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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 precipitation from Earth’s outer radiation belt heats the upper atmosphere and alters its chemical properties. The precipitating flux intensity, typically modelled using inputs from high-altitude, equatorial spacecraft, dictates the radiation belt’s energy contribution to the atmosphere and the strength of space-atmosphere coupling. The classical quasi-linear theory of electron precipitation through moderately fast diffusive interactions with plasma waves predicts that precipitating electron fluxes cannot exceed fluxes of electrons trapped in the radiation belt, setting an apparent upper limit for electron precipitation. Here we show from low-altitude satellite observations, that ~100 keV electron precipitation rates often exceed this apparent upper limit. We demonstrate that such superfast precipitation is caused by nonlinear electron interactions with intense plasma waves, which have not been previously incorporated in radiation belt models. The high occurrence rate of superfast precipitation suggests that it is important for modelling both radiation belt fluxes and space-atmosphere coupling.

     
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  3. Abstract

    Low‐altitude observations of magnetospheric particles provide a unique opportunity for remote probing of the magnetospheric and plasma states during active times. We present the first statistical analysis of a specific pattern in such observations, energetic electron flux dropouts in the low‐altitude projection of the plasma sheet. Using 3.5 years of data from the ELFIN CubeSats we report the occurrence distribution of 145 energetic electron flux dropout events and identify characteristics, including their prevalence in the dusk and premidnight sectors, their association with substorms and enhanced auroral activities, and their correlation with the region‐1 (R1) field‐aligned current region. We also investigate three representative dropout events which benefit from satellite conjunctions between ELFIN, GOES, and THEMIS, to better understand the magnetospheric drivers and magnetic field conditions that lead to such dropouts as viewed by ELFIN. One class of dropouts may be associated with magnetic field mapping distortions due to local enhancements and thinning of cross‐tail current sheets and amplification of R1 field‐aligned currents. The other class may be associated with the increase in perpendicular anisotropy of magnetospheric electrons due to magnetic field dipolarizations near premidnight. These plasma sheet flux dropouts at ELFIN provide a valuable tool for refining magnetospheric models, thereby improving the accuracy of field‐line mapping during substorms.

     
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  4. Abstract

    Resonant interactions with whistler‐mode waves are one of the most important drivers for rapid energetic electron precipitation. In this letter, we study a conjunction event, where bursts of energetic electron precipitation (50–800 keV) with timescales of several seconds are observed by the twin ELFIN Cubesats at low Earth orbit, while very‐oblique intense whistler‐mode waves are observed by the Time History of Events and Macroscale Interactions during Substorms E satellite at the conjugate magnetic equator. Our observation‐constrained test‐particle simulations reveal that the electron precipitation, particularly above 100 keV, results from high‐order resonant scattering by the very‐oblique whistler‐mode waves. Our study provides the first direct evidence for high‐order resonance driven precipitation, explaining a bursty precipitation event. The results demonstrate that high‐order resonant scattering could be important, not only in long‐term diffusion models, but also in models of short timescale events.

     
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  5. Abstract

    Electron resonant scattering by whistler‐mode waves is one of the most important mechanisms responsible for electron precipitation to the Earth's atmosphere. The temporal and spatial scales of such precipitation are dictated by properties of their wave source and background plasma characteristics, which control the efficiency of electron resonant scattering. We investigate these scales with measurements from the two low‐altitude Electron Losses and Fields Investigation (ELFIN) CubeSats that move practically along the same orbit, with along‐track separations ranging from seconds to tens of minutes. Conjunctions with the equatorial THEMIS mission are also used to aid our interpretation. We compare the variations in energetic electron precipitation at the sameL‐shells but on successive data collection orbit tracks by the two ELFIN satellites. Variations seen at the smallest inter‐satellite separations, those of less than a few seconds, 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 precipitationL‐shell profiles at intermediate inter‐satellite separations, a few seconds to about 1 min, are likely associated with whistler‐mode wave power modulations by ultra‐low frequency waves, that is, with the wave source region (from a few to tens of seconds to a few minutes in time and ∼1,000 km in space at the equator). During these two types of variations, consecutive crossings are associated with precipitationL‐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 ≈[103, 104] km spatial scales). The latter type of variations results in appreciable changes in the precipitationL‐shell profiles and can significantly modify the net electron losses during successive tracks. Thus, such mesoscale variations should be included in simulations of the radiation belt dynamics.

     
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  6. Abstract

    Energetic (≳50 keV) electron precipitation from the magnetosphere to the ionosphere during substorms can be important for magnetosphere‐ionosphere coupling. Using conjugate observations between the THEMIS, ELFIN, and DMSP spacecraft during a substorm, we have analyzed the energetic electron precipitation, the magnetospheric injection, and the associated plasma waves to examine the role of waves in pitch‐angle scattering plasma sheet electrons into the loss cone. During the substorm expansion phase, ELFIN‐A observed 50–300 keV electron precipitation from the plasma sheet that was likely driven by wave‐particle interactions. The identification of the low‐altitude extent of the plasma sheet from ELFIN is aided by DMSP global auroral images. Combining quasi‐linear theory, numerical test particle simulations, and equatorial THEMIS measurements of particles and fields, we have evaluated the relative importance of kinetic Alfvén waves (KAWs) and whistler‐mode waves in driving the observed precipitation. We find that the KAW‐driven bounce‐averaged pitch‐angle diffusion coefficientsnear the edge of the loss cone are ∼10−6–10−5s−1for these energetic electrons. Thedue to parallel whistler‐mode waves, observed at THEMIS ∼10‐min after the ELFIN observations, are ∼10−8–10−6s−1. Thus, at least in this case, the observed KAWs dominate over the observed whistler‐mode waves in the scattering and precipitation of energetic plasma sheet electrons during the substorm injection.

     
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  7. Abstract

    Relativistic electron losses in Earth's radiation belts are usually attributed to electron resonant scattering by electromagnetic waves. One of the most important wave modes 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 theforce bunchingeffect. In this study we investigate if this transport can block electron precipitation. We combine test particle simulations, low‐altitude observations of EMIC‐driven electron precipitation by the Electron Losses and Fields Investigations mission, and ground‐based EMIC observations. Comparing simulations and observations, we show that, despite 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.

     
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  8. Abstract

    We present statistical characteristics of electron microburst precipitation using high time‐resolution measurements from the low‐altitude Electron Losses and Fields InvestigatioN (ELFIN) CubeSats. The radial distribution of the equatorial projection of microbursts as a function of geomagnetic activity suggests that they are produced by resonant interaction with quasi‐parallel lower‐band chorus waves. ELFIN electron flux measurements provide the first statistical models of microburst energy spectra from 50 keV to 2 MeV. Microbursts with energies up to 150 keV have a relatively flat pitch‐angle spectrum. Estimates of scattering rates required to produce the observed flat spectra suggest that such precipitation signatures are due to near‐equatorial electron scattering by chorus wave packets with peak amplitudes of 0.4–0.9 nT, well above the threshold for nonlinear resonant interaction. More rare microbursts, exceeding 500 keV, are observed preferentially near dawn during disturbed periods. We interpret them as evidence of scattering by intense ducted chorus waves propagating from the equator up to middle latitudes with little attenuation.

     
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  9. Abstract

    Man‐made very low frequency (VLF) transmitter waves play a critical role in energetic electron scattering and precipitation from the inner radiation belt, a type of which is called wisp precipitation. Wisps exhibit dispersive energy‐versus‐Lspectra due to the evolution of electron cyclotron resonance conditions with near‐monochromatic VLF transmitter waves. Here, we report on such observations of inner belt wisp precipitation events with full pitch angle resolution in the energy range of 50 to ∼500 keV as measured by Electron Loss and Fields Investigation (ELFIN) atL < ∼2 between March 2021 and April 2022. Statistical observations (82 events) reveal occasional (18 events) wisp precipitation events with local bounce‐loss‐cone electron flux enhancements, which provide new information compared with flux enhancements measured in previous studies only in the drift loss cone. Based on magnetic field and plasmaspheric density models, quasilinear theory, and detailed pitch angle distributions of wisps from ELFIN, we have estimated the wisp electron bounce‐averaged pitch angle diffusion coefficients to be of the order of 10−4to 10−2 s−1. These are several orders of magnitude larger than the diffusion rates calculated from models using global statistical averages of VLF transmitter wave power. When using our estimated diffusion coefficients to deduce the associated local transmitter wave amplitudes near the equator, based on quasilinear calculations from a transmitter‐induced electron diffusion model, we find these wave amplitudes to be >1 mV/m. Although probable overestimates, such inferred wave amplitudes exceed the theoretical threshold amplitude for nonlinear interactions, strongly suggesting that it is necessary to include nonlinear effects for an accurate evaluation of energetic electron scattering by transmitter waves.

     
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