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Title: Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event: Drift Shell Splitting on the Dayside

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 (>1MeV) 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 themore » 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.« less

Authors:
 [1];  [2];  [3];  [4]; ORCiD logo [3];  [3];  [3];  [3];  [5];  [6]; ORCiD logo [7];  [8];  [9];  [9];  [9];  [10];  [10]
  1. Univ. of California, Los Angeles, CA (United States). Dept. of Atmospheric and Oceanic Sciences; Univ. of California, Los Angeles, CA (United States). Dept. of Earth, Planetary, and Space Sciences and Inst. of Geophysics and Space Physics
  2. Univ. of California, Los Angeles, CA (United States). Dept. of Atmospheric and Oceanic Sciences; Boston Univ., MA (United States). Center for Space Physics
  3. Univ. of California, Los Angeles, CA (United States). Dept. of Atmospheric and Oceanic Sciences
  4. Univ. of California, Los Angeles, CA (United States). Dept. of Earth, Planetary, and Space Sciences and Inst. of Geophysics and Space Physics
  5. Univ. of Texas at Dallas, Richardson, TX (United States). Dept. of Physics
  6. Univ. of Colorado, Boulder, CO (United States). Lab. for Atmospheric and Space Research
  7. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); New Mexico Consortium, Los Alamos, NM (United States). Space Sciences Division
  8. Univ. of New Hampshire, Durham, NH (United States). Inst. for the Study of Earth, Oceans, and Space
  9. Univ. of Iowa, Iowa City, IA (United States). Dept. of Physics and Astronomy
  10. Aerospace Corporation, Los Angeles, CA (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
National Aeronautic and Space Administration (NASA); National Science Foundation (NSF); US Air Force Office of Scientific Research (AFOSR)
OSTI Identifier:
1402610
Report Number(s):
LA-UR-16-23134
Journal ID: ISSN 2169-9380; TRN: US1703008
Grant/Contract Number:
AC52-06NA25396; 967399; 921647; NAS5-01072; FA9550-15-1-0158; NNX15AI96G; NNX15AF61G; NNX11AR64G; NNX13AI61G; NNX14AI18G; AGS 1405041; 1405054; 1564510
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Geophysical Research. Space Physics
Additional Journal Information:
Journal Volume: 121; Journal Issue: 9; Journal ID: ISSN 2169-9380
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Heliospheric and Magnetospheric Physics; dropouts; relativistic electron loss; drift shell splitting; magnetopause shadowing; outer radiation belt; magnetic storm

Citation Formats

Zhang, X. -J., Li, W., Thorne, R. M., Angelopoulos, V., Ma, Q., Li, J., Bortnik, J., Nishimura, Y., Chen, L., Baker, D. N., Reeves, Geoffrey D., Spence, H. E., Kletzing, C. A., Kurth, W. S., Hospodarsky, G. B., Blake, J. B., and Fennell, J. F.. Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event: Drift Shell Splitting on the Dayside. United States: N. p., 2016. Web. doi:10.1002/2016JA022517.
Zhang, X. -J., Li, W., Thorne, R. M., Angelopoulos, V., Ma, Q., Li, J., Bortnik, J., Nishimura, Y., Chen, L., Baker, D. N., Reeves, Geoffrey D., Spence, H. E., Kletzing, C. A., Kurth, W. S., Hospodarsky, G. B., Blake, J. B., & Fennell, J. F.. Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event: Drift Shell Splitting on the Dayside. United States. doi:10.1002/2016JA022517.
Zhang, X. -J., Li, W., Thorne, R. M., Angelopoulos, V., Ma, Q., Li, J., Bortnik, J., Nishimura, Y., Chen, L., Baker, D. N., Reeves, Geoffrey D., Spence, H. E., Kletzing, C. A., Kurth, W. S., Hospodarsky, G. B., Blake, J. B., and Fennell, J. F.. 2016. "Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event: Drift Shell Splitting on the Dayside". United States. doi:10.1002/2016JA022517. https://www.osti.gov/servlets/purl/1402610.
@article{osti_1402610,
title = {Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event: Drift Shell Splitting on the Dayside},
author = {Zhang, X. -J. and Li, W. and Thorne, R. M. and Angelopoulos, V. and Ma, Q. and Li, J. and Bortnik, J. and Nishimura, Y. and Chen, L. and Baker, D. N. and Reeves, Geoffrey D. and Spence, H. E. and Kletzing, C. A. and Kurth, W. S. and Hospodarsky, G. B. and Blake, J. B. and Fennell, J. F.},
abstractNote = {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 (>1MeV) 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.},
doi = {10.1002/2016JA022517},
journal = {Journal of Geophysical Research. Space Physics},
number = 9,
volume = 121,
place = {United States},
year = 2016,
month = 8
}

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  • We present a magnetic field drift shell--splitting model for the unusual butterfly and head-and-shoulder energetic (E>25 keV) particle pitch angle distributions (PADs) which appear deep within the dayside magnetosphere during the course of storms and substorms. Drift shell splitting separates the high and low pitch angle particles in nightside injections as they move to the dayside magnetosphere, so that the higher pitch angle particles move radially away from Earth. Consequently, butterfly PADs with a surplus of low pitch angle particles form on the inner edge of the injection, but head-and-shoulder PADs with a surplus of high pitch angle particles frommore » on the outer edge. A similar process removes high pitch angle particles from the inner dayside magnetosphere during storms, leaving the remaining lower pitch angle particles to form butterfly PADs on the inner edge of the ring current. A detailed case and statistical study of CCE/MEPA observations, as well as a review of previous work, shows most examples of unusual PADs to be consistent with the model. copyright American Geophysical Union 1987« less
  • Temporal and spectral signatures of a lightning-induced electron precipitation (LEP) burst observed on the S81-1 (SEEP) satellite are analyzed and compared with the predictions of a test particle model of the gyroresonant whistler-particle interaction in the magnetosphere. The flux to be detected by specific detectors on the low altitude (/similar to/220 km) satellite at Lapprox. =2.24 is calculated in terms of the integral counting rate as a function of time and in terms of the dynamic energy spectra during the initial /similar to/300-ms precipitation pulse. For a whistler wave packet with frequency range 500 Hz to 6 kHz the dynamicmore » energy spectra are found to depend sensitively on the electron angular distribution in the vicinity of the loss cone. In the case of a whistler wave originating in northern hemisphere lightning the maximum whistler-induced pitch angle scattering of electrons occurs near /similar to/10 /sup 0/S geomagnetic latitude. However, scattering occuring over the latitude range of /similar to/20 /sup 0/N to /similar to/20 /sup 0/S is found to be significant and contributes to the observed LEP pulse. The dynamic energy spectra of the LEP pulse and the temporal profile of the integral counting rate are consistent with the predictions of a test particle model of the gyroresonant scattering of the electrons by a whistler wave having an equatorial intensity at 6 kHz of /similar to/200 pT. The measured LEP pulse pitch angle distribution is wider than that estimated on the basis of the test particle model. copyright American Geophysical Union 1989« less
  • A statistical survey of electron pitch angle distributions (PADs) is performed based on the pitch angle-resolved flux observations from the Magnetic Electron Ion Spectrometer (MagEIS) instrument on board the Van Allen Probes during the period from 1 October 2012 to 1 May 2015. By fitting the measured PADs to a sin nα form, where α is the local pitch angle and n is the power law index, we investigate the dependence of PADs on electron kinetic energy, magnetic local time (MLT), the geomagnetic Kp index, and L shell. The difference in electron PADs between the inner and outer belt ismore » distinct. In the outer belt, the common averaged n values are less than 1.5, except for large values of the Kp index and high electron energies. The averaged n values vary considerably with MLT, with a peak in the afternoon sector and an increase with increasing L shell. In the inner belt, the averaged n values are much larger, with a common value greater than 2. The PADs show a slight dependence on MLT, with a weak maximum at noon. A distinct region with steep PADs lies in the outer edge of the inner belt where the electron flux is relatively low. The distance between the inner and outer belt and the intensity of the geomagnetic activity together determine the variation of PADs in the inner belt. Finally, besides being dependent on electron energy, magnetic activity, and L shell, the results show a clear dependence on MLT, with higher n values on the dayside.« less
  • During the event of April 16, 1970, energetic (approx.0.7 MeV) protons were observed by Vela 6B in the solar wind and by Vela 5B in the magnetosheath. During one 4-hour period the omnidirectional flux measured by 5B was a factor of 3 larger than that measured by 6B. The pitch angle distribution observed at this time by 6B was characterized by a strong peak at 0degree pitch angle. Transformation of this distribution according to the Liouville equation accounts for the enhanced omnidirectional flux and the shape of the pitch angle distributions observed at 5B. At a later time in themore » event, distorted pitch angle distributions, which were observed in the magnetosheath, are accounted for by a Liouville transformation of an assumed unidirectional distribution in interplanetary space. We conclude that particle entry into the magnetosheath was approximately adiabatic.« less