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Abstract 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. 

Abstract Magnetic field‐line curvature scattering (FLCS) of energetic particles in the equatorial magnetotail results in isotropization of pitch‐angle distributions, loss‐cone filling, and precipitation above a minimum energy at a given latitude. At a fixed energy, the lowest latitude of isotropization is the isotropy boundary (IB) for that energy. Nominally, the IB (latitude) exhibits a characteristic energy dependence due to the monotonic variation of the equatorial magnetic field intensity with radial distance. Deviations from this nominal IB dispersion can occur if the radial variation (spatial or temporal) is non‐mononotic and/or if other precipitation mechanisms prevail. With its sensitive and detailed measurements of electron spectra up to relativistic energies, ELFIN's recent observations reveal a variety of electron IBe patterns near magnetic midnight which are repeatable enough to warrant classification. This study aims to categorize the various IBe patterns observed by ELFIN's high‐fidelity but short lived dataset (a few months), compare them with simultaneous nearby POES observations, which are made with a limited energy coverage and resolution but last for decades, and discuss their possible interpretation. The general agreement between ELFIN and POES IB observations indicate a relatively large‐scale nature of IBe patterns. Surprisingly, there exists a large number (up to 2/3 of all events) of non‐monotonic‐or steep/multiple‐IB patterns. This suggest an abundance of non‐trivial tail current sheet structures or a mixed contribution of two mechanisms in the vicinity of IBe in these cases. 

Abstract Electromagnetic ion cyclotron (EMIC) waves effectively scatter relativistic electrons in Earth's radiation belts and energetic ions in the ring current. Empirical models parameterizing the EMIC wave characteristics are important elements of inner magnetosphere simulations. Two main EMIC wave populations included in such simulations are the population generated by plasma sheet injections and another population generated by magnetospheric compression due to the solar wind. In this study, we investigate a third class of EMIC waves, generated by hot plasma sheet ions modulated by compressional ultra‐low frequency (ULF) waves. Such ULF‐modulated EMIC waves are mostly observed on the dayside, between magnetopause and the outer radiation belt edge. We show that ULF‐modulated EMIC waves are weakly oblique (with a wave normal angle ) and narrow‐banded (with a spectral width of of the mean frequency). We construct an empirical model of the EMIC wave characteristics as a function of ‐shell and MLT. The low ratio of electron plasma frequency to electron gyrofrequency around the EMIC wave generation region does not allow these waves to scatter energetic electrons. However, these waves provide very effective (comparable to strong diffusion) quasi‐periodic precipitation of plasma sheet protons. 

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. 

Electromagnetic ion cyclotron (EMIC) waves are a key plasma mode affecting radiation belt dynamics. These waves are important for relativistic electron losses through scattering and precipitation into Earth's ionosphere. Although theoretical models of such resonant scattering predict a low-energy cut-off of ∼1 MeV for precipitating electrons, observations from low-altitude spacecraft often show simultaneous relativistic and sub-relativistic electron precipitation associated with EMIC waves. Recently, nonresonant electron scattering by EMIC waves has been proposed as a possible solution to the above discrepancy. We employ this model and a large database of EMIC waves to develop a universal treatment of electron interactions with EMIC waves, including nonresonant effects. We use the Green's function approach to generalize EMIC diffusion rates foregoing the need to modify existing codes or recompute empirical wave databases. Comparison with observations from the electron losses and fields investigation mission demonstrates the efficacy of the proposed method for explaining sub-relativistic electron losses by EMIC waves. 

Abstract In planetary radiation belts, the Kennel‐Petschek flux limit is expected to set an upper limit on trapped electron fluxes at 80–600 keV in the presence of efficient electron loss through pitch‐angle diffusion by whistler‐mode chorus waves generated around the magnetic equator by the same 80–600 keV electron population. Comparisons with maximum measured fluxes have been relatively successful, but several key assumptions of the Kennel‐Petschek model have not been experimentally tested. The Kennel‐Petschek model notably assumes an exponential growth of chorus waves as the trapped electron flux increases, and a fixed maximum wave power gain of about 3. Here, we describe a method for inferring the near‐equatorial wave power gain using only measurements of trapped, precipitating, and backscattered electron fluxes at low altitude. Next, we make use of Electron Losses and Fields Investigation (ELFIN) CubeSats measurements of such electron fluxes during two moderate geomagnetic storms with sustained electron injections to infer the corresponding chorus wave power gains as a function of time, energy, and equatorial trapped electron flux. We show that wave power increases exponentially with trapped flux, with a wave power gain roughly proportional to the theoretical linear convective gain, and that the maximum inferred gain near the upper flux limit is roughly 10, with a factor of 2 uncertainty. Therefore, two key theoretical underpinnings of the Kennel‐Petschek model are borne out by the present results, although the strong inferred gains should correspond to higher flux limits than in traditional estimates. 

Abstract We present statistical distributions of whistler‐mode chorus and hiss waves at frequencies ranging from the local proton gyrofrequency to the equatorial electron gyrofrequency (f_ce,eq) in Jupiter's magnetosphere based on Juno measurements. The chorus wave power spectral densities usually follow thef_ce,eqvariation with major wave power concentrated in the 0.05f_ce,eq–f_ce,eqfrequency range. The hiss wave frequencies are less dependent onf_ce,eqvariation than chorus with major power concentrated below 0.05f_ce,eq, showing a separation from chorus atM < 10. Our survey indicates that chorus waves are mainly observed at 5.5 < M < 13 from the magnetic equator to 20° latitude, consistent with local wave generation near the equator and damping effects. The hiss wave powers extend to 50° latitude, suggesting longer wave propagation paths without attenuation. Our survey also includes the whistler‐mode waves at high latitudes which may originate from the Io footprint, auroral hiss, or propagating hiss waves reflected to highMshells. 

Abstract Precipitation of relativistic electrons into the Earth's atmosphere regulates the outer radiation belt fluxes and contributes to magnetosphere‐atmosphere coupling. One of the main drivers of such precipitation is electron scattering by whistler‐mode waves. Such waves typically originate at the equator, where they can resonate with and scatter sub‐relativistic (tens to a few hundred keV) electrons. However, they can occasionally propagate far away from the equator along field lines, reaching middle latitudes, where they can resonate with and scatter relativistic (>500 keV) electrons. Such a propagation is typical for the dayside, but statistically has not been found on the nightside where the waves are quickly damped along their propagation due to Landau damping. Here we explore two events of relativistic electron precipitation from low‐altitude observations on the nightside. Combining measurements of whistler‐mode waves from ground observatories, relativistic electron precipitation from low‐altitude satellites, total electron content maps from GPS receivers, and magnetic field and electron flux from equatorial satellites, we show wave ducting by plasma density gradients is the possible channel that allows the waves to reach middle latitudes and scatter relativistic electrons. We suggest that both whistler‐mode wave generation and ducting can be driven by equatorial mesoscale (with spatial scales of about one Earth radius) transient structures during nightside injections. We also compare these nightside events with observations of ducted waves and relativistic electron precipitation at the dayside, where wave generation and ducting are driven by ultra‐low‐frequency waves. This study demonstrates the potential importance of mesoscale transients in relativistic electron precipitation, but does not however unequivocally establish that ducted whistler‐mode waves are the primary cause of the observed electron precipitation. 

Electron fluxes in Earth's radiation belts are significantly affected by their resonant interaction with whistler-mode waves. This wave-particle interaction often occurs via first cyclotron resonance and, when intense and nonlinear, can accelerate subrelativistic electrons to relativistic energies while also scattering them into the atmospheric loss cone. Here, we model Electron Losses and Fields INvestgation’s (ELFIN) low-altitude satellite measurements of precipitating electron spectra with a wave-electron interaction model to infer the profiles of whistler-mode intensity along magnetic latitude assuming realistic waveforms and statistical models of plasma density. We then compare these profiles with a wave power spatial distribution along field lines from an empirical model. We find that this empirical model is consistent with observations of subrelativistic (<200 keV) electron precipitation events, but deviates significantly for relativistic (>200 keV) electron precipitation events at all MLTs, especially on the nightside. This may be due to the sparse coverage of wave measurements at mid-to-high latitudes which causes statistically averaged wave power to be likely underestimated in current empirical wave models. As a result, this discrepancy suggests that intense waves likely do propagate to higher latitudes, although further investigation is required to quantify how well this high-latitude population can account for the observed relativistic electron precipitation. 

Abstract The Poynting vector (Poynting flux) from Earth's magnetosphere downward toward its ionosphere carries the energy that powers the Joule heating in the ionosphere and thermosphere. The Joule heating controls fundamental ionospheric properties affecting the entire magnetosphere‐ionosphere‐thermosphere system, so it is necessary to understand when and where the Poynting flux is significant. Taking advantage of new data sets generated from DMSP (Defense Meteorological Satellite Program) observations, we investigate the Poynting flux distribution within and around the auroral zone, where most magnetosphere‐ionosphere (M‐I) dynamics and thus Joule heating occurs. We find that the Poynting flux, which is generally larger under more active conditions, is concentrated in the sunlit cusp and near the interface between Region 1 and 2 currents. The former concentration suggests voltage generators drive the cusp dynamics. The latter concentration shows asymmetries with respect to the interface between the Region 1 and 2 currents. We show that these reflect the controlling impact of subauroral polarization streams and dawnside auroral polarization streams on the Poynting flux.