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  1. Abstract

    The two most important wave modes responsible for energetic electron scattering to the Earth's ionosphere are electromagnetic ion cyclotron (EMIC) waves and whistler‐mode waves. These wave modes operate in different energy ranges: whistler‐mode waves are mostly effective in scattering sub‐relativistic electrons, whereas EMIC waves predominately scatter relativistic electrons. In this study, we report the direct observations of energetic electron (from 50 keV to 2.5 MeV) scattering driven by the combined effect of whistler‐mode and EMIC waves using ELFIN measurements. We analyze five events showing EMIC‐driven relativistic electron precipitation accompanied by bursts of whistler‐driven precipitation over a wide energy range. These events reveal an enhancement of relativistic electron precipitation by EMIC waves during intervals of whistler‐mode precipitation compared to intervals of EMIC‐only precipitation. We discuss a possible mechanism responsible for such precipitation. We suggest that below the minimum resonance energy (Emin) of EMIC waves, the whistler‐mode wave may both scatter electrons into the loss‐cone and accelerate them to higher energy (1–3 MeV). Electrons accelerated aboveEminresonate with EMIC waves that, in turn, quickly scatter those electrons into the loss‐cone. This enhances relativistic electron precipitation beyond what EMIC waves alone could achieve. We present theoretical support for this mechanism, along with observational evidence from the ELFIN mission. We discuss methodologies for further observational investigations of this combined whistler‐mode and EMIC precipitation.

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    Free, publicly-accessible full text available May 1, 2025
  2. Abstract

    Energetic electron losses by pitch‐angle scattering and precipitation to the atmosphere from the radiation belts are controlled, to a great extent, by resonant wave particle interactions with whistler‐mode waves. The efficacy of such precipitation is primarily modulated by wave intensity, although its relative importance, compared to other wave and plasma parameters, remains unclear. Precipitation spectra from the low‐altitude, polar‐orbiting ELFIN mission have previously been demonstrated to be consistent with energetic precipitation modeling derived from empirical models of field‐aligned wave power across a wide swath of local‐time sectors. However, such modeling could not explain the intense, relativistic electron precipitation observed on the nightside. Therefore, this study aims to additionally consider the contributions of three modifications—wave obliquity, frequency spectrum, and local plasma density—to explain this discrepancy on the nightside. By incorporating these effects into both test particle simulations and quasi‐linear diffusion modeling, we find that realistic implementations of each individual modification result in only slight changes to the electron precipitation spectrum. However, these modifications, when combined, enable more accurate modeling of ELFIN‐observed spectra. In particular, a significant reduction in plasma density enables lower frequency waves, oblique, or even quasi field‐aligned waves to resonate with near ∼1 MeV electrons closer to the equator. We demonstrate that the levels of modification required to accurately reproduce the nightside spectra of whistler‐mode wave‐driven relativistic electron precipitation match empirical expectations and should therefore be included in future radiation belt modeling.

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    Free, publicly-accessible full text available March 1, 2025
  3. In this study, we present simultaneous multi-point observations of magnetospheric oscillations on a time scale of tens of minutes (forced-breathing mode) and modulated whistler-mode chorus waves, associated with concurrent energetic electron precipitation observed through enhanced BARREL X-rays. Similar fluctuations are observed in X-ray signatures and the compressional component of magnetic oscillations, spanning from ∼9 to 12 h in MLT and 5 to 11 inLshell. Such magnetospheric oscillations covering an extensive region in the pre-noon sector have been suggested to play a potential role in precipitating energetic electrons by either wave scattering or loss cone modulation, showing a high correlation with the enhancement in X-rays. In this event, the correlation coefficients between chorus waves (smoothed over 8 min), ambient magnetic field oscillations and X-rays are high. We perform an in-depth quasi-linear modeling analysis to evaluate the role of magnetic field oscillations in modulating energetic electron precipitation in the Earth’s magnetosphere through modulating whistler-mode chorus wave amplitude, resonance condition between chorus waves and electrons, as well as loss cone size. Model results further show that the modulation of chorus wave amplitude plays a dominant role in modulating the electron precipitation. However, the effect of the modulation in the resonant energy between chorus waves and energetic electrons due to the background magnetic field oscillations cannot be neglected. The bounce loss cone modulation, affected by the magnetic oscillations, has little influence on the electron precipitation modulation. Our results show that the low frequency magnetospheric oscillations could play a significant role in modulating the electron precipitation through modulating chorus wave intensity and the resonant energy between chorus waves and electron.

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    Free, publicly-accessible full text available February 22, 2025
  4. Abstract

    The ion foreshock, filled with backstreaming foreshock ions, is very dynamic with many transient structures that disturb the bow shock and the magnetosphere‐ionosphere system. It has been shown that foreshock ions can be generated through either solar wind reflection at the bow shock or leakage from the magnetosheath. While solar wind reflection is widely believed to be the dominant generation process, our investigation using Time History of Events and Macroscale Interactions during Substorms mission observations reveals that the relative importance of magnetosheath leakage has been underestimated. We show from case studies that when the magnetosheath ions exhibit field‐aligned anisotropy, a large fraction of them attains sufficient field‐aligned speed to escape upstream, resulting in very high foreshock ion density. The observed foreshock ion density, velocity, phase space density, and distribution function shape are consistent with such an escape or leakage process. Our results suggest that magnetosheath leakage could be a significant contributor to the formation of the ion foreshock. Further characterization of the magnetosheath leakage process is a critical step toward building predictive models of the ion foreshock, a necessary step to better forecast foreshock‐driven space weather effects.

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    Free, publicly-accessible full text available February 1, 2025
  5. Abstract

    Shock waves are sites of intense plasma heating and charged particle acceleration. In collisionless solar wind plasmas, such acceleration is attributed to shock drift or Fermi acceleration but also to wave–particle resonant interactions. We examine the latter for the case of electrons interacting with one of the most commonly observed wave modes in shock environments, the whistler mode. Such waves are particularly intense in dynamic, localized regions upstream of shocks, arising from the kinetic interaction of the shock with solar wind discontinuities. These regions, known as foreshock transients, are also sites of significant electron acceleration by mechanisms not fully understood. Using in situ observations of such transients in the Earth’s foreshock, we demonstrate that intense whistler-mode waves can resonate nonlinearly with >25 eV solar wind electrons and accelerate them to ∼100–500 eV. This acceleration is mostly effective for the 50–250 eV energy range, where the accelerated electron population exhibits a characteristic butterfly pitch-angle distribution consistent with theoretical predictions. Such nonlinear resonant acceleration is very fast, implying that this mechanism may be important for injecting suprathermal electrons of solar wind origin into the shock region, where they can undergo further, efficient shock-drift acceleration to even higher energies.

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  6. Abstract

    Electromagnetic ion cyclotron (EMIC) waves can very rapidly and effectively scatter relativistic electrons into the atmosphere. EMIC‐driven precipitation bursts can be detected by low‐altitude spacecraft, and analysis of the fine structure of such bursts may reveal unique information about the near‐equatorial EMIC source region. In this study, we report, for the first time, observations of EMIC‐driven electron precipitation exhibiting energy,E, dispersion as a function of latitude (and henceL‐shell): two predominant categories exhibitdE/dL > 0 anddE/dL < 0. We interpret precipitation withdE/dL < 0 as due to the typical inward radial gradient of cold plasma density and equatorial magnetic field (∼65% of the statistics). Precipitation withdE/dL > 0 is interpreted as due to an outward radial gradient of the equatorial magnetic field, likely produced by energetic ions freshly injected into the ring current (∼35% of the statistics). The observed energy dispersion of EMIC‐driven electron precipitation was reproduced in simulations.

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  7. This paper reviews key properties and major unsolved problems about Strong Thermal Emission Velocity Enhancement (STEVE) and the picket fence. We first introduce the basic characteristics of STEVE and historical observations of STEVE-like emissions, particularly the case on 11 September 1891. Then, we discuss major open questions about STEVE: 1) Why does STEVE preferentially occur in equinoxes? 2) How do the solar wind and storm/substorm conditions control STEVE? 3) Why is STEVE rare, despite that STEVE does not seem to require extreme driving conditions? 4) What are the multi-scale structures of STEVE? 5) What mechanisms determine the properties of the picket fence? 6) What are the chemistry and emission mechanisms of STEVE? 7) What are the impacts of STEVE on the ionosphere−thermosphere system? Also, 8) what is the relation between STEVE, stable auroral red (SAR) arcs, and the subauroral proton aurora? These issues largely concern how STEVE is created as a unique mode of response of the subauroral magnetosphere−ionosphere−thermosphere coupling system. STEVE, SAR arcs, and proton auroras, the three major types of subauroral emissions, require energetic particle injections to the pre-midnight inner magnetosphere and interaction with cold plasma. However, it is not understood why they occur at different times and why they can co-exist and transition from one to another. Strong electron injections into the pre-midnight sector are suggested to be important for driving intense subauroral ion drifts (SAID). A system-level understanding of how the magnetosphere creates distinct injection features, drives subauroral flows, and disturbs the thermosphere to create optical emissions is required to address the key questions about STEVE. The ionosphere−thermosphere modeling that considers the extreme velocity and heating should be conducted to answer what chemical and dynamical processes occur and how much the STEVE luminosity can be explained. Citizen scientist photographs and scientific instruments reveal the evolution of fine-scale structures of STEVE and their connection to the picket fence. Photographs also show the undulation of STEVE and the localized picket fence. High-resolution observations are required to resolve fine-scale structures of STEVE and the picket fence, and such observations are important to understand underlying processes in the ionosphere and thermosphere. 
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  8. Abstract

    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) on board spacecraft at different points in the heliosphere on 2022 October 9. 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’s magnetosphere and solar wind missions THEMIS and Wind; meteorological satellites POES15, POES19, and MetOp3; and MAVEN—a NASA mission orbiting Mars. GRB221009A had a structure of four bursts: the less intense Pulse 1—the triggering impulse—was detected by gamma-ray observatories atT0= 13:16:59 UT (near the Earth); the most intense Pulses 2 and 3 were detected on board all the spacecraft from the list; and Pulse 4 was detected in more than 500 s after Pulse 1. Due to their different scientific objectives, the spacecraft, whose data were used in this study, were separated by more than 1 au (Solar Orbiter and MAVEN). This enabled the tracking of GRB221009A as it was propagating across the heliosphere. STEREO-A was the first to register Pulse 2 and 3 of the GRB, almost 100 s 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 s after their detection near Earth. By processing the observed time delays, we show that the source location of the GRB221009A was at R.A. 288.°5, decl. 18.°5 ± 2° (J2000).

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  9. Accepted, not yet published 
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