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Various studies have been dedicated to quantifying the atmospheric chemical effects of energetic electron precipitation (EEP), but the contribution from relativistic electron precipitation (REP) was largely overlooked. Based on the precipitating fluxes estimated from Polar‐orbiting Observational Environmental Satellites, we quantify the REP‐induced atmospheric chemical effects using the Whole Atmosphere Community Climate Model. Present results show that direct stratospheric ionization caused by REP can enhance the NOx concentration by a factor of ∼2.58 at ∼37 km altitude, and the HOx concentration by a factor of ∼6.41 at ∼44 km altitude. As for the annual variation, REP causes an additional ozone loss of ∼16.2%–17.1% at ∼30–35 km altitude during winter. Moreover, REP's impact is not confined to winter since the resultant NOx and HOx production occurs in situ. Therefore, neglecting REP would significantly underestimate EEP's total effects on the stratospheric ozone. Plain Language Summary Electrons in the Earth's radiation belts can precipitate into the atmosphere and induce significant chemical changes, especially the ozone concentration in the stratosphere. In this study, we have utilized numerical models to simulate these effects, with particular attention to relativistic precipitating electrons, which are well known to be capable of penetrating into the stratosphere, but their effects were largely overlooked. The fluxes of precipitating electrons were taken from the measurement by Polar‐orbiting Observational Environmental Satellites. We use a climate‐chemistry model, the so‐called Whole Atmosphere Community Climate Model, to resimulate how the atmospheric species change under the influence of these precipitating electrons. Present results show that relativistic precipitating electrons can directly cause ozone depletion in the stratosphere, and previous studies have significantly underestimated the chemical effects of precipitating electrons. Key Points A scheme to simulate the chemical effects of relativistic precipitating electrons has been implemented in the Whole Atmosphere Community Climate Model Relativistic precipitating electrons can reduce the ozone by ∼16.2%–17.1% at ∼30–35 km altitude in addition to medium‐energy electrons Relativistic electron precipitation's impact on the concentration of stratospheric ozone is not confined to winter since the resultant NOx and HOx production occurs in situ

The precipitation of energetic electrons from the outer radiation belt plays a crucial role in regulating their flux dynamics and influencing ionospheric properties. While theoretical and numerical studies, often based on near‐equatorial spacecraft measurements, are essential for assessing the precipitation efficiency, low‐altitude observations offer a direct and effective method for monitoring these losses. This study analyzes a new data set of energetic electron precipitation captured by the CALorimetric Electron Telescope (CALET) and the Monitor of All‐sky X‐ray Image (MAXI) aboard the International Space Station. By comparing CALET and MAXI measurements with low‐altitude observations from ELFIN, we identified three distinct precipitation patterns: those driven by electron scattering from whistler‐mode waves, electromagnetic ion cyclotron waves, and field‐line curvature scattering (FLCS). Each pattern exhibits unique characteristics, allowing us to construct a statistical picture of precipitation based on CALET and MAXI data. A key finding from this analysis is the substantial contribution of FLCS‐driven precipitation within the outer radiation belt. These results highlight the necessity of incorporating FLCS as a significant loss mechanism in radiation belt models. Plain Language Summary Energetic electrons populate the donut‐shaped region around Earth known as the outer radiation belt. When these electrons precipitate into Earth's atmosphere, they can significantly alter atmosphere's chemical composition. The primary driver of this precipitation is the resonant interaction between electrons and electromagnetic waves within the outer radiation belt. However, the natural curvature of Earth's magnetic field lines can also scatter electrons and contribute to their loss. In this study, we used measurements of precipitating electrons from the International Space Station and the low‐altitude ELFIN CubeSat mission to assess the relative importance of this curvature‐driven scattering. Our findings showcase that curvature‐driven scattering is highly effective in causing energetic and relativistic electron precipitation from the outer radiation belt. Key Points Three types of electron precipitation are investigated: those driven by EMIC waves, whistler‐mode waves, and field‐line curvature scattering We use measurements from CALET&MAXI onboard the ISS to statistically compare the occurrence rates of these precipitation types We use ELFIN observations to develop an algorithm for identifying different precipitation types in CALET&MAXI/ISS data

Electromagnetic ion cyclotron (EMIC) waves are recognized as one of the primary drivers of energetic electron precipitation (EEP) into the Earth's atmosphere. A problematic discrepancy has remained between the occurrence frequency of waves in magnetic local time (MLT) and the resultant precipitation observed by satellites. This study attempts to characterize the ionization profiles induced by EMIC‐wave‐driven EEP from ground‐based measurements. Our combined observational data sets, comprising imaging riometer, atmospheric radar, and magnetometer data obtained at Syowa Station from 2016 to 2019, enable us to find more than 850 events of EMIC‐driven EEP identified through concurrent ionization and EMIC wave activity. The MLT distribution of events peaks in the afternoon sector (13:00–14:00 MLT), consistent with the MLT distributions of waves previously reported. More than 60% of the events exhibit ionization above 60 km altitude caused by sub‐MeV EEP, which is attributed to non‐resonant scattering by EMIC waves. Plain Language Summary Electromagnetic ion cyclotron (EMIC) waves are a type of plasma waves emitted from the Earth's magnetosphere which drive precipitation of radiation belt electrons through interactions between the electrons and the waves. It is now known that such energetic electron precipitation (EEP) into the Earth's atmosphere from space also affects the variability of atmospheric composition, such as ozone depletion, which is responsible for changes in the Earth's climate and weather. To properly understand this effects on the atmosphere, it is necessary to build a basic knowledge of when, where, and how often EEPs occur. This study aims to contribute to these understanding by statistical analysis of atmospheric ionization caused by EMIC waves observed from the ground. In this study, we find that most of the events occurred in the 12–18 hr in local time, and that EMIC‐driven EEP of several hundred keV expected from previous theoretical and observational studies are common since ionization are detected above 60 km in more than 60% of the events. Key Points EMIC‐wave‐driven EEP events are statistically investigated from ground‐based observations of the atmospheric ionization They were preferentially observed in the afternoon sector at 12–18 MLT, consistent with the known MLT distribution of the waves More than 60% of the events are with ionization above 60 km altitude, supporting sub‐MeV EEP caused by non‐resonant scattering

We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ∼17 s, or Δ L ∼ 0.56 ) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ∼50 events of strong EMIC wave-driven precipitation. Most reside at L ∼ 5 − 7 at dusk, while a smaller subset exists at L ∼ 8 − 12 at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an L -shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio’s spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of ∼ 1.45 MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. It should be noted though that this diffusive treatment likely includes effects from nonlinear resonant interactions (especially at high energies) and nonresonant effects from sharp wave packet edges (at low energies). Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven >1 MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to ∼ 200-300 keV by much less intense higher frequency EMIC waves at dusk (where such waves are most frequent). At ∼100 keV, whistler-mode chorus may be implicated in concurrent precipitation. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation.

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. Plain Language Summary During magnetospheric substorms, energetic electrons in the Earth's plasma sheet (PS), the night‐side magnetosphere region filled by hot plasma, precipitate to the ionosphere. Energetic electron precipitation (EEP) affects the density, temperature, and composition of the ionosphere. However, the exact process that causes such precipitation is not well understood due to observational constraints. The challenge lies in simultaneously measuring the EEP properties at the ionosphere and the plasma and wave properties in the PS. We analyze a fortuitous satellite conjunction during a substorm, during which EEP was simultaneously captured by ELFIN, Swarm, and POES at low altitudes, and THEMIS in the equatorial PS. EEP was observed to extend across a broad equatorial domain, projecting into a wide ionospheric region and encompassing the PS region and a significant portion of the inner magnetosphere. High‐energy‐resolution measurements from ELFIN reveal that the main driver of precipitation is the scattering of energetic electrons by strongly curved magnetic field lines in the PS, as opposed to the more commonly suggested scattering mechanisms associated with wave‐particle interactions. We also show that the EEP drastically altered the ionization profile of the ionosphere. Key Points We investigate the radial location of energetic (50–1,000 keV) electron precipitation (EEP) during a substorm We compare the impact of plasma sheet electron precipitation (<30 keV) and EEP (50–1,000 keV) on the altitudinal profile of ionization Our results underscore the key role of curvature scattering in energetic electron precipitation during substorms

This chapter reviews fundamental properties and recent advances of diffuse and pulsating aurora. Diffuse and pulsating aurora often occurs on closed field lines and involves energetic electron precipitation by wave-particle interaction. After summarizing the definition, large-scale morphology, types of pulsation, and driving processes, we review observation techniques, occurrence, duration, altitude, evolution, small-scale structures, fast modulation, relation to high-energy precipitation, the role of ECH waves, reflected and secondary electrons, ionosphere dynamics, and simulation of wave-particle interaction. Finally we discuss open questions of diffuse and pulsating aurora.

We report a rare quiet‐time observation by NOAA‐18 spacecraft flying nearly along the electron isotropy boundary (IBe). Complemented by data from four nearby meridional IBe crossings of other POES spacecraft, this case study provides a novel quantitative information concerning the regular energy dependence of the loss‐cone filling ratio (Jprec/Jtrap) variations near the IBe foot. Specific strong energy‐dependent difference of loss‐cone filling rates is manifested at different spacecraft and in various events in the same way, which is consistent with the pitch‐angle scattering in the magnetotail field reversal region as confirmed by the numerical simulation. Therefore it provides a landmark to distinguish the field line curvature scattering (FLCS) mechanism from wave‐induced precipitation. In FLCS case, a strongly fluctuating loss cone filling ratio implies that equatorial magnetic field is inherently structured in the magnetotail in the IBe foot region.

We report a statistical result of electrons inside the loss cone with energies of 67 eV–88 keV using electron measurements obtained in situ by the Arase satellite in the inner magnetosphere around the magnetic equator for 60 months. Loss cone electrons are found with a high occurrence probability from the nightside to the dawnside at approximately L = 6. For 641 eV–88 keV electrons, the high‐occurrence region shifts toward later magnetic local times (MLTs) with increasing loss cone electron energy. The spatial distribution of the occurrence probability around MLT = 22–3 at L = 5–6 is consistent with the calculated average resonance energy distribution of whistler mode chorus waves near the magnetic equator. These results suggest that pitch angle scattering driven by chorus waves plays the main role in electron precipitation in this region. Plain Language Summary Energetic electrons originating from the magnetosphere precipitate and transport energy into the upper atmosphere, modifying ionospheric conditions and affecting the magnetosphere‐ionosphere coupling system. Pitch angle scattering caused by wave‐particle interactions in the magnetosphere plays a significant role in generating electrons inside the loss cone, namely, precipitating electrons. However, owing to the few direct observations of loss cone electrons near the magnetic equator, the contribution of wave‐particle interactions to electron precipitation remains an unsolved problem. Recently, energetic electron analyzers with high angular resolutions onboard the Arase satellite have enabled in situ observations of loss cone electron fluxes in the inner magnetosphere. In this study, we conducted a statistical survey of loss cone electrons using these in situ electron measurements in the inner magnetosphere. We discovered that the spatial distribution of precipitating electrons with a high occurrence frequency overlaps with the region where whistler mode chorus waves are detected in the night and early morning sectors. The distribution of the average resonance energies of chorus waves can explain the peak distribution of the precipitating electron energy. These results suggest that chorus waves strongly contribute to electron precipitation with energies of hundreds of eV to tens of keV in this region. Key Points The low‐ and medium‐energy electrons inside the loss cone observed in situ in the inner magnetosphere were statistically investigated The distribution of loss cone electrons coincides with those where chorus waves are observed in the night and early morning sectors The local time dependence of the peak energy of loss cone electrons can be explained by the resonance energy of chorus waves

Electromagnetic ion cyclotron (EMIC) waves are commonly observed electromagnetic emissions in Earth's magnetosphere and are widely considered to efficiently scatter relativistic electrons into bounce loss cones. However, their precise scattering effects remain highly debated due to limited energy coverage and coarse resolution of previous measurements. Here, we present high‐energy‐resolution measurements of EMIC‐induced relativistic electron precipitation from the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. A long duration >1 MeV electron precipitation event was measured by CIRBE/REPTile‐2 in both the northern and southern hemispheres on 25 April 2023. The energy versus L dispersions of these >1 MeV precipitating electrons show good agreement with minimum resonance energies of electrons interacting with He+ band EMIC waves at specific frequencies. These novel observations unveil the detailed scattering effect of EMIC waves and provide important clues regarding wave‐particle interaction processes near the equator.

Relativistic electron precipitation (REP) refers to the release of high‐energy electrons initially trapped in the outer radiation belt, which then precipitate into Earth's upper atmosphere, contributing significantly to the rapid depletion of radiation belt electron flux. This study presents a statistical analysis of REP observations collected by the Calorimetric Electron Telescope (CALET) experiment aboard the International Space Station from 2015 to the present day. Specifically, the analysis utilizes count rates acquired from the two top scintillators constituting the top charge detector, each sensitive to electrons with energies above 1.5 and 3.4 MeV, respectively. Analysis of CALET data reveals a previously unreported semi‐annual variation in the occurrence of REP events. REP periodicities resemble those observed for trapped electron fluxes in the outer belt. Furthermore, their amplitude follows the overall trend of solar wind high‐speed streams and the solar activity.