Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event

X.‐J Zhang; W Li; R. M Thorne; V Angelopoulos; Q Ma; J Li; J Bortnik; Y Nishimura; L Chen; D. N Baker; G. D Reeves; H. E Spence; C. A Kletzing; W. S Kurth; G. B Hospodarsky; J. B Blake; J. F Fennell

doi:10.1002/2016JA022517

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Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event

Journal article

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Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event

X.‐J Zhang, W Li, R. M Thorne, V Angelopoulos, Q Ma, J Li, J Bortnik, Y Nishimura, L Chen, D. N Baker, …

Journal of geophysical research. Space physics, Vol.121(9), pp.8300-8316

09/2016

DOI: 10.1002/2016JA022517

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https://www.osti.gov/biblio/1402610View

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Abstract

Three mechanisms have been proposed to explain relativistic electron flux depletions (dropouts) in the Earth's outer radiation belt during storm times: adiabatic expansion of electron drift shells due to a decrease in magnetic field strength, magnetopause shadowing and subsequent outward radial diffusion, and precipitation into the atmosphere (driven by EMIC wave scattering). Which mechanism predominates in causing electron dropouts commonly observed in the outer radiation belt is still debatable. In the present study, we evaluate the physical mechanism that may be primarily responsible for causing the sudden change in relativistic electron pitch angle distributions during a dropout event observed by Van Allen Probes during the main phase of the 27 February 2014 storm. During this event, the phase space density of ultrarelativistic (>1 MeV) electrons was depleted by more than 1 order of magnitude over the entire radial extent of the outer radiation belt (3 < L* < 5) in less than 6 h after the passage of an interplanetary shock. We model the electron pitch angle distribution under a compressed magnetic field topology based on actual solar wind conditions. Although these ultrarelativistic electrons exhibit highly anisotropic (peaked in 90°), energy‐dependent pitch angle distributions, which appear to be associated with the typical EMIC wave scattering, comparison of the modeled electron distribution to electron measurements indicates that drift shell splitting is responsible for this rapid change in electron pitch angle distributions. This further indicates that magnetopause loss is the predominant cause of the electron dropout right after the shock arrival. Key Points Drift shell splitting associated with the magnetopause shadowing can result in “top‐hat” distributions on the dayside Drift shell splitting significantly modulates particle dynamics at high L shells Drift shell splitting is independent of particle species, but it leads to larger pitch angle anisotropies for multi‐MeV electrons

drift shell splitting

dropouts

magnetic storm

magnetopause shadowing

outer radiation belt

relativistic electron loss

Details

Title: Subtitle: Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event
Creators: X.‐J Zhang - University of California
W Li - Boston University
R. M Thorne - University of California, Los Angeles
V Angelopoulos - University of California, Los Angeles
Q Ma - University of California, Los Angeles
J Li - University of California, Los Angeles
J Bortnik - University of California, Los Angeles
Y Nishimura - University of California, Los Angeles
L Chen - University of Texas at Dallas
D. N Baker - University of Colorado Boulder
G. D Reeves - Los Alamos National Laboratory
H. E Spence - University of New Hampshire
C. A Kletzing - University of Iowa
W. S Kurth - University of Iowa
G. B Hospodarsky - University of Iowa
J. B Blake - The Aerospace Corporation
J. F Fennell - The Aerospace Corporation
Resource Type: Journal article
Publication Details: Journal of geophysical research. Space physics, Vol.121(9), pp.8300-8316
DOI: 10.1002/2016JA022517
ISSN: 2169-9380
eISSN: 2169-9402
Number of pages: 17
Grant note: NSF (AGS 1405041; 1405054; 1564510) AFOSR (FA9550‐15‐1‐0158) JHU/APL (967399; 921647) NASA (NNX15AI96G; NNX15AF61G; NNX11AR64G; NNX13AI61G; NNX14AI18G) NASA's prime (NAS5‐01072)
Language: English
Date published: 09/2016
Academic Unit: Physics and Astronomy
Record Identifier: 9984199936602771

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