providing an analytical model of omnidirectional electron flux along each geomagnetic field line, based on measurements from only one spacecraft in low Earth orbit. The modeled electron fluxes are provided as a function of $L$-shell, altitude, energy, and two different indices of past substorm activity, computed over the preceding 4 hours or 3 days, potentially allowing to disentangle impulsive processes (such as rapid injections) from cumulative processes (such as inward radial diffusion and wave-driven energization). The model is validated through comparisons with equatorial measurements from the Van Allen Probes, demonstrating the broad applicability of the present method. The model indicates that both impulsive and time-integrated substorm activity partly control electron fluxes in the outer radiation belt and in the plasma sheet.

Recent study has demonstrated that electron cyclotron harmonic (ECH) waves can be excited by a low energy electron beam. Such waves propagate at moderately oblique wave normal angles (~70). The potential effects of beam-driven ECH waves on electron dynamics in Earth's plasma sheet is not known. Using two-dimensional Darwin particle-in-cell simulations with initial electron distributions that represent typical plasma conditions in the plasma sheet, we explore the excitation and saturation of such beam-driven ECH waves. Both ECH and electron acoustic waves are excited in the simulation and propagate at oblique wave normal angles. Compared with the electron acoustic waves, ECH waves grow much faster and have more intense saturation amplitudes. Cold, stationary electrons are first accelerated by ECH waves through cyclotron resonance and then accelerated in the parallel direction by both the ECH and electron acoustic waves through Landau resonance. Beam electrons, on the other hand, are decelerated in the parallel direction and scattered to larger pitch angles. The relaxation of the electron beam and the continuous heating of the cold electrons contribute to ECH wave saturation and suppress the excitation of electron acoustic waves. When the ratio of plasma to electron cyclotron frequency wpe/wce increases, the ECH wave amplitude increases while the electron acoustic wave amplitude decreases. Our work reveals the importance of ECH and electron acoustic waves in reshaping sub-thermal electron distributions and improves our understanding on the potential effects of wave-particle interactions in trapping ionospheric electron outflows and forming anisotropic (field-aligned) electron distributions in the plasma sheet.

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 anisotropy towards the upstream direction, which may be responsible for enhancing magnetosheath leakage and therefore foreshock ion density. To understand the conditions leading to such an anisotropy and the potential for enhanced leakage, we perform case studies and a statistical study of magnetosheath and foreshock region data surrounding ~500 THEMIS bow shock crossings. We quantify the anisotropy 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 anisotropy events exhibit heat flux preferentially towards 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 anisotropy 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., Comment: Under review by JGR Space Physics

Resonant interactions between relativistic electrons and electromagnetic ion cyclotron (EMIC) waves provide an effective loss mechanism for this important electron population in the outer radiation belt. The diffusive regime of electron scattering and loss has been well incorporated into radiation belt models within the framework of the quasi-linear diffusion theory, whereas the nonlinear regime has been mostly studied with test particle simulations. There is also a less investigated, nonresonant regime of electron scattering by EMIC waves. All three regimes should be present, depending on the EMIC waves and ambient plasma properties, but the occurrence rates of these regimes have not been previously quantified. This study provides a statistical investigation of the most important EMIC wave-packet characteristics for the diffusive, nonlinear, and nonresonant regimes of electron scattering. We utilize 3 years of Van Allen Probe observations to derive distributions of wave amplitudes, wave-packet sizes, and rates of frequency variations within individual wave-packets. We demonstrate that EMIC waves typically propagate as wave-packets with $\sim 10$ wave periods each, and that $\sim 3-10$\% of such wave-packets can reach the regime of nonlinear resonant interaction with 2 to 6 MeV electrons. We show that EMIC frequency variations within wave-packets reach $50-100$\% of the center frequency, corresponding to a significant high-frequency tail in their wave power spectrum. We explore the consequences of these wave-packet characteristics for high and low energy electron precipitation by H-band EMIC waves and for the relative importance of quasi-linear and nonlinear regimes of wave-particle interactions.

Certain forms of solar wind transients contain significant enhancements of dynamic pressure and may effectively drive magnetosphere dynamics, including substorms and storms. An integral element of such driving is the generation of a wide range of electromagnetic waves within the inner magnetosphere, either by compressionally heated plasma or by substorm plasma sheet injections. Consequently, solar wind transient impacts are traditionally associated with energetic electron scattering and losses into the atmosphere by electromagnetic waves. In this study, we show the first direct measurements of two such transient-driven precipitation events as measured by the low-altitude Electron Losses and Fields Investigation (ELFIN) CubeSats. The first event demonstrates storm-time generated electromagnetic ion cyclotron waves efficiently precipitating relativistic electrons from >300 keV to 2 MeV at the duskside. The second event demonstrates whistler-mode waves leading to scattering of electrons from 50 keV to 700 keV on the dawnside. These observations confirm the importance of solar wind transients in driving energetic electron losses and subsequent dynamics in the ionosphere.

Electromagnetic Ion Cyclotron (EMIC) waves can drive radiation belt depletion and Low-Earth Orbit (LEO) satellites can detect the resulting electron and proton precipitation. The ELFIN (Electron Losses and Fields InvestigatioN) CubeSats provide an excellent opportunity to study the properties of EMIC-driven electron precipitation with much higher energy and pitch-angle resolution than previously allowed. We collect EMIC-driven electron precipitation events from ELFIN observations and use POES (Polar Orbiting Environmental Satellites) to search for 10s-100s keV proton precipitation nearby as a proxy of EMIC wave activity. Electron precipitation mainly occurs on localized radial scales (0.3 L), over 15-24 MLT and 5-8 L shells, stronger at MeV energies and weaker down to 100-200 keV. Additionally, the observed loss cone pitch-angle distribution agrees with quasilinear predictions at >250 keV (more filled loss cone with increasing energy), while additional mechanisms are needed to explain the observed low-energy precipitation., Comment: This manuscript has been accepted by Geophysical Research Letters in June 2023, currently pending publication. The version here is the accepted version

We present the results of processing the effects of the powerful Gamma Ray Burst GRB221009A captured by the charged particle detectors (electrostatic analyzers and solid-state detectors) onboard spacecraft at different points in the heliosphere on October 9, 2022. To follow the GRB221009A propagation through the heliosphere we used the electron and proton flux measurements from solar missions Solar Orbiter and STEREO-A; Earth magnetosphere and the solar wind missions THEMIS and Wind; meteorological satellites POES15, POES19, MetOp3; and MAVEN - a NASA mission orbiting Mars. GRB221009A had a structure of four bursts: less intense Pulse 1 - the triggering impulse - was detected by gamma-ray observatories at 131659 UT (near the Earth); the most intense Pulses 2 and 3 were detected on board all the spacecraft from the list, and Pulse 4 detected in more than 500 s after Pulse 1. Due to their different scientific objectives, the spacecraft, which data was used in this study, were separated by more than 1 AU (Solar Orbiter and MAVEN). This enabled tracking GRB221009A as it was propagating across the heliosphere. STEREO-A was the first to register Pulse 2 and 3 of the GRB, almost 100 seconds before their detection by spacecraft in the vicinity of Earth. MAVEN detected GRB221009A Pulses 2, 3, and 4 at the orbit of Mars about 237 seconds after their detection near Earth. By processing the time delays observed we show that the source location of the GRB221009A was at RA 288.5 degrees, Dec 18.5 degrees (J2000) with an error cone of 2 degrees

Electron resonant scattering by high-frequency electromagnetic whistler-mode waves has been proposed as a mechanism for solar wind electron scattering and pre-acceleration to energies that enable them to participate in shock drift acceleration around the Earth's bow shock. However, observed whistler-mode waves are often sufficiently intense to resonate with electrons nonlinearly, which prohibits the application of quasi-linear diffusion theory. This is the second of two accompanying papers devoted to developing a new theoretical approach for quantifying the electron distribution evolution subject to multiple resonant interactions with intense whistler-mode wave-packets. In the first paper, we described a probabilistic approach, applicable to systems with short wave-packets. For such systems, nonlinear resonant effects can be treated by diffusion theory, but with diffusion rates different from those of quasi-linear diffusion. In this paper we generalize this approach by merging it with a mapping technique. This technique can be used to model the electron distribution evolution in the presence of significantly non-diffusive resonant scattering by intense long wave-packets. We verify our technique by comparing its predictions with results from a numerical integration approach.

Adiabatic heating of solar wind electrons at the Earth's bow shock and its foreshock region produces transversely anisotropic hot electrons that, in turn, generate intense high-frequency whistler-mode waves. These waves are often detected by spacecraft as narrow-band, electromagnetic emissions in the frequency range of [0.1,0.5] of the local electron gyrofrequency. Resonant interactions between these waves and electrons may cause electron acceleration and pitch-angle scattering, which can be important for creating the electron population that seeds shock drift acceleration. The high intensity and coherence of the observed whistler-mode waves prohibit the use of quasi-linear theory to describe their interaction with electrons. In this paper, we aim to develop a new theoretical approach to describe this interaction, that incorporates nonlinear resonant interactions, gradients of the background density and magnetic field, and the fine structure of the waveforms that usually consist of short, intense wave-packet trains. This is the first of two accompanying papers. It outlines a probabilistic approach to describe the wave-particle interaction. We demonstrate how the wave-packet size affects electron nonlinear resonance at the bow shock and foreshock regions, and how to evaluate electron distribution dynamics in such a system that is frequented by short, intense whistler-mode wave-packets. In the second paper, this probabilistic approach is merged with a mapping technique, which allows us to model systems containing short and long wave-packets.

Electron acoustic waves (EAWs), as well as electron-acoustic solitary structures, play a crucial role in thermalization and acceleration of electron populations in Earth's magnetosphere. These waves are often observed in association with whistler-mode waves, but the detailed mechanism of EAW and whistler wave coupling is not yet revealed. We investigate the excitation mechanism of EAWs and their potential relation to whistler waves using particle-in-cell simulations. Whistler waves are first excited by electrons with a temperature anisotropy perpendicular to the background magnetic field. Electrons trapped by these whistler waves through nonlinear Landau resonance form localized field-aligned beams, which subsequently excite EAWs. By comparing the growth rate of EAWs and the phase mixing rate of trapped electron beams, we obtain the critical condition for EAW excitation, which is consistent with our simulation results across a wide region in parameter space. These results are expected to be useful in the interpretation of concurrent observations of whistler-mode waves and nonlinear solitary structures, and may also have important implications for investigation of cross-scale energy transfer in the near-Earth space environment.

Electromagnetic ion cyclotron (EMIC) waves lead to rapid scattering of relativistic electrons in Earth's radiation belts, due to their large amplitudes relative to other waves that interact with electrons of this energy range. A central feature of electron precipitation driven by EMIC waves is deeply elusive. That is, moderate precipitating fluxes at energies below the minimum resonance energy of EMIC waves occur concurrently with strong precipitating fluxes at resonance energies in low-altitude spacecraft observations. This paper expands on a previously reported solution to this problem: nonresonant scattering due to wave packets. The quasi-linear diffusion model is generalized to incorporate nonresonant scattering by a generic wave shape. The diffusion rate decays exponentially away from the resonance, where shorter packets lower decay rates and thus widen the energy range of significant scattering. Using realistic EMIC wave packets from $\delta f$ particle-in-cell simulations, test particle simulations are performed to demonstrate that intense, short packets extend the energy of significant scattering well below the minimum resonance energy, consistent with our theoretical prediction. Finally, the calculated precipitating-to-trapped flux ratio of relativistic electrons is compared to ELFIN observations, and the wave power spectra is inferred based on the measured flux ratio. We demonstrate that even with a narrow wave spectrum, short EMIC wave packets can provide moderately intense precipitating fluxes well below the minimum resonance energy., Comment: 32 pages, 9 figures

Utilizing observations from the ELFIN satellites, we present a statistical study of $\sim$2000 events in 2019-2020 characterizing the occurrence in magnetic local time (MLT) and latitude of $\geq$50 keV electron isotropy boundaries (IBs) at Earth, and the dependence of associated precipitation on geomagnetic activity. The isotropy boundary for an electron of a given energy is the magnetic latitude poleward of which persistent isotropized pitch-angle distributions ($J_{prec}/J_{perp}\sim 1$) are first observed to occur, interpreted as resulting from magnetic field-line curvature scattering (FLCS) in the equatorial magnetosphere. We find that energetic electron IBs can be well-recognized on the nightside from dusk until dawn, under all geomagnetic activity conditions, with a peak occurrence rate of almost 90% near $\sim$22 hours in MLT, remaining above 80% from 21 to 01 MLT. The IBs span a wide range of IGRF magnetic latitudes from $60^\circ$-$74^\circ$, with a maximum occurrence between $66^\circ$-$71^\circ$ (L of 6-8), shifting to lower latitudes and pre-midnight local times with activity. The precipitating energy flux of $\geq$50 keV electrons averaged over the IB-associated latitudes varies over four orders of magnitude, up to $\sim$1 erg/cm$^2$-s, and often includes electron energies exceeding 1 MeV. The local time distribution of IB-associated energies and precipitating fluxes also exhibit peak values near midnight for low activity, shifting toward pre-midnight for elevated activity. The percentage of the total energy deposited over the high-latitude regions ($55^\circ$ to $80^\circ$; or IGRF $L\gtrsim 3$) attributed to IBs is 10-20%, on average, or about 10 MW of total atmospheric power input, but at times can be up to $\sim$100% of the total $\geq$50 keV electron energy deposition over the entire sub-auroral and auroral zone region, exceeding 1 GW in atmospheric power input.

Force-free current sheets are local plasma structures with field-aligned electric currents and approximately uniform plasma pressures. Such structures, widely found throughout the heliosphere, are sites for plasma instabilities and magnetic reconnection, the growth rate of which is controlled by the structure's current sheet configuration. Despite the fact that many kinetic equilibrium models have been developed for one-dimensional (1D) force-free current sheets, their two-dimensional (2D) counterparts, which have a magnetic field component normal to the current sheets, have not received sufficient attention to date. Here, using particle-in-cell simulations, we search for such 2D force-free current sheets through relaxation from an initial, magnetohydrodynamic equilibrium. Kinetic equilibria are established toward the end of our simulations, thus demonstrating the existence of kinetic force-free current sheets. Although the system currents in the late equilibrium state remain field aligned as in the initial configuration, the velocity distribution functions of both ions and electrons systematically evolve from their initial drifting Maxwellians to their final time-stationary Vlasov state. The existence of 2D force-free current sheets at kinetic equilibrium necessitates future work in discovering additional integrals of motion of the system, constructing the kinetic distribution functions, and eventually investigating their stability properties., Comment: Published in ApJ, 20 pages with 14 figures

Electron resonant scattering by whistler-mode waves is one of the most important mechanisms responsible for electron precipitation to the Earth's atmosphere. We investigate temporal and spatial scales of such precipitation with measurements from the two low-altitude ELFIN CubeSats. We compare the variations in energetic electron precipitation at the same L-shells but on successive data collection orbit tracks by the two ELFIN satellites. Variations seen at the smallest inter-satellite separations are likely associated with whistler-mode chorus elements or with the scale of chorus wave packets (0.1 - 1 s in time and 100 km in space at the equator). Variations between precipitation L-shell profiles at intermediate inter-satellite separations are likely associated with whistler-mode wave power modulations by ultra-low frequency (ULF) waves, i.e., with the wave source region (from a few to tens of seconds to a few minutes in time and 1000km in space at the equator). During these two types of variations, consecutive crossings are associated with precipitation L-shell profiles very similar to each other. Therefore the spatial and temporal variations at those scales do not change the net electron loss from the outer radiation belt. Variations at the largest range of inter-satellite separations, several minutes to more than 10 min, are likely associated with mesoscale equatorial plasma structures that are affected by convection (at minutes to tens of minutes temporal variations and [1000,10000]km spatial scales). The latter type of variations results in appreciable changes in the precipitation L-shell profiles and can significantly modify the net electron losses during successive tracks.

Thermalization and heating of plasma flows at shocks result in unstable charged-particle distributions which generate a wide range of electromagnetic waves. These waves, in turn, can further accelerate and scatter energetic particles. Thus, the properties of the waves and their implication for wave-particle interactions are critically important for modeling energetic particle dynamics in shock environments. Whistler-mode waves, excited by the electron heat flux or a temperature anisotropy, arise naturally near shocks and foreshock transients. As a result, they can often interact with supra-thermal electrons. The low background magnetic field typical at the core of such transients and the large wave amplitudes may cause such interactions to enter the nonlinear regime. In this study, we present a statistical characterization of whistler-mode waves at foreshock transients around Earth bow shock, as they are observed under a wide range of upstream conditions. We find that a significant portion of them are sufficiently intense and coherent to warrant nonlinear treatment. Copious observations of background magnetic field gradients and intense whistler wave amplitudes suggest that phase trapping, a very effective mechanism for electron acceleration in inhomogeneous plasmas, may be the cause. We discuss the implications of our findings for electron acceleration in planetary and astrophysical shock environments., Comment: Submitted to APJ

Electromagnetic ion cyclotron waves are expected to pitch-angle scatter and cause atmospheric precipitation of relativistic ($> 1$ MeV) electrons under typical conditions in Earth's radiation belts. However, it has been a longstanding mystery how relativistic electrons in the hundreds of keV range (but $<1$ MeV), which are not resonant with these waves, precipitate simultaneously with those $>1$ MeV. We demonstrate that, when the wave packets are short, nonresonant interactions enable such scattering of $100$s of keV electrons by introducing a spread in wavenumber space. We generalize the quasi-linear diffusion model to include nonresonant effects. The resultant model exhibits an exponential decay of the scattering rates extending below the minimum resonant energy depending on the shortness of the wave packets. This generalized model naturally explains observed nonresonant electron precipitation in the hundreds of keV concurrent with $>1$ MeV precipitation.

Magnetic field line reconnection is a universal plasma process responsible for the magnetic field topology change and magnetic field energy dissipation into charged particle heating and acceleration. In many systems, the conditions leading to the magnetic reconnection are determined by the pre-reconnection configuration of a thin layer with intense currents -- otherwise known as the thin current sheet. In this study we investigate two such systems: Earth's magnetotail and helmet streamers in the solar corona. The pre-reconnection current sheet evolution has been intensely studied in the magnetotail, where in-situ spacecraft observations are available; but helmet streamer current sheets studies are fewer, due to lack of in-situ observations -- they are mostly investigated with numerical simulations and information that can be surmised from remote sensing instrumentation. Both systems exhibit qualitatively the same behavior, despite their largely different Mach numbers, much higher at the solar corona than at the magnetotail. Comparison of spacecraft data (from the magnetotail) with numerical simulations (for helmet streamers) shows that the pre-reconnection current sheet thinning, for both cases, is primarily controlled by plasma pressure gradients. Scaling laws of the current density, magnetic field, and pressure gradients are the same for both systems. We discuss how magnetotail observations and kinetic simulations can be utilized to improve our understanding and modeling of the helmet streamer current sheets.

Relativistic electron losses in Earth's radiation belts are usually attributed to electron resonant scattering by electromagnetic waves. One of the most important wave mode for such scattering is the electromagnetic ion cyclotron (EMIC) mode. Within the quasi-linear diffusion framework, the cyclotron resonance of relativistic electrons with EMIC waves results in very fast electron precipitation to the atmosphere. However, wave intensities often exceed the threshold for nonlinear resonant interaction, and such intense EMIC waves have been shown to transport electrons away from the loss cone due to the force bunching effect. In this study we investigate if this transport can block electron precipitation. We combine test particle simulations, low-altitude ELFIN observations of EMIC-driven electron precipitation, and ground-based EMIC observations. Comparing simulations and observations, we show that, despite of the low pitch-angle electrons being transported away from the loss cone, the scattering at higher pitch angles results in the loss cone filling and electron precipitation.

The magnetotail current sheet configuration determines magnetic reconnection properties that control the substorm onset, one of the most energetic phenomena in the Earth's magnetosphere. The quiet-time current sheet is often approximated as a two-dimensional (2D) magnetic field configuration balanced by isotropic plasma pressure gradients. However, reconnection onset is preceded by the current sheet thinning and the formation of a nearly one-dimensional (1D) magnetic field configuration. In this study, using particle-in-cell simulations, we investigate the force balance of such thin current sheets when they are driven by plasma inflow. We demonstrate that the magnetic field configuration transitions from 2D to 1D thanks to the formation of plasma pressure nongyrotropy and reveal its origin in the nongyrotropic terms of the ion distributions. We show that substorm onset may be controlled by the instability and dynamics of such nongyrotropic current sheets, having properties much different from the most commonly investigated 2D isotropic configuration., Comment: 16 pages, 4 figures

Non-Gaussian distributions are commonly observed in collisionless space plasmas. Generating samples from non-Gaussian distributions is critical for the initialization of particle-in-cell simulations that investigate their driven and undriven dynamics. To this end, we report a computationally efficient, robust tool, Chebsampling, to sample general distribution functions in one and two dimensions. This tool is based on inverse transform sampling with function approximation by Chebyshev polynomials. We demonstrate practical uses of Chebsampling through sampling typical distribution functions in space plasmas., Comment: 15 pages, 4 figures

Many in-situ spacecraft observations have demonstrated that magnetic reconnection in the Earth's magnetotail is largely controlled by the pre-reconnection current sheet configuration. One of the most important thin current sheet characteristics is the preponderance of electron currents driven by strong polarized electric fields, which are commonly observed in the Earth's magnetotail well before the reconnection. We use particle-in-cell simulations to investigate magnetic reconnection in the 2D magnetotail current sheet with a finite magnetic field component normal to the current sheet and with the current sheet polarization. Under the same external driving conditions, reconnection in a polarized current sheet is shown to occur at a lower rate than in a nonpolarized current sheet. The reconnection rate in a polarized current sheet decreases linearly as the electron current's contribution to the cross-tail current increases. In simulations with lower background temperature the reconnection electric field is higher. We demonstrate that after reconnection in such a polarized current sheet, the outflow energy flux is mostly in the form of ion enthalpy flux, followed by electron enthalpy flux, Poynting flux, ion kinetic energy flux and electron kinetic energy flux. These findings are consistent with spacecraft observations. Because current sheet polarization is not uniform along the magnetotail, our results suggest that it may slow down reconnection in the most polarized near-Earth magnetotail and thereby move the location of reconnection onset downtail., Comment: 37 pages, 11 figures

Magnetic field-line reconnection is a universal plasma process responsible for the conversion of magnetic field energy to the plasma heating and charged particle acceleration. Solar flares and Earth's magnetospheric substorms are two most investigated dynamical systems where magnetic reconnection is believed to be responsible for global magnetic field reconfiguration and energization of plasma populations. Such a reconfiguration includes formation of a long-living current systems connecting the primary energy release region and cold dense conductive plasma of photosphere/ionosphere. In both flares and substorms the evolution of this current system correlates with formation and dynamics of energetic particle fluxes. Our study is focused on this similarity between flares and substorms. Using a wide range of datasets available for flare and substorm investigations, we compare qualitatively dynamics of currents and energetic particle fluxes for one flare and one substorm. We showed that there is a clear correlation between energetic particle bursts (associated with energy release due to magnetic reconnection) and magnetic field reconfiguration/formation of current system. We then discuss how datasets of in-situ measurements in the magnetospheric substorm can help in interpretation of datasets gathered for the solar flare.

Electron cyclotron harmonic (ECH) waves play a significant role in driving the diffuse aurora, which constitutes more than 75% of the particle energy input into the ionosphere. ECH waves in magnetospheric plasmas have long been thought to be excited predominantly by the loss cone anisotropy (velocity-space gradients) that arises naturally in a planetary dipole field. Recent THEMIS observations, however, indicate that an electron beam can also excite such waves in Earth's magnetotail. The ambient and beam plasma conditions under which electron beam excitation can take place are unknown. Knowledge of such conditions would allow us to further explore the relative contribution of this excitation mechanism to ECH wave scattering of magnetospheric electrons at Earth and the outer planets. Using the hot plasma dispersion relation, we address the nature of beam-driven ECH waves and conduct a comprehensive parametric survey of this instability. We find that growth is provided by beam electron cyclotron resonances of both first and higher orders. We also find that these waves are unstable under a wide range of plasma conditions. The growth rate increases with beam density, beam velocity, and hot electron temperature; it decreases with increasing beam temperature and beam temperature anisotropy, hot electron density, and cold electron density and temperature. Such conditions abound in Earth's magnetotail, where magnetospheric electrons heated by earthward convection and magnetic reconnection coexist with colder ionospheric electrons.

Upstream of shocks, the foreshock is filled with hot ions. When these ions are concentrated and thermalized around a discontinuity, a diamagnetic cavity bounded by compressional boundaries, referred to as a foreshock transient, forms. Sometimes, the upstream compressional boundary can further steepen into a secondary shock, which has been observed to accelerate particles and contribute to the primary shock acceleration. However, secondary shock formation conditions and processes are not fully understood. Using particle-in-cell simulations, we reveal how secondary shocks are formed. From 1D simulations, we show that electric fields play a critical role in shaping the shock's magnetic field structure, as well as in coupling the energy of hot ions to that of the shock. We demonstrate that larger thermal speed and concentration ratio of hot ions favors the formation of a secondary shock. From a more realistic 2D simulation, we examine how a discontinuity interacts with foreshock ions leading to the formation of a foreshock transient and a secondary shock. Our results imply that secondary shocks are more likely to occur at primary shocks with higher Mach number. With the secondary shock's previously proven ability to accelerate particles in cooperation with a planetary bow shock, it is even more appealing to consider them in particle acceleration of high Mach number astrophysical shocks., Comment: Accepted for publication in the Astrophysical Journal

Magnetosheath jets are localized fast flows with enhanced dynamic pressure. When they supermagnetosonically compress the ambient magnetosheath plasma, a bow wave or shock can form ahead of them. Such a bow wave was recently observed to accelerate ions and possibly electrons. The ion acceleration process was previously analyzed, but the electron acceleration process remains largely unexplored. Here we use multi-point observations by Time History of Events and Macroscale during Substorms from three events to determine whether and how magnetosheath jet-driven bow waves can accelerate electrons. We show that when suprathermal electrons in the ambient magnetosheath convect towards a bow wave, some electrons are shock-drift accelerated and reflected towards the ambient magnetosheath and others continue moving downstream of the bow wave resulting in bi-directional motion. Our study indicates that magnetosheath jet-driven bow waves can result in additional energization of suprathermal electrons in the magnetosheath. It implies that magnetosheath jets can increase the efficiency of electron acceleration at planetary bow shocks or other similar astrophysical environments., Comment: Under review by JGR space physics

When a magnetosheath jet (localized dynamic pressure enhancements) compresses ambient magnetosheath at a (relative) speed faster than the local magnetosonic speed, a bow wave or shock can form ahead of the jet. Such bow waves or shocks were recently observed to accelerate particles, thus contributing to magnetosheath heating and particle acceleration in the extended environment of Earth bow shock. To further understand the characteristics of jet-driven bow waves, we perform a statistical study to examine which solar wind conditions favor their formation and whether it is common for them to accelerate particles. We identified 364 out of 2859 (13%) magnetosheath jets to have a bow wave or shock ahead of them with Mach number typically larger than 1.1. We show that large solar wind plasma beta, weak interplanetary magnetic field (IMF) strength, large solar wind Alfven Mach number, and strong solar wind dynamic pressure present favorable conditions for their formation. We also show that magnetosheath jets with bow waves or shocks are more frequently associated with higher maximum ion and electron energies than those without them, confirming that it is common for these structures to accelerate particles. In particular, magnetosheath jets with bow waves have electron energy flux enhanced on average by a factor of 2 compared to both those without bow waves and the ambient magnetosheath. Our study implies that magnetosheath jets can contribute to shock acceleration of particles especially for high Mach number shocks. Therefore, shock models should be generalized to include magnetosheath jets and concomitant particle acceleration., Comment: Under review by JGR space physics

This White Paper outlines the importance of addressing the fundamental science theme <> through a future ESA mission. The White Paper presents five compelling science questions related to particle energization by shocks, reconnection,waves and turbulence, jets and their combinations. Answering these questions requires resolving scale coupling, nonlinearity and nonstationarity, which cannot be done with existing multi-point observations. In situ measurements from a multi-point, multi-scale L-class plasma observatory consisting of at least 7 spacecraft covering fluid, ion and electron scales are needed. The plasma observatory will enable a paradigm shift in our comprehension of particle energization and space plasma physics in general, with very important impact on solar and astrophysical plasmas. It will be the next logical step following Cluster, THEMIS and MMS for the very large and active European space plasmas community. Being one of the cornerstone missions of the future ESA Voyage 2035-2050 science program, it would further strengthen the European scientific and technical leadership in this important field.

We analyze statistically extreme time-integrated Ap events in 1958-2007, which occurred during both strong and weak geomagnetic storms. The tail of the distribution of such events can be accurately fitted by a power-law with a sharp upper cutoff, in close agreement with a second fit inferred from Extreme Value Theory. Such a behavior is suggestive of a self-organization of the solar wind-magnetosphere-ionosphere system appearing during strong and sustained solar wind driving. The 1 in 10 years to 1 in 100 years return levels of such extreme events are calculated, taking into account possible solar cycle modulations. The huge October 2003 event turns out to be a 1 in 100 (+/- 40) years event. Comparisons with the distribution of extreme time-integrated aa events collected in 1870-2010 support the reliability of our results over the long run. Using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites and the Van Allen Probes, we show that extreme time-integrated $ap$ events produce hard fluxes of energetic electrons and ions in the magnetotail and high fluxes (>1000 000 e/cm2/sr/s/MeV) of 1.8 MeV electrons in the heart of the outer radiation belt., Comment: 36 pages, 5 Figures

The foreshock region involves localized and transient structures such as foreshock cavities and hot flow anomalies due to solar wind-bow shock interactions, and foreshock transients have been shown to lead to magnetospheric and ionospheric responses. In this paper, the interaction between a foreshock transient and the magnetosphere-ionosphere system is investigated using dayside aurora imagers revealing structures and propagation in greater detail than previously possible. A foreshock transient was detected by THEMIS-B and C during 1535-1545 UT on June 25, 2008. THEMIS-A, D and E observed magnetopause compression, cold plasma enhancement and ULF waves in the dayside magnetosphere. The all-sky imager (ASI) at South Pole observed that both diffuse and discrete aurora brightened locally soon after the appearance of this foreshock transient. The diffuse aurora brightening, which corresponded to a region a few Re size in GSM-Y in the equatorial plane, propagated duskward with an average speed of ~100 km/s. Soon after the diffuse aurora brightened, discrete aurora also brightened and extended duskward, which was consistent with the motion of the foreshock transient as it swept through the magnetosheath while impacting the magnetopause. Equivalent horizontal currents measured by magnetometers revealed a pair of field-aligned currents (FACs) moving duskward consistent with motion of the discrete aurora patterns. We conclude that the high-resolution and two-dimensional observation of auroral responses by ground-based ASI can help to estimate the evolution and propagation of upstream foreshock transients and their substantial impacts on the magnetosphere-ionosphere coupling system, including magnetospheric compression and currents in the ionosphere.

Field-aligned currents in the Earth's magnetotail are traditionally associated with transient plasma flows and strong plasma pressure gradients in the near-Earth side. In this paper we demonstrate a new field-aligned current system present at the lunar orbit tail. Using magnetotail current sheet observations by two ARTEMIS probes at $\sim60 R_E$, we analyze statistically the current sheet structure and current density distribution closest to the neutral sheet. For about half of our 130 current sheet crossings, the equatorial magnetic field component across-the tail (along the main, cross-tail current) contributes significantly to the vertical pressure balance. This magnetic field component peaks at the equator, near the cross-tail current maximum. For those cases, a significant part of the tail current, having an intensity in the range 1-10nA/m$^2$, flows along the magnetic field lines (it is both field-aligned and cross-tail). We suggest that this current system develops in order to compensate the thermal pressure by particles that on its own is insufficient to fend off the lobe magnetic pressure., Comment: 7 pages, 3 figures, preparing to submit to GRL

Foreshock transients upstream of Earth's bow shock have been recently observed to accelerate electrons to many times their thermal energy. How such acceleration occurs is unknown, however. Using THEMIS case studies, we examine a subset of acceleration events (31 of 247 events) in foreshock transients with cores that exhibit gradual electron energy increases accompanied by low background magnetic field strength and large-amplitude magnetic fluctuations. Using the evolution of electron distributions and the energy increase rates at multiple spacecraft, we suggest that Fermi acceleration between a converging foreshock transient's compressional boundary and the bow shock is responsible for the observed electron acceleration. We then show that a one-dimensional test particle simulation of an ideal Fermi acceleration model in fluctuating fields prescribed by the observations can reproduce the observed evolution of electron distributions, energy increase rate, and pitch-angle isotropy, providing further support for our hypothesis. Thus, Fermi acceleration is likely the principal electron acceleration mechanism in at least this subset of foreshock transient cores., Comment: Submitted to JGR

Several types of foreshock transients upstream of Earth's bow shock possessing a tenuous, hot core have been observed and simulated. Because of the low dynamic pressure in their cores, these phenomena can significantly disturb the bow shock and the magnetosphere-ionosphere system. Recent observations have also demonstrated that foreshock transients can accelerate particles which, when transported earthward, can affect space weather. Understanding the potential of foreshock transients to accelerate particles can help us understand shock acceleration at Earth and at other planetary and astrophysical systems. To further investigate foreshock transients' potential for acceleration we conduct a statistical study of ion and electron energization in the core of foreshock transients. We find that electron energies typically increase there, evidently due to an internal acceleration process, whereas, as expected, ion energies most often decrease to support transient formation and expansion. Nevertheless, ion energy enhancements can be seen in some events suggesting an internal ion acceleration process as well. Formation conditions of foreshock transients are related to weak solar wind magnetic field strength and fast solar wind speed. Ion and electron energization are also positively correlated with solar wind speed., Comment: Submitted to JGR