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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 above Eminresonate 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. Energetic electron precipitation into the upper atmosphere is an important loss process of outer radiation belt fluxes. Whistler‐mode and electromagnetic ion cyclotron (EMIC) waves are two of the most important wave modes responsible for energetic electron scattering to the Earth's ionosphere through wave‐particle interaction. These wave modes typically drive losses of electrons in different energy ranges (above 1 MeV for EMIC waves and tens to hundreds of keV for whistler‐mode waves), occurring in different spatial regions. We report the first observations of energetic electron scattering driven by the combined effect of whistler‐mode and EMIC waves. Our results from equatorial and low‐altitude observations, and a data‐driven test particle simulation explain the wide energy range of electron precipitation from tens of keVs to a few MeVs due to the combined whistler‐mode and EMIC waves effect and explain the unusually high intensity of relativistic electron precipitation at such times. We report observations of energetic electron precipitation likely driven by concurrent whistle‐mode and electromagnetic ion cyclotron (EMIC) wavesThe combined scattering of whistler‐mode and EMIC waves leads to electron precipitation over a wide energy range of 50 keVs to a few MeVsThis study highlights the potential nonlinear effects for explaining the observed energetic electron fluxes in the inner magnetosphere We report observations of energetic electron precipitation likely driven by concurrent whistle‐mode and electromagnetic ion cyclotron (EMIC) waves The combined scattering of whistler‐mode and EMIC waves leads to electron precipitation over a wide energy range of 50 keVs to a few MeVs This study highlights the potential nonlinear effects for explaining the observed energetic electron fluxes in the inner magnetosphere

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. Low‐Earth Orbit (LEO) observations of magnetospheric particles can sometimes provide critical information on complex high‐altitude processes that are challenging to access. Here we introduce a novel type of LEO particle measurement: energetic electron flux dropouts in the low‐altitude projection of the plasma sheet. These particle observations may prove valuable for remotely probing magnetospheric configurations during geomagnetically disturbed times. Utilizing 3.5 years of data from the ELFIN CubeSats, we conduct the first statistical analysis of these flux dropout events. Our findings reveal that they predominantly occur in the dusk and premidnight magnetic local time sectors during substorms, and exhibit a well‐defined relationship with the large‐scale field‐aligned current region. We also examine three representative dropout events to gain a deeper understanding of the magnetospheric drivers and magnetic field conditions that lead to such dropouts at low altitudes. These plasma sheet flux dropouts at ELFIN provide a valuable tool for refining magnetospheric models, consequently enhancing the accuracy of field‐line mapping during disturbed times. We report statistical observations of energetic electron flux dropouts at the low‐altitude extent of the plasma sheet by ELFINFlux dropouts occur preferentially at dusk and premidnight and are collocated with R1 FAC poleward of the electron isotropy boundaryTwo types of magnetospheric drivers of ELFIN flux dropouts are discussed by analyzing three conjunction events with GOES and THEMIS We report statistical observations of energetic electron flux dropouts at the low‐altitude extent of the plasma sheet by ELFIN Flux dropouts occur preferentially at dusk and premidnight and are collocated with R1 FAC poleward of the electron isotropy boundary Two types of magnetospheric drivers of ELFIN flux dropouts are discussed by analyzing three conjunction events with GOES and THEMIS

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 coefficients Dα0α0${D}_{{\alpha }_{0}{\alpha }_{0}}$near the edge of the loss cone are ∼10−6–10−5s−1for these energetic electrons. The Dα0α0${D}_{{\alpha }_{0}{\alpha }_{0}}$due 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. One of the key elements of magnetosphere‐ionosphere coupling is energetic electron precipitation from the magnetosphere to the ionosphere. This precipitation becomes particularly significant and geoeffective during substorms, when it can cause radio transmission interference by increasing ionospheric radio absorption. Understanding the mechanisms of such precipitation is thus critical to space weather nowcasting and forecasting. In this paper, using magnetically conjugate observations between the THEMIS, ELFIN, and DMSP satellites in both the Earth's magnetosphere and ionosphere, and aided with numerical simulations and theoretical estimates, we examine the relative importance of different plasma waves in the scattering and precipitation of energetic electrons from the plasma sheet during a substorm. By comparing the electron scattering efficiencies due to the observed kinetic Alfvén waves (KAWs) and whistler‐mode waves, we conclude that KAWs dominate over whistler‐mode waves, at least in this case, in driving the observed energetic (50–300 keV) electron precipitation by ELFIN during the substorm expansion. Thus, the KAW may be an important, previously overlooked contributor to energetic electron precipitation from the plasma sheet during substorms. Conjugate spacecraft observations allow evaluating the relative importance of kinetic Alfvén waves (KAWs) and whistlers in driving energetic electron precipitationKAW‐driven electron bounce‐averaged diffusion rates above 50 keV are one order of magnitude larger than those due to whistler‐mode wavesKAWs can contribute to and may dominate the scattering and precipitation of energetic electrons from the substorm plasma sheet Conjugate spacecraft observations allow evaluating the relative importance of kinetic Alfvén waves (KAWs) and whistlers in driving energetic electron precipitation KAW‐driven electron bounce‐averaged diffusion rates above 50 keV are one order of magnitude larger than those due to whistler‐mode waves KAWs can contribute to and may dominate the scattering and precipitation of energetic electrons from the substorm plasma sheet

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 same L‐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 precipitation L‐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 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 ≈[103, 104] 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. Thus, such mesoscale variations should be included in simulations of the radiation belt dynamics. Temporal scales of electron precipitating flux variations are studied with Electron Losses and Fields Investigation CubeSatsMain patterns include microbursts of ∼1 s, modulated precipitation of ∼1 min, and equatorial parameter variations of ∼10 minPossible explanations of such time scales are discussed Temporal scales of electron precipitating flux variations are studied with Electron Losses and Fields Investigation CubeSats Main patterns include microbursts of ∼1 s, modulated precipitation of ∼1 min, and equatorial parameter variations of ∼10 min Possible explanations of such time scales are discussed

Energetic electron precipitation to the Earth's atmosphere is a key process controlling radiation belt dynamics and magnetosphere‐ionosphere coupling. One of the main drivers of precipitation is electron resonant scattering by whistler‐mode waves. Low‐altitude observations of such precipitation often reveal quasi‐periodicity in the ultra‐low‐frequency (ULF) range associated with whistler‐mode waves, causally linked to ULF‐modulated equatorial electron flux and its anisotropy. Conjunctions between ground‐based instruments and equatorial spacecraft show that low‐altitude precipitation concurrent with equatorial whistler‐mode waves also exhibits a spatial periodicity as a function of latitude over a large spatial region. Whether this spatial periodicity might also be due to magnetospheric ULF waves spatially modulating electron fluxes and whistler‐mode chorus has not been previously addressed due to a lack of conjunctions between equatorial spacecraft, low earth orbit spacecraft, and ground‐based instruments. To examine this question, we combine ground‐based and equatorial observations magnetically conjugate to observations of precipitation at the low‐altitude, polar‐orbiting CubeSats Electron Losses and Fields Investigation‐A and ‐B. As they sequentially cross the outer radiation belt with a temporal separation of minutes to tens of minutes, they can easily reveal the spatial quasi‐periodicity of electron precipitation. Our combined data sets confirm that ULF waves may modulate whistler‐mode wave generation within a large magnetic local time and L‐shell domain in the equatorial magnetosphere, and thus lead to significant aggregate energetic electron precipitation exhibiting both temporal and spatial periodicity. Our results suggest that the coupling between ULF and whistler‐mode waves is important for outer radiation belt dynamics. We report quasi‐periodic energetic electron precipitation observed by low‐altitude Electron Losses and Fields Investigation CubeSatsQuasi‐periodicity of the precipitation is due to periodicity of whistler‐mode wave generation at the equatorWhistler‐mode wave generation is modulated by ultra‐low‐frequency waves seen within a large magnetic local time, L‐shell domain We report quasi‐periodic energetic electron precipitation observed by low‐altitude Electron Losses and Fields Investigation CubeSats Quasi‐periodicity of the precipitation is due to periodicity of whistler‐mode wave generation at the equator Whistler‐mode wave generation is modulated by ultra‐low‐frequency waves seen within a large magnetic local time, L‐shell domain

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) at L< ∼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 10−2s−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. Man‐made ground‐based very low frequency (VLF) transmitter waves can scatter and precipitate energetic radiation belt electrons, which pose a threat to telecommunication and human assets in space. Therefore, understanding the controlling parameters and the efficiency of energetic electron scattering by VLF transmitter waves are of both theoretical and practical importance. Our paper reports statistical analyses of inner belt wisp precipitation events due to scattering by VLF transmitter waves as observed by the Electron Loss and Fields Investigation CubeSats. Occasional strong local wisp precipitation events have been found to occur at preferential longitudes east of the South Atlantic Anomaly in the Northern Hemisphere and were primarily observed in the morning magnetic local time. These local wisp precipitation events have been used to infer energetic electron diffusion rates and VLF transmitter wave amplitudes near the equator based on quasilinear diffusion theory. 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. We report statistical observations of inner belt wisps with full pitch angle distributions measured by Electron Loss and Fields InvestigationLocal bounce‐loss‐cone wisp precipitation events allow inference of quasilinear diffusion rates and transmitter wave amplitudesInferred transmitter wave amplitudes exceed nonlinear interaction threshold amplitudes near the equator We report statistical observations of inner belt wisps with full pitch angle distributions measured by Electron Loss and Fields Investigation Local bounce‐loss‐cone wisp precipitation events allow inference of quasilinear diffusion rates and transmitter wave amplitudes Inferred transmitter wave amplitudes exceed nonlinear interaction threshold amplitudes near the equator

Electron injections are critical processes associated with magnetospheric substorms, which deposit significant electron energy into the ionosphere. Although wave scattering of <10 keV electrons during injections has been well studied, the link between magnetotail electron injections and energetic (≥100 keV) electron precipitation remains elusive. Using conjugate observations between the Electron Loss and Fields Investigation (ELFIN) and Magnetospheric Multiscale (MMS) missions, we present evidence of tens to hundreds of keV electron precipitation to the ionosphere potentially driven by kinetic Alfvén waves (KAWs) associated with magnetotail electron injections and magnetic field gradients. Test particle simulations adapted to observations show that dipolarization‐front magnetic field gradients and associated ∇Bdrifts allow Doppler‐shifted Landau resonances between the injected electrons and KAWs, producing electron spatial scattering across the front which results in pitch‐angle decreases and subsequent precipitation. Test particle results show that such KAW‐driven precipitation can account for ELFIN observations below ∼300 keV. Energetic electron precipitation from magnetospheric injections has a major impact on magnetosphere‐ionosphere coupling. This energy deposition is largely in the form of electron precipitation driven by wave‐particle interactions in the magnetotail. Although wave‐driven precipitation with energies less than approximately ∼10 keV has been studied extensively, the link between energetic electron precipitation (≥∼100 keV) and electron injections remains elusive. Combining observations and simulations, this paper provides evidence of such precipitation driven by kinetic Alfvén waves, which have been previously observed to be ubiquitously associated with magnetospheric electron injections but have not been considered as an important driver for precipitation of such electrons. Conjugate Electron Loss and Fields Investigationand Magnetospheric Multiscale observations reveal evidence of tens to hundreds of keV electron precipitation associated with kinetic Alfvén wavesDipolarized magnetic field gradients and perpendicular magnetic drifts allow Landau resonance between waves and injected electronsAdiabatic transport and spatially varying E× Band grad‐Bdrifts induce perpendicular momentum scattering and electron losses Conjugate Electron Loss and Fields Investigationand Magnetospheric Multiscale observations reveal evidence of tens to hundreds of keV electron precipitation associated with kinetic Alfvén waves Dipolarized magnetic field gradients and perpendicular magnetic drifts allow Landau resonance between waves and injected electrons Adiabatic transport and spatially varying E× Band grad‐Bdrifts induce perpendicular momentum scattering and electron losses

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. Electron microbursts are extremely intense and transient bursts of precipitation from the Earth's radiation belts into the atmosphere, with typical sizes of 20–30 km at low Earth orbit and 100–200 km near the equator. The integral contribution of such precipitation bursts can be substantial for radiation belt dynamics. Their bursty nature, however, questions the applicability of standard electron diffusive models. Different spacecraft have detected microbursts in very different energy ranges, from tens of keV to MeVs, and most of these observations suggest that microbursts are generated by electron resonant scattering by very intense packets of whistler‐mode chorus waves. In this study, we provide the first statistical model of microburst energy spectra from 50 keV to 2 MeV. We have also shown that the wave amplitude required to produce the observed microbursts is so large that classical diffusive models of electron scattering by waves would not work. This implies that microbursts are likely produced by electron nonlinear resonant interactions with intense chorus waves that probably propagate ducted inside field‐aligned density enhancements up to the latitude of resonance with electrons. Statistical study of microburst energy spectrum from 50 keV to 2 MeV shows that the mean energy increases with substorm activityRelatively flat pitch‐angle distributions inside the loss cone suggest nonlinear electron precipitation by intense chorus wave packetsHigh precipitating‐to‐trapped electron flux ratios above 500 keV provide evidence of ducted chorus wave propagation to middle latitudes Statistical study of microburst energy spectrum from 50 keV to 2 MeV shows that the mean energy increases with substorm activity Relatively flat pitch‐angle distributions inside the loss cone suggest nonlinear electron precipitation by intense chorus wave packets High precipitating‐to‐trapped electron flux ratios above 500 keV provide evidence of ducted chorus wave propagation to middle latitudes