A Statistical Survey of Radiation Belt Dropouts Observed by Van Allen Probes

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.

Modeling the Magnetopause Shadowing Loss During the June 2015 Dropout Event

“…The NASA Van Allen Probes mission has enabled new and higher‐fidelity description of the radiation belts. One type of the remarkable variations of Earth's outer radiation belt is called radiation belt dropout, during which the MeV electron fluxes are observed to drop by orders of magnitude on time scales of a few hours or less (e.g., Morley et al, ; Morley et al, ; Onsager et al, ; Shprits et al, ; Tu et al, , , ; Turner et al, ; Xiang et al, , ). The fundamental question is where the electrons go during the dropouts.…”

Magnetic Local Time‐Resolved Examination of Radiation Belt Dynamics during High‐Speed Solar Wind Speed‐Triggered Substorm Clusters

“…However, the cost of such high‐quality scientific platforms limits the number which will operate at any given time. In recent years there has been a wealth of exceptional in situ observations deepening our understanding (e.g., Aseev et al, ; Jaynes et al, ; Kasahara et al, ; Turner et al, ; Xiang et al, ; Zhao et al, ) but which are limited in their ability to provide simultaneous MLT and L coverage.…”

Precipitation of MeV and Sub‐MeV Electrons Due to Combined Effects of EMIC and ULF Waves

“…Using statistics of Van Allen Probes observations, Xiang et al () have recently found that the minimum energy of electron dropouts varies with equatorial pitch angle α 0 roughly like the theoretically expected E min ( α 0 ) for cyclotron resonance with hydrogen‐band EMIC waves (Cao et al, ; Kersten et al, ; Mourenas et al, ; Ni et al, ; Zhang et al, ) at L * ≤ 4 ( L * is the adiabatically invariant L ‐shell) but that it decreases below E min ( α 0 ) at L * =5. This indicates that low‐energy electron losses related to EMIC waves are mainly localized at high L * ≥ 5.…”

“…We conjecture that such a strong peak could potentially lead to a much stronger impact of ULF waves on EMIC‐driven electron precipitation at these L ‐shells. It could partially explain the electron losses observed at energies below the theoretical threshold E min for EMIC waves alone (Xiang et al, ). The same effect could also account for a statistical increase of ~0.7–2 MeV electron losses when both EMIC and ULF waves are present, as compared with EMIC waves alone (Simms et al, ).…”

“…This suggests a much stronger statistical impact of ULF waves on EMIC‐driven electron precipitation at L ∼5.5–6.0. Such a stronger presence of intense compressional ULF waves together with EMIC waves at L ≥ 5.5 might also account for the decrease of the minimum energy of electron dropouts below the theoretical threshold for EMIC waves alone, observed at L * =5 but not at lower L * ≤ 4.5 (Xiang et al, ).…”