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Understanding the Mechanisms of Radiation Belt Dropouts Observed by Van Allen Probes


AuthorXiang, Zheng; Tu, Weichao; Li, Xinlin; Ni, Binbin; Morley, S.; Baker, D.;
KeywordsEMIC wave; last closed drift shell; magnetopause shadowing; Phase space density; radiation belt dropout; Van Allen Probes
AbstractTo achieve a better understanding of the dominant loss mechanisms for the rapid dropouts of radiation belt electrons, three distinct radiation belt dropout events observed by Van Allen Probes are comprehensively investigated. For each event, observations of the pitch angle distribution of electron fluxes and electromagnetic ion cyclotron (EMIC) waves are analyzed to determine the effects of atmospheric precipitation loss due to pitch angle scattering induced by EMIC waves. Last closed drift shells (LCDS) and magnetopause standoff position are obtained to evaluate the effects of magnetopause shadowing loss. Evolution of electron phase space density (PSD) versus L* profiles and the μ and K (first and second adiabatic invariants) dependence of the electron PSD drops are calculated to further analyze the dominant loss mechanisms at different L*. Our findings suggest that these radiation belt dropouts can be classified into distinct classes in terms of dominant loss mechanisms: magnetopause shadowing dominant, EMIC wave scattering dominant, and combination of both mechanisms. Different from previous understanding, our results show that magnetopause shadowing can deplete electrons at L* < 4, while EMIC waves can efficiently scatter electrons at L* > 4. Compared to the magnetopause standoff position, it is more reliable to use LCDS to evaluate the impact of magnetopause shadowing. The evolution of electron PSD versus L* profile and the μ, K dependence of electron PSD drops can provide critical and credible clues regarding the mechanisms responsible for electron losses at different L* over the outer radiation belt.
Year of Publication2017
JournalJournal of Geophysical Research: Space Physics
Volume
Number of Pages
Section
Date Published08/2017
ISBN
URLhttp://onlinelibrary.wiley.com/doi/10.1002/2017JA024487/full
DOI10.1002/2017JA024487