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Field‐line curvature scattering (FLCS) is believed to be the primary mechanism forming electron isotropy boundaries (IB) and can rapidly scatter relativistic electrons from the outer radiation belt. However, its direct and quantitative impact on controlling outer belt electron lifetimes has never been directly assessed. Using simultaneous observations of IBs from low‐altitude satellites and in situ electron fluxes from equatorial satellites, we report IBs intruding into the outer belt (reaching L ∼ 4.5), closely synchronized with sharp flux radial gradients near IBs, caused by significant electron loss outside IBs during a 4‐day storm recovery period. By combining observations with simulations, we provide the first direct and quantitative evidence that FLCS‐induced electron loss outside the IB dominantly controls the outer belt electron lifetimes. Our findings reveal that this simple yet fundamental physical process, which has been historically neglected in global radiation belt models, can explain the outer electron belt configuration.

We have analyzed the role of auroral processes in the formation of the outer radiation belt, considering that the main part of the auroral oval maps to the outer part of the ring current, instead of the plasma sheet as is commonly postulated. In this approach, the outer ring current is the region where transverse magnetospheric currents close inside the magnetosphere. Specifically, we analyzed the role of magnetospheric substorms in the appearance of relativistic electrons in the outer radiation belt. We present experimental evidence that the presence of substorms during a geomagnetic storm recovery phase is, in fact, very important for the appearance of a new radiation belt during this phase. We discuss the possible role of adiabatic acceleration of relativistic electrons during storm recovery phase and show that this mechanism may accelerate the relativistic electrons by more than one order of magnitude.

The plasma of the solar wind incident upon the Earth’s magnetosphere can produce several types of geoeffective events. Among them, an important phenomenon consists of the interrelation of the magnetospheric–ionospheric current systems and the charged-particle population of the Earth’s Van Allen radiation belts. Ultra-low-frequency (ULF) waves resonantly interacting with such particles have been claimed to play a major role in the energetic particle flux changes, particularly at the outer radiation belt, which is mainly composed of electrons at relativistic energies. In this article, we use global magnetohydrodynamic simulations along with in situ and ground-based observations to evaluate the ability of two different solar wind transient (SWT) events to generate ULF (few to tens of mHz) waves in the equatorial region of the inner magnetosphere. Magnetic field and plasma data from the Advanced Composition Explorer (ACE) satellite were used to characterize these two SWT events as being a sector boundary crossing (SBC) on 24 September 2013, and an interplanetary coronal mass ejection (ICME) in conjunction with a shock on 2 October 2013. Associated with these events, the twin Van Allen Probes measured a depletion of the outer belt relativistic electron flux concurrent with magnetic and electric field power spectra consistent with ULF waves. Two ground-based observatories apart in 90 ∘ longitude also showed evidence of ULF-wave activity for the two SWT events. Magnetohydrodynamic (MHD) simulation results show that the ULF-like oscillations in the modeled electric and magnetic fields observed during both events are a result from the SWT coupling to the magnetosphere. The analysis of the MHD simulation results together with the observations leads to the conclusion that the two SWT structures analyzed in this article can be geoeffective on different levels, with each one leading to distinct ring current intensities, but both SWTs are related to the same disturbance in the outer radiation belt, i.e. a dropout in the relativistic electron fluxes. Therefore, minor disturbances in the solar wind parameters, such as those related to an SBC, may initiate physical processes that are able to be geoeffective for the outer radiation belt.

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. Energetic electron densities in the radiation belt increases during geomagnetic storms. Here, the authors show oblique whistler mode waves enhance electron losses and create strong fluxes of about 100 keV electrons precipitating into the atmosphere, that should be considered in radiation belt models.

The flux of energetic electrons in the Earth's outer radiation belt can vary by several orders of magnitude over time scales less than a day, in response to changes in properties of the solar wind instigated by solar activity. Variability in the radiation belts is due to an imbalance between the dominant source and loss processes, caused by a violation of one or more of the adiabatic invariants. For radiation belt electrons, non‐adiabatic behavior is primarily associated with energy and momentum transfer during interactions with various magnetospheric waves. A review is presented here of recent advances in both our understanding and global modeling of such wave‐particle interactions, which have led to a paradigm shift in our understanding of electron acceleration in the radiation belts; internal local acceleration, rather than radial diffusion now appears to be the dominant acceleration process during the recovery phase of magnetic storms.

Low‐altitude measurements of energetic electron fluxes offer insight into the dynamics of the radiation belts and plasma sheet. However, distinguishing between key magnetospheric regions–such as inner belt, slot region, outer belt, and plasma sheet–based on low‐altitude data remains challenging, particularly for missions lacking pitch‐angle resolution. A commonly used boundary indicator, the flux anisotropy (i.e., precipitating‐to‐trapped flux ratio), is unavailable in such cases. In this study, we propose an alternative approach to identify the plasma sheet—radiation belt interface, commonly associated with the isotropy boundary, using high energy‐resolution measurements from the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. We demonstrate that this boundary is marked by a distinct spectral change: a power‐law form in the plasma sheet transitions to an exponential form within the outer radiation belt. These findings are supported by comparisons with Polar‐orbiting Operational Environmental Satellites (POES) observations, and we discuss the potential mechanisms responsible for the spectral transition.

Electron microburst precipitation has been shown to have significant potential for depletion of the outer radiation belt. We present observations from the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) of six (five unique and one dual‐balloon observation) microburst events, each containing minutes to hours of persistent microbursts. We find that each event included a slowly‐varying smooth precipitation component underlying the bursty component. The smooth component has not yet been fully characterized in the literature; we have written a program to identify microburst events and quantify the relative contributions of each component in the BARREL data. In all six events analyzed, the smooth component contributed more to the total X‐ray counts measured, indicating that the smooth component could contribute significantly to radiation belt loss. Plain Language Summary Earth's radiation belts have been of interest since their discovery in 1958. Understanding and predicting the particle dynamics of the radiation belts can help us to better understand the space environment around Earth, improving our ability to protect the satellites on which our technological society depends. The microburst events studied here are known to have a significant impact on the radiation belts, with the potential to completely deplete them in as little as a few hours. We report here previously unclassified precipitation that occurs concurrently with microbursts. Unlike the short microbursts (each lasting less than a second) the additional component is less intense but is sustained for much longer periods (minutes to several hours). The analysis presented indicates that this smooth component comprises a significant fraction of the depletion potential, implying that the smooth component should be included in radiation belt models and loss rate estimates. Key Points Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) microburst events comprise at least two precipitation components: a smooth component (minutes–hours) and a bursty component (∼100 ms) An automated algorithm was created to identify microburst events in BARREL data and separate the smooth and bursty precipitation components For the events studied, the smooth component contributed more to the measured X‐ray count rate than the bursty component

A statistical survey using 3 years of Van Allen Probes data from 2013 to 2015 is conducted to investigate the impact of broadband kinetic Alfvén waves (KAWs) on the pitch angle distributions (PADs) of relativistic electrons. 62 events exhibiting distinct KAW signatures, identified when other wave modes known to generate butterfly distributions were absent, are examined along with the corresponding PADs of electrons. The results reveal a relationship between the spectral energy density of KAWs and PAD of relativistic electrons, with butterfly PAD features becoming more pronounced and showing larger dip‐sizes as the spectral energy density of KAWs increases, particularly for electrons in 0.5–3.4 MeV energy range. At these times the magnetopause sub‐solar stand‐off distance renders magnetopause shadowing an unlikely formation mechanism. This suggests the interaction of relativistic electrons with broadband KAWs could be a significant mechanism, alongside drift‐shell splitting, contributing to the formation of butterfly PADs in the night‐side outer radiation belt of Earth.

Deep penetration of outer radiation belt electrons to low L (<3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lower L. This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lower L to match the observations of storm time values of E‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthal E‐fields contribute preferentially to the deep penetration of lower‐energy electrons. Plain Language Summary Electrons trapped by the Earth's magnetic field gather in two regions known as the Van Allen radiation belts. It is well reported that electrons can be transported radially inward from the outer radiation belt during geomagnetically active times. More specifically, low energy (100 s of keV) electrons can be moved radially deeper than higher energy (∼1 MeV) electrons. Previous studies suggested that enhanced convection electric fields could contribute to the earthward transport of low energy (<200 keV) electrons. However, the mechanism which leads to different efficiencies of electron transport at different energies has not been quantified. This study expands the traditional radial diffusion model with an empirically determined convection term and shows that the net convection velocity increases for lower energy electrons. For the first time, we quantitatively modeled the energy‐dependent penetration of radiation belt electrons in a wide energy range (10 s of keV to 2 MeV) in the presence of enhanced large‐scale electric fields, during two geomagnetic storm events observed by the Van Allen Probes mission. Key Points Convective radial transport of storm‐time enhanced large‐scale E‐fields is an efficient inward transport mechanism of 10–100 s keV electrons The energy‐dependent electron penetration can be explained by the relation between the timescales of electron drift and large‐scale E‐fields A radial diffusion‐convection model is developed to reproduce the storm‐time penetration of lower energy electrons to lower L

Electromagnetic ion cyclotron waves in the Earth's outer radiation belt drive rapid electron losses through wave‐particle interactions. The precipitating electron flux can be high in the hundreds of keV energy range, well below the typical minimum resonance energy. One of the proposed explanations relies on nonresonant scattering, which causes pitch‐angle diffusion away from the fundamental cyclotron resonance. Here we propose the fractional sub‐cyclotron resonance, a second‐order nonlinear effect that scatters particles at resonance order n = 1/2, as an alternate explanation. Using test‐particle simulations, we evaluate the precipitation ratios of sub‐MeV electrons for wave packets with various shapes, amplitudes, and wave normal angles. We show that the nonlinear sub‐cyclotron scattering produces larger ratios than the nonresonant scattering when the wave amplitude reaches sufficiently large values. The ELFIN CubeSats detected several events with precipitation ratio patterns matching our simulation, demonstrating the importance of sub‐cyclotron resonances during intense precipitation events. Plain Language Summary High‐energy electrons in the Earth's radiation belt are constantly being scattered by the ubiquitous electromagnetic plasma waves. A portion of these scattered electrons is lost to the atmosphere, where the particles deposit their energy and cause a chain of chemical reactions, possibly contributing to ozone destruction. The energy and flux of the precipitating electrons depend on the nature of the wave‐particle interactions in the radiation belt. The electromagnetic ion cyclotron wave (EMIC), known to be responsible for scattering relativistic electrons, has been observed to cause precipitation at energies much lower than expected by the standard theory. We numerically investigate two types of interactions, the nonresonant scattering and the nonlinear sub‐cyclotron scattering, and show how both influence the relative precipitating fluxes. We demonstrate that sub‐cyclotron interactions driven by intense EMIC waves can cause stronger precipitation than nonresonant scattering at sub‐MeV energies. The dual ELFIN CubeSats detected precipitation profiles that match our numerical results, confirming the importance of nonlinear sub‐cyclotron scattering in the analysis of intense precipitation events. Key Points Electrons resonate with intense quasiparallel electromagnetic ion cyclotron wave waves at fractions of the minimum resonance energy Fractional resonant scattering causes significant precipitation when the wave amplitude reaches above 1% of the ambient field Precipitating electron flux spectrum observed by the ELFIN CubeSats supports the estimated influence of fractional resonances