Radiation Belt Relativistic Electrons Research Articles

Surfing Acceleration of Radiation Belt Relativistic Electrons Induced by the Propagation of Interplanetary Shock

AbstractInterplanetary shocks (IPS) can initiate prompt acceleration of relativistic electrons in the Earth's radiation belt, which is related to the generation and propagation of impulsive electric field (IEF). We investigate the effect of IEF on accelerating radiation belt electrons in the 6 September 2017 IPS event. A “surfing” effect of electrons with respect to the electric field, referring to electrons that drift together with the tailward‐propagating IEF in the duskside, is investigated in this study. Our results show that the maximum increase of electron differential flux is at 3.4 MeV by a factor of 2.2, corresponding to a drift velocity of 531 km/s, which is more consistent with the IPS propagating speed of 621 km/s rather than the fast‐mode speed of 1,074 km/s. We suggest that the effect of IPS propagation is important for radiation belt dynamics, and we highlight the potential importance of the parameter of IPS propagation speed.

BARREL Observations of Microburst Events With a Slowly‐Varying Component

AbstractElectron 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.

Cross‐Scale Modeling of Storm‐Time Radiation Belt Variability

AbstractDuring geomagnetic storms relativistic outer radiation belt electron flux exhibits large variations on rapid time scales of minutes to days. Many competing acceleration and loss processes contribute to the dynamic variability of the radiation belts; however, distinguishing the relative contribution of each mechanism remains a major challenge as they often occur simultaneously and over a wide range of spatiotemporal scales. In this study, we develop a new comprehensive model for storm‐time radiation belt dynamics by incorporating electron wave‐particle interactions with parallel propagating whistler mode waves into our global test‐particle model of the outer belt. Electron trajectories are evolved through the electromagnetic fields generated from the Multiscale Atmosphere‐Geospace Environment (MAGE) global geospace model. Pitch angle scattering and energization of the test particles are derived from analytical expressions for quasi‐linear diffusion coefficients that depend directly on the magnetic field and density from the magnetosphere simulation. Using a study of the 17 March 2013 geomagnetic storm, we demonstrate that resonance with lower band chorus waves can produce rapid relativistic flux enhancements during the main phase of the storm. While electron loss from the outer radiation belt is dominated by loss through the magnetopause, wave‐particle interactions drive significant atmospheric precipitation. We also show that the storm‐time magnetic field and cold plasma density evolution produces strong, local variations of the magnitude and energy of the wave‐particle interactions and is critical to fully capturing the dynamic variability of the radiation belts caused by wave‐particle interactions.

Comparison of radiation belt electron fluxes simultaneously measured with PROBA-V/EPT and RBSP/MagEIS instruments

Abstract. Relativistic radiation belt electron observations from the Energetic Particle Telescope (EPT) on board the PROBA-V (Project for On-Board Autonomy and Vegetation) satellite are compared to those performed by the Magnetic Electron Ion Spectrometer (MagEIS) on board the Van Allen Probes formerly known as the Radiation Belt Storm Probes (RBSP). Despite their very different orbits, both instruments are able to measure fluxes of electrons trapped on a given magnetic shell. In the outer belt, the comparison of high- and low-altitude fluxes is performed during the first 3 months of 2014, featuring the most intense storms of the year. In the inner belt, measurements from the two instruments are compared only at conjunction, when the satellites are physically close to each other. Due to the low number of conjunctions, the whole period of mutual operation of both instruments is used (i.e., May 2013–October 2019). The comparisons show that flux variations appear simultaneously on both spacecraft, but the fluxes observed by the EPT are almost always lower than for MagEIS, as expected from their different orbits. In addition, this difference in flux intensity increases with electron energy. During geomagnetic storms, it is also shown that dropout events (i.e., sudden depletion of electrons) in the outer belt are more pronounced at low altitudes than near the geomagnetic Equator. The effect of the equatorial pitch angle value of electrons is investigated in the outer belt. Despite the difference in flux intensity observed by the two instruments, especially at high energies, a linear relationship with a linear correlation higher than 0.7 was found. The correlation is at its maximum when low-pitch-angle electrons near the Equator are considered.

Spatio‐Temporal Characteristics of IPDP‐Type EMIC Waves on April 19, 2017: Implications for Loss of Relativistic Electrons in the Outer Belt

AbstractTo understand the mechanism of the increased frequency of intervals of pulsations of diminishing periods (IPDPs), we analyzed IPDP‐type electromagnetic ion cyclotron (EMIC) waves that occurred on 19 April 2017, using ground and satellite observations. Observations by low‐altitude satellites and ground‐based magnetometers indicate that the increased IPDP frequency is caused by an inward (i.e., Earthward) shift of the EMIC wave source region. The EMIC wave source region moves inward along the mid‐latitude trough, which we used as a proxy for the plasmapause location. A statistical analysis shows that increases in the IPDP frequency showed a positive correlation with polar cap potentials. These results suggest an enhanced convection electric field causes an inward shift of the source region. The inward shift of the source region allows EMIC waves to scatter relativistic electrons over a wide range of radial distances during the IPDP event. This mechanism suggests that IPDP‐type EMIC waves are more likely to scatter relativistic electrons than other EMIC waves. We also show that the decreased phase‐space density of relativistic electrons in the outer radiation belt is consistent with the extent of the source region and the resonant energy of EMIC waves, implying a possible contribution of EMIC waves to outer radiation belt loss during the main phase of geomagnetic storms.

The Effects of Geomagnetic Activities on Acceleration Regions of Radiation Belt Electrons

AbstractSolar wind and magnetospheric processes play important roles in relativistic electron acceleration in the Earth's outer radiation belt. However, how geomagnetic activity affects the acceleration region of relativistic electrons is still not clear. Here we first report the local acceleration regions during the strong storm of 1 June 2013. The first local acceleration region occurs at the lower L shell (L* ∼ 4.0) during the early recovery phase, which is associated with non‐continuous substorm activities. The second local acceleration region occurs at the higher L shell (L* ∼ 5.0) during the late recovery phase, which is associated with continuous substorm activities. To provide a more comprehensive picture, we additionally analyze two storm events with different geomagnetic activities. These events suggest that (a) the strong storm makes it possible for the local acceleration regions to occur at different L shells; (b) the timing, intensity, and duration of substorms during the recovery phase are crucial to the location and dynamics of the local acceleration region. These results can help us further understand the dynamics of relativistic electrons in the Earth's outer radiation belt.

The Evolutions of the Seed and Relativistic Electrons in the Earth's Outer Radiation Belt During the Geomagnetic Storms: A Statistical Study

AbstractUsing the Van Allen Probes data, we statistically study the evolutions of the seed and relativistic electrons in the Earth's outer radiation belt during the storms. Based on the different storm intensities and durations of the main phase, the storm events are divided into “moderate and short (MS) events,” “moderate and long (ML) events,” “strong and short (SS) events,” and “strong and long (SL) events.” The results from a superposed epoch analysis show that (a) For most MS and SS events, the seed electron fluxes can exceed pre‐storm levels, and enhanced fluxes mainly occur around the storm peak. (b) For the ML and SL events, the seed electron fluxes greatly exceed pre‐storm levels by more than an order of magnitude, and enhanced fluxes mainly occur during the main phase. (c) For most MS events, MeV electron fluxes are difficult to return to or exceed pre‐storm levels. While MeV electron fluxes at some L* shells for some SS events can exceed pre‐storm levels. (d) For most ML or SL events, MeV electron fluxes at L* ≥4.0 or ≥3.5 can exceed pre‐storm levels, and enhanced fluxes mainly start to occur from the main phase. These results suggest that the duration of the main phase and storm intensity play important roles in the evolutions of the seed and MeV electrons in the Earth's outer radiation belt during the storms. These are important for our further understanding of the electron dynamics of the outer radiation belt.

Testing the key processes that accelerate outer radiation belt relativistic electrons during geomagnetic storms

Since the discovery of the Earth’s radiation belts in 1958, it has always been a challenge to determine the dominant physical mechanisms, whether local acceleration by chorus or inward radial diffusion, that leads to outer radiation belt relativistic electron flux enhancements. In this study, we test a chain of processes with several potential successive steps that is believed to accelerate outer belt relativistic electrons. By performing correlation analysis of different part of this chain, including the geomagnetic condition, evolution of source and seed electron fluxes, chorus wave activity, and maximum fluxes (jmax) of relativistic electrons, we aim to identify the critical steps that lead to acceleration of MeV electrons. Based on 5-years of Van Allen Probes observations, our results confirm the repeatable response of both source and seed electrons to the storms, showing a significant flux enhancement during the main phase of storms, followed by either a gradual decay or flux persistence at a stable level. However, it is the intense and prolonged occurrence of substorms that contributes to the long-lasting existence of both source and seed electrons, which is also strongly associated with the jmax of relativistic electrons. The significant correlation (Correlation Coefficient, CC∼0.8) between the seed electron fluxes and jmax reveal that the prolonged and pronounced seed electrons are the prerequisite for the significant flux enhancement of relativistic electrons regardless of the acceleration mechanism. The slightly smaller CC (∼0.5–0.7) between source electron fluxes and jmax of relativistic electrons indicates that while local acceleration by chorus wave plays an important role to accelerate relativistic electrons to jmax, other mechanisms such as inward radial diffusion are still needed in this process. The CC between the source electrons and the chorus wave amplitude increases with increasing levels of substorms, showing (CC)max of ∼0.8, which further supports the crucial role of chorus waves in accelerating the relativistic electrons during intense substroms.

Evidence of H+‐Band EMIC Waves in the Inner Radiation Belt Observed by Van Allen Probes During Magnetic Storms

AbstractElectromagnetic ion cyclotron (EMIC) waves play an important role in the losses of ring current ions and outer radiation belt relativistic electrons. While EMIC waves have been reported to occur frequently in the outer belt and slot region, there is little information about the occurrence of EMIC waves in the inner radiation belt. On basis of the magnetometer observations from Van Allen Probes, we analyze two representative events of H+‐band EMIC waves that occurred in the inner zone on December 21 and June 22, 2015. We find that these waves occurred synchronously accompanying with 5–35 keV proton dispersive injections at the dawn and night sectors. During the periods of EMIC waves, the perpendicular temperature of injected protons increased, while the corresponding parallel component decreased. By analyzing the relation between the temperature anisotropy and parallel plasma beta for the two cases, we suggest that the evident enhancements of the temperature anisotropy (T⊥/T∥) of 5–35 keV protons may provide the energy source to excite the H+‐band EMIC waves. A total of 16 left‐hand H+‐band EMIC wave events have been found in the inner radiation belt during the 4‐year (2013–2016) interval and all occur during magnetic storms. The statistical results implicated that left‐hand H+‐band EMIC waves in the inner radiation belt is associated with storm‐time ion injections.

The dependence of radial diffusion coefficients on solar/interplanetary drivers

Radial diffusion driven by Ultra Low Frequency (ULF) waves is very important for magnetospheric dynamics, as it contributes to both acceleration and loss of relativistic electrons in the outer Van Allen radiation belt, as well as to other planetary magnetospheres. The dependence of ULF wave power spectral density and radial diffusion coefficients (DLL) on solar wind parameters has already been investigated but their dependence on the various solar and interplanetary drivers is poorly studied. In this study, we conduct a statistical analysis of radial diffusion coefficients DLL to look into their dependence on different interplanetary drivers, i.e. Interplanetary Coronal Mass Ejections (ICMEs) and Stream Interaction Regions (SIRs), which originate from different solar drivers: CMEs and coronal holes. We use the ”SafeSpace” database (https://synergasia.uoa.gr/modules/document/?course=PHYS120), which includes radial diffusion coefficients DLL and ULF wave power spectral density, created using magnetic and electric field measurements by the THEMIS satellites in the 2011–2019 time period. We study how the properties of these solar wind drivers influence the behavior of radial diffusion coefficients. The results indicate significant differences between drivers of different solar origin at the ratio of the electric over the magnetic DLL component (DLLE over DLLB), especially when the solar wind dynamic pressure is maximized due to shock existence. Furthermore, differences are observed between SIRs with shock and SIRs without shock. This feature introduces a significant energy dependence to the radial diffusion coefficients, which is further depending on the radial distance and the different first adiabatic invariant (μ) values. These results emphasize the importance of the energy dependence of the magnetic component of the DLL, especially during the occurrence of interplanetary shocks, which is neglected by most semi-empirical models used in radiation belt simulations.

Rapid Enhancements of Relativistic Electrons in the Earth's Outer Radiation Belt Caused by the Intense Substorms: A Statistical Study

AbstractUsing the data from Van Allen Probe A and B, we investigate rapid enhancements of relativistic electrons in the Earth's outer radiation belt caused by the intense substorms (AEmax > ∼900 nT) for 29 events from January 2013 to April 2015. These intense substorms may occur during the storm main phase or recovery phase. Based on the different substorm evolution characteristics, the intense substorms are divided into non‐continuous intense substorm activities and continuous intense substorm activities. In this study, we set a criterion for rapid enhancements when the electron phase space densities for μ = 1,096, 2,290, and 3,311 MeV/G increased by more than two times in 9 hr. In the time interval of 9 hr, the local acceleration by chorus waves is the dominant process for accelerating the seed populations (hundred kiloelectron volts) up to MeV energies. Our statistical results show that enhanced chorus waves and seed electrons during the intense substorms are observed in the outer radiation belt. Continuous intense substorm activities can more rapidly (<9 hr) and efficiently accelerate relativistic electrons in the outer radiation belt than non‐continuous intense substorm activities. During the intense substorms, the electric field acceleration could contribute to rapid enhancements of relativistic electrons in the outer radiation belt. Our statistical study suggests that the intense substorms during geomagnetic storms have a significant effect on the rapid variations of relativistic electron dynamics.

Off-equatorial effects of the nonlinear interaction of VLF chorus waves with radiation belt electrons

Nonlinear processes are involved in both the growth of VLF chorus waves and the energization of radiation belt electrons trapped in the wave potential. Nonlinear theory has led to analytic formulae describing both these processes. To investigate these processes, observations from the Van Allen Probes twin spacecraft provide simultaneous in situ information on VLF chorus waves, radiation belt and injected electrons, and local plasma parameters. We combine the theoretical treatment summarized by Omura (2021) with these in situ observations to investigate the characteristics and effects of nonlinear radiation belt processes at the off-equatorial location of the spacecraft observations. We show the smooth phase transition between initial subpackets of chorus wave elements, conducive to extended trapping and enhancement of resonant electrons. The structure of the chorus wave element changes as it propagates away from the equator. Frequency dispersion due to the variation of parallel wave group velocity with frequency contributes to the chorus waveform frequency sweep rate observed at an off-equatorial location. Nonlinear damping at the local value of ½ fce progressively erodes wave amplitude at frequencies above ½ fceEQ. We examine the important dependencies of the nonlinear inhomogeneity factor on the time rate of change of the wave frequency and the field-aligned gradient of the magnetic field and discuss their implication for the energization of trapped non-relativistic and MeV electrons. The 0.5–2% energy gain we find for 3–6 MeV seed electrons indicates that prompt local acceleration of highly relativistic and ultra-relativistic radiation belt electrons can take place directly through their nonlinear interaction with an individual VLF chorus wave element.

Medium-term prediction of the fluence of relativistic electrons in geostationary orbit using solar wind streams forecast based on solar observations

The new approaches to the modelling of the Sun-solar wind-magnetosphere-radiation belts chain and their implementation at MSU's Space weather center (Space Monitoring Data Center, SMDC) are presented. The main idea is to use data from medium-term (3–5 days) forecasts of the speed of quasi-stationary solar wind streams based on data obtained from solar images in the UV wavelength range to improve the quality of forecasts of the magnetospheric factors of the space environment. An operational model for predicting the daily fluence of relativistic electrons (>2 MeV) in the Earth's outer radiation belt has been developed, with the extended set of input parameters that include the forecast of the solar wind velocity. To validate the model, the period from October 11, 2016 to February 18, 2017 was considered, during which the Sun made several revolutions and it was possible to observe the evolution of coronal holes and the development of solar sporadic phenomena. The arrival of high-speed solar wind streams from coronal holes to the Earth's magnetosphere, as well as, as impact of coronal mass ejections, provoked geomagnetic disturbances of various intensity, and caused significant variations of the electron fluxes of the Earth's outer radiation belt. SMDC website implements solar wind model based on the data of solar observations by the AIA instrument of the SDO space observatory at wavelengths of 19.3 and 21.1 nm that forecasts the daily fluence of relativistic electrons (E > 2 MeV) in geostationary orbit. It is shown that the use of the results of medium-term forecasting of the solar wind speed as an input parameter in the daily electron fluence forecast model can significantly improve its quality with a forecast horizon of up to 4 days.

Electron Flux Variability and Ultra‐Low Frequency Wave Activity in the Outer Radiation Belt Under the Influence of Interplanetary Coronal Mass Ejections and High‐Speed Solar Wind Streams: A Statistical Analysis From the Van Allen Probes Era

AbstractThe ultra‐low frequency (ULF) waves can stochastically accelerate radiation belt electrons. Radial diffusion is a well‐established mechanism that can enhance or reduce the electron population in combination with other processes. Using data from the Van Allen Probes, we investigated the response of the 2.10 MeV energy electrons and ULF waves to two types of solar wind structures interacting with Earth's magnetosphere, namely, interplanetary coronal mass ejections (ICMEs) and High‐Speed solar wind streams (HSS). We use measured electron differential flux and ULF waves in the Pc4–Pc5 frequency range from October 2012 to May 2019. We examine 155 events with changes in the outer radiation belt electron differential flux. Results considering all ICMEs and HSSs during the Van Allen Probes era show that for both solar wind structures, solar wind interplanetary magnetic field Bz, solar wind proton density, and speed are related to the outer radiation belt relativistic electrons' response. The persistent ULF power is present during enhancement cases, while for reduction, the ULF waves power is concentrated at the initial reduction on the outer radiation belt electron flux.

A Natural Limit to the Spectral Hardness of Worst Case Electron Radiation in the Terrestrial Van Allen Belt

AbstractThe flux of relativistic electrons in the terrestrial Van Allen radiation belt can vary by orders of magnitude during a geomagnetic storm. The response is typically assumed to be controlled by an often delicate balance between acceleration and loss processes. Here we analyze all 133 magnetic storms from the NASA Van Allen Probes era. We show how the belts demonstrate a repeatable response which limits not only the worst case flux, but also controls and delivers a limit to the severity of the spectral hardness, consistent with the theory of Kennel and Petschek first developed over 50 years ago. When this theory is extended to relativistic energies, the observed electron flux is seen to reach a naturally limited worst case with a clear energy dependence. Here we show how the Kennel‐Petschek theory can explain the evolution and hardening of the electron spectrum during geomagnetic storms. It also introduces an energy‐dependent maximum flux limit. This limit is never reached during the Van Allen Probes era at energies above 2.6 MeV, meanwhile, it is reached within hours at lower energies (∼100 keV) in almost every storm. No current radiation belt models include this effect; without it, they cannot accurately predict the consequent radiation dynamics. Overall, our results demonstrate how this creates a remarkable absolute natural limit to both the spectral hardness and the severity of extreme electron space radiation.

Modeling Radiation Belt Electrons With Information Theory Informed Neural Networks

AbstractAn 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).

A Rapid Localized Deceleration of Earth's Radiation Belt Relativistic Electrons Driven by Storm Proton Injection

AbstractEarth's radiation belt relativistic electron dropouts frequently occur during the main phase of geomagnetic storms, which have been partially attributed to the ring current ion enhancements causing geomagnetic field reconfigurations. In contrast to early studies on the global radiation belt response to the ring current buildup on a timescale of several hours, we here describe a rapid, localized, significant decay of relativistic electrons driven by freshly injected energetic protons during the 27 May 2017 geomagnetic storm. Near the proton injection front, the magnetic field lines were stretched outward, with an inferred migration of their equatorial crossings by ∼0.5 RE during several minutes. Because of the betatron and Fermi decelerations, the relativistic electron fluxes decreased by 2–3 orders of magnitude with pitch‐angle distributions evolving from pancake/flat‐top type to cigar type. This rapid localized response pattern of relativistic electrons could be general over strong particle injections during both geomagnetic storms and substorms.

Relativistic Electron Enhancements Through Successive Dipolarizations During a CIR‐Driven Storm

AbstractRelativistic 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.

Near-Earth Sub-Alfvénic Solar Winds: Interplanetary Origins and Geomagnetic Impacts

Abstract The near-Earth solar wind is in general super-Alfvénic and supermagnetosonic. Using all available near-Earth solar wind measurements between 1973 and 2020, we identified 30 intervals with sub-Alfvénic solar winds. The majority (83%) of the events occurred within interplanetary coronal mass ejection magnetic clouds (MCs)/driver gases. These MC sub-Alfvénic events are characterized by exceptionally low plasma densities (N sw) of ∼0.04–1.20 cm−3, low temperatures (T sw) of ∼0.08 × 105 K to 12.46 × 105 K, enhanced magnetic field intensities (B 0) of ∼8.3–53.9 nT, and speeds (V sw) of ∼328–949 km s−1. The resultant high Alfvén wave speeds (V A) ranged from ∼410 to 1471 km s−1. This is consistent with a mechanism of the MC expansions as they propagate radially outward, causing small pockets of sub-Alfvénic wind regions within the MCs. The remainder of the sub-Alfvénic intervals (17%) occurred within the extreme trailing portions of solar wind high-speed streams (HSSs). These HSS sub-Alfvénic winds had low N sw of ∼0.04–0.97 cm−3, low T sw of ∼0.06 × 105 K to 0.46 × 105 K, B 0 of ∼6.3–18.2 nT, V sw of ∼234–388 km s−1, and a V A range of ∼364–626 km s−1. This is consistent with a mechanism of solar wind super-radial expansions in the trailing HSS regions. During sub-Alfvénic solar wind intervals, Earth's bow shock nose exhibited rapid evanescence, and the estimated geocentric magnetopause distance increased by ∼33%–86%. The inner magnetosphere was more or less unaffected by the sub-Alfvénic solar winds. No significant impact was observed in the outer radiation belt relativistic electrons, and no geomagnetic storms or substorms were triggered during the sub-Alfvénic solar wind events.

Early-Time Non-Equilibrium Pitch Angle Diffusion of Electrons by Whistler-Mode Hiss in a Plasmaspheric Plume Associated with BARREL Precipitation

In August 2015, the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) observed precipitation of energetic (<200 keV) electrons magnetically conjugate to a region of dense cold plasma as measured by the twin Van Allen Probes spacecraft. The two spacecraft passed through the high density region during multiple orbits, showing that the structure was spatial and relatively stable over many hours. The region, identified as a plasmaspheric plume, was filled with intense hiss-like plasma waves. We use a quasi-linear diffusion model to investigate plume whistler-mode hiss waves as the cause of precipitation observed by BARREL. The model input parameters are based on the observed wave, plasma and energetic particle properties obtained from Van Allen Probes. Diffusion coefficients are found to be largest in the same energy range as the precipitation observed by BARREL, indicating that the plume hiss waves were responsible for the precipitation. The event-driven pitch angle diffusion simulation is also used to investigate the evolution of the electron phase space density (PSD) for different energies and assumed initial pitch angle distributions. The results show a complex temporal evolution of the phase space density, with periods of both growth and loss. The earliest dynamics, within the ∼5 first minutes, can be controlled by a growth of the PSD near the loss cone (by a factor up to ∼2, depending on the conditions, pitch angle, and energy), favored by the absence of a gradient at the loss cone and by the gradients of the initial pitch angle distribution. Global loss by 1-3 orders of magnitude (depending on the energy) occurs within the first ∼100 min of wave-particle interaction. The prevalence of plasmaspheric plumes and detached plasma regions suggests whistler-mode hiss waves could be an important driver of electron loss even at high L-value (L ∼6), outside of the main plasmasphere.