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The reprocessing of radiation belt electron flux measurements into phase space density (PSD) as a function of the adiabatic invariants is a widely-used method to address major questions regarding electron energization and loss in the outer radiation belt. In this reprocessing, flux measurements j(a,E) at local pitch angles α, energies E, and optionally magnetometer measurements B, are combined with a global magnetic field model to express the phase space density f(C) in terms of the third invariant Φ α: 1/L* at fixed first and second invariants M and K. While the general framework of the calculation is agreed upon, implementation details vary amongst the literature, and the issue of magnetic field model dependence is rarely addressed. This work reviews the steps of the calculation with lists of commonly used implementation options. For the first time, analysis is presented to display the effect of doing the calculation with different implementation options and with different backing models (including both empirical and MHDdriven models). The results are summarized to inform evaluation of existing results and future efforts calculating and analyzing radiation belt electron phase space density. Three events are analyzed, and while differences are found, the primary structural interpretations of the phase space density analysis exhibit model independence. [ABSTRACT FROM AUTHOR]

The editorial discusses the loss and acceleration mechanisms of energetic electrons in the Earth's outer radiation belt during geomagnetic storms. Various mechanisms, such as wave-particle interactions with different types of waves, contribute to the loss and acceleration of electrons in the outer radiation belt. The research topic aims to advance understanding and modeling capabilities related to the evolution of electrons in the outer radiation belt, with contributions from multiple studies exploring different aspects of this topic. The authors acknowledge the contributions of all researchers involved in the Frontiers Research Topic. [Extracted from the article]

Earth's radiation belt and ring current are donut-shaped regions of energetic and relativistic particles, trapped by the geomagnetic field. The strengthened solar wind dynamic pressure (Pdyn) can alter the structure of the geomagnetic field, which can bring about the dynamic variation of radiation belt and ring current. In the study, we firstly utilize group test particle simulations to investigate the phase space density (PSD) under the varying geomagnetic field modeled by the International Geomagnetic Reference Field (IGRF) and T96 magnetic field models from 19 December 2015 to 20 December 2015. Combining the observation of the Van Allen Probe, we find that the PSD of outer radiation belt electrons evolves towards different states under different levels of Pdyn. In the first stage, the Pdyn (~7.94 nPa) results in the obvious rise of electron anisotropy. In the second stage, there is a significant reduction in PSD for energetic electrons at all energy levels and pitch angles under the action of intense Pdyn (~22 nPa), which suggests that the magnetopause shadowing and outward radial diffusion play important roles in the second process. The result of the study can help us further understand the dynamic evolution of the radiation belt and ring current during a period of geomagnetic disturbance. [ABSTRACT FROM AUTHOR]

Turbulent and compressed sheath regions preceding interplanetary coronal mass ejections strongly impact electron dynamics in the outer radiation belt. Changes in electron flux can occur on timescales of tens of minutes, which are unlikely to be captured by a two‐satellite mission. The recently released Global Positioning System (GPS) data set generally has shorter revisit times (at L ∼ 4–8) owing to the large number of satellites in the constellation equipped with energetic particle detectors. Investigating electron fluxes at energies from 140 keV to 4 MeV and sheaths observed in 2012–2018, we show that the flux response to sheaths on a timescale of 6 hr, previously reported from Van Allen Probes (RBSP) data, is reproduced by GPS measurements. Furthermore, GPS data enables derivation of the response on a timescale of 30 min, which further confirms that the energy and L‐shell dependent changes in electron flux are associated with the impact of the sheath. Sheath‐driven loss is underestimated over longer timescales as the electrons recover during the ejecta. We additionally show the response of electron phase space density (PSD), which is a key quantity in identifying non‐adiabatic loss from the system and electron energization through wave‐particle interactions. The PSD response is calculated from both RBSP and GPS data for the 6 hr timescale, as well as from GPS data for the 30 min timescale. The response is divided based on the geoeffectiveness of the sheaths revealing that electrons are effectively accelerated only during geoeffective sheaths, while loss commonly occurs during all sheaths. Key Points: Global Positioning System measurements confirm 6 hr RBSP outer belt electron flux response to interplanetary coronal mass ejection‐driven sheaths at 6 hr and 30 min timescalesPhase space density (PSD) response shows that electron energization is associated only with geoeffective sheaths but loss occurs in response to all sheathsImpacts in electron flux and PSD presented here are related to sheaths, and the lost electrons are replenished during the early ejecta [ABSTRACT FROM AUTHOR]

We present joint observations of a substorm injection event on 5 February 2016. The enhancements of energetic electron (∼30 to ∼300 keV) fluxes, the drift echoes of the injected electrons, and the quasi‐monochromatic ultralow frequency oscillations embedded in the flux variations were simultaneously observed by the BeiDa Imaging Electron Spectrometer and the Van Allen Probe B. These vivid features of substorm injection were concretely captured by multiple spacecraft within the geosynchronous distance for the first time. The radial separation of the spacecraft (∼0.3 RE) allowed for a quantitative comparison of the electron flux and phase space density (PSD) across different drift shells. It is found that a more intense increase of equatorial PSD occurred at the lower L‐shell (L* ∼5.5), which indicated a non‐adiabatic acceleration of the injected electrons in the inner magnetosphere. Plain Language Summary: Energetic charged particles can be transported from the magnetotail into the inner magnetosphere via a process named substorm injection. The injection process is usually considered adiabatic, as the magnetic moment of the particles remained conserved. In this study, however, we present observational evidence of non‐adiabatic acceleration of substorm injected electrons. Specifically, we derive the radial gradient of electron phase space density using the dual‐spacecraft data obtained by BeiDa Imaging Electron Spectrometer and Van Allen Probe B. The phase space density gradient turned into negative by the arrival of the injected electrons, which is indicative of a non‐adiabatic acceleration. Key Points: Substorm injection of energetic electrons was simultaneously observed by radially separated spacecraft within the geosynchronous distanceThe radial phase space density gradient turned into negative by the arrival of the injected electronsThe negative phase space density gradient indicated a non‐adiabatic acceleration of the injected electrons [ABSTRACT FROM AUTHOR]

Cross‐satellite calibration of energetic particle fluxes is essential to understanding Earth's radiation belt dynamics and modeling the space radiation environment. Using the method of comparing phase space density (PSD) conjunctions in the same set of phase space coordinates, we perform a cross‐satellite calibration of energetic electron fluxes measured by the Low Earth Orbit Chinese FengYun‐3B satellite (FY‐3B) and the near‐equatorial Highly Eccentric Orbit Van Allen Probe‐A (VAP‐A), respectively. VAP‐A provides pitch angle resolved electron (directional) differential fluxes while FY‐3B provides omnidirectional electron differential fluxes. Before calculating the PSDs from FY‐3B high energy electron detector (HEED), a method is introduced to estimate the local pitch angle of FY‐3B based on the installation direction information of HEED and the T89c magnetic field model. We calculate the calibration factors by using the PSD conjunctions inferred from the two satellites for three fixed sets of (μ, K, L*) during the geomagnetic storm times. The calibration factors range from 0.76 to 0.86 varying with fixed sets of (μ, K, L*), which indicates that the electron differential fluxes from FY‐3B are higher than that from VAP‐A. During geomagnetic quiet times, we perform calibration by using independent Gaussian fitting analysis of the number of points with the unaveraged PSDs of FY‐3B and VAP‐A for three fixed sets of (μ, K, L*), with the corresponding calibration factors of 0.78, 0.72, and 0.46. The calibrated electron differential fluxes of FY‐3B measured at the high magnetic latitudes, can be used in the future modeling of the radiation belt electron dynamics. Key Points: Cross‐satellite calibrations of electron fluxes measured by FengYun‐3B satellite and Van Allen Probe‐A are conducted using electron phase space densities (PSDs)We compare PSD conjunctions and perform independent Gaussian fitting analysis in the fixed sets of (μ, K, L*)The cross‐satellite calibration factors are 0.76–0.86 and 0.46–0.78 during geomagnetic storm times and quiet times, respectively [ABSTRACT FROM AUTHOR]

Jupiter is known to have a complex magnetosphere containing energetic (above 10s of keV) electrons, protons, and heavy ions. However, a global distribution of these energetic particles is not fully understood before the era of the polar‐orbiting Juno mission. In this study, we focus on the energetic proton distribution at M < 50 by taking advantage of Juno's measurement covering various magnetic latitudes and M‐shells, and find that energetic proton fluxes are higher off‐equator than that near the equator at M > ∼20, and become comparable or lower at high latitudes than those near the equator at low M‐shells. Pitch angle distributions of energetic protons are field‐aligned, isotropic, and weak pancake‐like from high to low M‐shells. Proton phase space density shows a weak M‐shell dependence at M > 30 and a large positive slope at M < 30, suggesting their potential source and loss processes. Plain Language Summary: Jupiter has a strongly magnetized magnetosphere, which traps electrons and ions with a broad energy range. Before the arrival of Juno at Jupiter, there were limited observations of particles at high latitudes. By taking advantage of high‐quality energetic proton observations from Juno, we evaluate the global distribution of energetic protons at Jupiter. The statistical results suggest that energetic proton flux is mostly comparable or even higher at high latitudes compared to that near the equator, which is consistent with their equatorial pitch angle distributions showing mostly field‐aligned and isotropic, except for a weak pancake feature at low M‐shells. Furthermore, the radial proton phase space density profile shows a weak M‐shell dependence at M > 30 and a large positive slope at M < 30. The underlying mechanisms related to the observed statistical pattern of proton distributions are discussed. Key Points: A statistical survey of energetic proton flux at various M‐shells (M < 50) and magnetic latitudes is performed using Juno observationsPitch angle distribution of energetic protons is mostly field aligned at M > 25 and tends to become pancake‐shaped at smaller M‐shellsPhase space density of equatorial protons increases with increasing M‐shell at M ∼7–30 and keeps almost the same beyond M ∼ 30 [ABSTRACT FROM AUTHOR]

An empirical model of radiation belt relativistic electrons (μ = 560–875 MeV G−1 and I = 0.088–0.14 RE G0.5) with average energy ∼1.3 MeV is developed. The model inputs solar wind parameters (velocity, density, interplanetary magnetic field (IMF) |B|, Bz, and By), magnetospheric state parameters (SYM‐H and AL), and L*. The model outputs the radiation belt electron phase space density (PSD). The model is operational from L* = 3 to 6.5. The model is constructed with neural networks assisted by information theory. Information theory is used to select the most effective and relevant solar wind and magnetospheric input parameters plus their lag times based on their information transfer to the PSD. Based on the test set, the model prediction efficiency (PE) increases with increasing L*, ranging from −0.043 at L* = 3 to 0.76 at L* = 6.5. The model PE is near 0 at L* = 3–4 because at this L* range, the solar wind and magnetospheric parameters transfer little information to the PSD. Using solar wind observations at L1 and magnetospheric index (AL and SYM‐H) models solely driven by solar wind, the radiation belt model can be used to forecast PSD 30–60 min ahead. This baseline model can potentially complement a class of empirical models that input data from low earth orbit (LEO). Plain Language Summary: An empirical model of radiation belt relativistic electrons with an energy of 1–2 MeV is developed. The model inputs solar wind parameters, magnetospheric state parameters, and L*. L* gives a measure of radial distance from the center of the Earth with a unit of RE (radius of the Earth = 6,378 km). The model outputs the radiation belt electron phase space density (PSD). The model is operational from L* = 3 to L* 6.5. The model is constructed with an information theory informed neural networks. Information theory is used to select the relevant solar wind and magnetospheric parameters and their lag times based on the amount of information they provide to the radiation belt electrons. The model performance increases with increasing radial distance (L*) because at distances close to Earth (L* = 3–4), the solar wind and magnetospheric parameters provide little information about the radiation belt electron PSD. The model can be used to forecast radiation belt PSD 30–60 min ahead. Key Points: An empirical model to predict state of radiation belt relativistic electrons is developedThe model prediction efficiency increases with increasing L* with a PE > 0.6 at L* > 5The model can potentially complement a class of empirical models that input observations from low earth orbit [ABSTRACT FROM AUTHOR]

During July to October of 2019, a sequence of isolated Corotating Interaction Regions (CIRs) impacted the magnetosphere, for four consecutive solar rotations, without any interposed Interplanetary Coronal Mass Ejections. Even though the series of CIRs resulted in relatively weak geomagnetic storms, the net effect of the outer radiation belt during each disturbance was different, depending on the electron energy. During the August-September CIR group, significant multi-MeV electron enhancements occurred, up to ultra-relativistic energies of 9.9 MeV in the heart of the outer Van Allen radiation belt. These characteristics deemed this time period a fine case for studying the different electron acceleration mechanisms. In order to do this, we exploited coordinated data from the Van Allen Probes, the Time History of Events and Macroscale Interactions during Substorms Mission (THEMIS), Arase and Galileo satellites, covering seed, relativistic and ultra-relativistic electron populations, investigating their Phase Space Density (PSD) profile dependence on the values of the second adiabatic invariant K, ranging from near-equatorial to off equatorial mirroring populations. Our results indicate that different acceleration mechanisms took place for different electron energies. The PSD profiles were dependent not only on the μ value, but also on the K value, with higher K values corresponding to more pronounced local acceleration by chorus waves. The 9.9 MeV electrons were enhanced prior to the 7.7 MeV, indicating that different mechanisms took effect on different populations. Finally, all ultra-relativistic enhancements took place below geosynchronous orbit, emphasizing the need for more Medium Earth Orbit (MEO) missions.

Relativistic electrons in the Earth's radiation belts are highly dynamic on a variety of timescales during the geomagnetic storm. Using Van Allen Probe spacecraft data, we investigate rapid enhancements of relativistic electrons in the outer radiation belt during a corotating interaction region (CIR) driven storm. Successive dipolarizations associated with 100keV‐MeV electron injections are identified. The evolution of energetic electrons is analyzed in the space of adiabatic invariants (μ, K and L*). Within less than a few hours, the phase space density (PSD) of the relativistic electrons promptly increases corresponding to injections of MeV electrons. The PSD of MeV electrons cumulatively increases by a factor of 4–10 at L* = 4.5–5.8 which is likely due to successive groups of dipolarizations and injections. Both near‐equatorial (small K) and off‐equatorial (large K) energetic electrons increase significantly. The increases in the near‐equatorial electrons are still dominant, suggesting the operation of betatron acceleration. The event study shows that successive dipolarizations associated with the CIR‐driven storm may rapidly affect relativistic electrons of the outer radiation belt over a wide range in the phase space. Key Points: Relativistic electrons increase significantly through a cumulative effect of consecutive dipolarizations associated with a corotating interaction region‐driven stormThe phase space density of MeV electrons increases rapidly over a wide range of adiabatic invariantsThe increase of off‐equatorial electrons is nearly half of that of the near‐equatorial electrons [ABSTRACT FROM AUTHOR]

Coronal mass ejection driven sheath regions are one of the key drivers of drastic outer radiation belt responses. The response can however be significantly different based on the sheath properties and the associated inner magnetospheric wave activity. We performed two case studies on the effects of sheaths on outer belt electrons of various energies using data from the Van Allen Probes. One sheath caused a major geomagnetic disturbance and the other had only a minor impact. We especially investigated the phase space density (PSD) of seed, core, and ultrarelativistic electrons to determine the dominant energization and loss processes taking place during the events. Both sheaths produced substantial variation in the electron fluxes from tens of kiloelectronvolts up to ultrarelativistic energies. The responses were however the opposite: the geoeffective sheath mainly led to enhancement, while the nongeoeffective one caused a depletion throughout most of the outer belt. The case studies highlight that both inward and outward radial transport driven by ultra‐low frequency waves played an important role in both electron energization and loss. Additionally, PSD radial profiles revealed a local peak that indicated significant acceleration to core energies by chorus waves during the geoeffective event. The distinct responses and different mechanisms in action during these events were related to the timing of the peaked solar wind dynamic pressure causing magnetopause compression, and the differing levels of substorm activity. The most remarkable changes in the radiation belt system occurred in key sheath sub‐regions near the shock and the ejecta leading edge. Key Points: Opposite outer belt response caused by two sheaths: mainly enhancement by geoeffective sheath and depletion by nongeoeffective sheathPhase space density analyses of seed, core, and ultrarelativistic electrons reveal importance of ultra‐low frequency‐driven diffusion and chorus accelerationMajor variations in wave activity and electron fluxes occur during key sub‐regions near the start and end of a sheath [ABSTRACT FROM AUTHOR]

Earth’s radiation belt and ring current are donut-shaped regions of energetic and relativistic particles, trapped by the geomagnetic field. The strengthened solar wind dynamic pressure (Pdyn) can alter the structure of the geomagnetic field, which can bring about the dynamic variation of radiation belt and ring current. In the study, we firstly utilize group test particle simulations to investigate the phase space density (PSD) under the varying geomagnetic field modeled by the International Geomagnetic Reference Field (IGRF) and T96 magnetic field models from 19 December 2015 to 20 December 2015. Combining the observation of the Van Allen Probe, we find that the PSD of outer radiation belt electrons evolves towards different states under different levels of Pdyn. In the first stage, the Pdyn (~7.94 nPa) results in the obvious rise of electron anisotropy. In the second stage, there is a significant reduction in PSD for energetic electrons at all energy levels and pitch angles under the action of intense Pdyn (~22 nPa), which suggests that the magnetopause shadowing and outward radial diffusion play important roles in the second process. The result of the study can help us further understand the dynamic evolution of the radiation belt and ring current during a period of geomagnetic disturbance.

Models of monochromatic Pc5 (2–7 mHz) ultralow frequency (ULF) wave interactions with high energy (greater than ∼1 MeV) electrons predict drift resonant interactions that can cause rapid radial transport and acceleration. There are few reports of electron drift resonance at energies greater than ∼1 MeV, in contrast to lower energies; moreover, all previous reports occur in the aftermath of interplanetary shocks. These two facts are difficult to reconcile with theory and numerical simulations predicting that greater than ∼1 MeV drift resonances should occur more often and in a wider variety of driving conditions. In this study, we show that a combination of observational sampling biases and nominal radial phase space density gradients is one explanation for this discrepancy between theory and observations. In particular, we examine electron dynamics in two case studies with very similar satellite coverage, solar wind conditions, and Pc5 wave properties, yet with different radial phase space density profiles. Using global wave and particle observations, we show that the events have vastly different particle responses despite having similar wave properties. Placing these results in context with past studies, we further show that nominal radial PSD gradients near geostationary orbit can mask the expected drift resonance particle response and explain (1) the small number of past greater than ∼1 MeV drift resonance reports and (2) the restriction of these reports to interplanetary shock events. We argue that future observational studies characterizing radial transport via drift resonance should examine global particle dynamics, including observations of the radial phase space density profile. Plain Language Summary: The Earth's radiation belts are dynamic, with variations in radiation intensity depending on many factors. During certain conditions radiation in this region can damage satellite electronics. A variety of plasma waves can interact with this radiation and affect its overall intensity; thus, a characterization of these waves and their related interactions is needed to predict radiation belt dynamics. In this study, we examine a resonant interaction between large‐scale plasma waves—wavelengths of 10,000 km or more—and high‐energy electrons near geostationary orbit (GEO). We find that one of the predicted observational signatures for the resonant interaction can be masked by background radiation conditions if observations are only collected near GEO. We also find that past reports of these resonances favor a particular set of solar wind driving conditions because of this masking effect and that these resonances may in fact occur in a broader range of conditions and spatial regions. We propose that future studies seeking to characterize how these resonances affect radiation belt dynamics should use observations from a broader spatial region whenever possible and/or incorporate background radiation conditions into their analysis. Key Points: Pc5 wave events with similar properties/satellite locations yet different radial PSD gradients yield vastly different particle responsesRadial PSD gradients mask expected drift resonance particle response near ∼1 MeV, reducing likelihood of detection near geostationary orbitA small number of ∼1 MeV electron drift resonance reports and restriction to shock events can be explained by nominal radial PSD gradients [ABSTRACT FROM AUTHOR]

Electrons of ∼102 keV in the inner magnetosphere are suggested potentially to be the "seeds" of outer radiation belt electrons. Investigations into them could shed light on the origination of the outer radiation belt. Here we conduct statistics on seed electron phase space density (PSD) and its radial gradient, using 2‐year data obtained by Fly's Eye Energetic Particle Spectrometer (FEEPS) onboard Magnetospheric Multiscale (MMS). The radial distributions of the PSD are μ and K dependent, and azimuthally asymmetric. In the nightside, as L* increases, PSD at lower μ always increases, while PSD at higher μ first increases and then decreases. In contrast, in the noonside, PSD first increases and then decreases at all covered μ. Crossover μ, defined as μ at which PSD radial gradient is equal to 0, is identified from the statistics. It relatively stays a constant of ∼300 MeV/G inside L*∼6, but decreases as L* increases outside L*∼6. Crossover μ is larger in the nightside than in the noonside and is larger at smaller K. The radial structures of seed electron PSD should be taken into account when considering the dynamics of the outer radiation belt. Key Points: Phase space density (PSD) of 25‐ to 650‐keV electrons in L*∼ 4–9 is statistically derived from 2‐year MMS/FEEPS dataElectron PSD outside the geosynchronous orbit (GSO) is highly azimuthally asymmetricCrossover μ at which PSD radial gradient approaches 0, is relatively constant inside the GSO but decreases as L * increases outside the GSO [ABSTRACT FROM AUTHOR]

Systematic uncertainties in the conversion of measured counts to phase space density by charged particle instrumentation result in errors in reported plasma moments (e.g., density, velocity, and temperature). Unlike previous particle instrumentation that relied on a spacecraft spin to sample all look‐directions, the Fast Plasma Investigation (FPI) suite on NASA's Magnetospheric Multiscale mission nearly simultaneously images the full sky. This configuration results in unprecedented time resolution but also introduces the possibility of spin tones in plasma moments, in particular electron bulk velocity. Here we characterize the effect of systematic linear errors of corrected FPI phase space densities on its reported plasma moments. We find that the flat‐fielding correction factors (i.e., scale factor errors) of FPI are typically accurate to within a few percent but can nonetheless result in significant spin tones in magnetospheric plasmas. Key Points: The flat‐field correction factors for the Fast Plasma Investigation suite on MMS are typically accurate to within a few percentBackground errors can be corrected for at the moment level if appropriate models are availableImperfect flat‐field correction factors result in systematic biases in plasma moments [ABSTRACT FROM AUTHOR]

Using energetic particle and wave measurements from the Van Allen Probes, Polar Orbiting Environmental Satellites (POES), and Geostationary Operational Environmental Satellite (GOES), the acceleration mechanism of ultrarelativistic electrons (>3 MeV) in the center of the outer radiation belt is investigated statistically. A superposed epoch analysis is conducted using 19 storms, which caused flux enhancements of 1.8–7.7 MeV electrons. The evolution of electron phase space density radial profile suggests an energy‐dependent acceleration of ultrarelativistic electrons in the outer belt. Especially, for electrons with very high energies (~7 MeV), prevalent positive phase space density radial gradients support inward radial diffusion being responsible for electron acceleration in the center of the outer belt (L*~3–5) during most enhancement events in the Van Allen Probes era. We propose a two‐step acceleration process to explain the acceleration of ~7 MeV electrons in the outer belt: intense and sustained chorus waves locally energize core electron populations to ultrarelativistic energies at high L region beyond the Van Allen Probes' apogee, followed by inward radial diffusion which further energizes these populations to even higher energies. Statistical results of chorus wave activity inferred from POES precipitating electron measurements as well as core electron populations observed by the Van Allen Probes and GOES support this hypothesis. Key Points: Electron phase space density (PSD) evolution was studied for 19 storms with 1.8–7.7 MeV electron enhancements in the Van Allen Probes eraPrevalent positive PSD radial gradients support inward radial diffusion being responsible for ~7 MeV electron acceleration at L*~3–5Two‐step acceleration mechanism is proposed to explain the acceleration of very high energy electrons in the outer belt [ABSTRACT FROM AUTHOR]

We investigate the response of the outer Van Allen belt electrons to various types of solar wind and magnetospheric disturbances. We use electron phase space density calculations as well as concurrent Pc5 and chorus wave activity observations in the outer belt during the Van Allen Probes era to compare 20 electron enhancement and 8 depletion events. Results indicate that the combined effect of magnetopause shadowing and outward diffusion driven by Pc5 waves is present in both groups of events. Furthermore, in the case of enhancement events, the synergy of enhanced seed population levels and chorus activity—due to the enhanced substorm activity—can effectively replenish the losses of relativistic electrons, while inward diffusion can further accelerate them. Key Points: The number of events with PSD enhancements is L* and μ dependentLoss, as a combined effect of magnetopause shadowing and outward diffusion, is a common feature in both depletion and enhancement eventsThe synergy of enhanced seed population and chorus activity is what distinguishes relativistic electrons enhancements from depletions [ABSTRACT FROM AUTHOR]

We present the temporal evolution of electron Phase Space Density (PSD) in the outer radiation belt during the intense March 2015 geomagnetic storm. Comparing observed PSD profiles as a function of L* at fixed first, M, and second, K, adiabatic invariants with those produced by simulations is critical for determining the physical processes responsible for the outer radiation belt dynamics. Here we show that the bulk of the accelerated and enhanced outer radiation belt population consists of electrons with K < 0.17 G1/2Re. For these electrons, the observed PSD versus L* profiles during the recovery phase of the storm have a positive radial gradient. We compare the observed temporal evolution of the PSD profiles during the recovery phase with those produced by radial diffusion simulations driven by observed Ultralow Frequency wave power as measured on the ground. Our results indicate that the dominant flux enhancement, inside L* < 5, in the heart of the outer radiation belt during the March 2015 geomagnetic storm is consistent with that produced by fast inward radial diffusion of electrons from a dynamic outer boundary driven by enhanced Ultralow Frequency wave power. Key Points: The March 2015 outer radiation belt flux enhancement was dominated by a population of accelerated low‐K electrons with K < 0.17 G1/2ReDuring the flux recovery phase no growing PSD peaks occurred inside L*≲5, suggesting local heating was not the dominant acceleration processThe observed flux enhancement inside L* < 5 is reproduced by our ULF wave radial diffusion simulation without including local acceleration [ABSTRACT FROM AUTHOR]

Abstract: A statistical analysis on the radiation belt dropouts is performed based on 4 years of electron phase space density data from the Van Allen Probes. The μ, K, and L* dependence of dropouts and their driving mechanisms and geomagnetic and solar wind conditions are investigated using electron phase space density data sets for the first time. Our results suggest that electronmagnetic ion cyclotron (EMIC) wave scattering is the dominant dropout mechanism at low L* region, which requires the most active geomagnetic and solar wind conditions. In contrast, dropouts at high L* have a higher occurrence and are due to a combination of EMIC wave scattering and outward radial diffusion associated with magnetopause shadowing. In addition, outward radial diffusion at high L* is found to cause larger dropouts than EMIC wave scattering and is accompanied with active geomagnetic and solar wind drivers. [ABSTRACT FROM AUTHOR]

Abstract: We survey radiation belt enhancement events during the Van Allen Probes era to determine what mechanism is the dominant cause of enhancements and where it is most effective. Two primary mechanisms have been proposed: (1) betatron/Fermi acceleration due to the Earthward radial transport of electrons, which produces monotonic gradients in phase space density (PSD), and (2) “local acceleration” due to gyro/Landau resonant interaction with electromagnetic waves, which produces radially localized growing peaks in PSD. To differentiate between these processes, we examine radial profiles of PSD in adiabatic coordinates using data from the Van Allen Probes and Time History of Events and Macroscale Interactions during Substorms satellites for 80 outer belt enhancement events from October 2012 to April 2017 This study shows that local acceleration is the dominant acceleration mechanism for MeV electrons in the outer belt, with 87% of the enhancement events exhibiting growing peaks. The strong correlation of the location of these with geomagnetic activity further supports this conclusion. [ABSTRACT FROM AUTHOR]