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Title: Relativistic electron dynamics produced by azimuthally localized poloidal mode ULF waves: Boomerang-shaped pitch angle evolutions

Abstract

Here, we present an analysis of “boomerang-shaped” pitch angle evolutions of outer radiation belt relativistic electrons observed by the Van Allen Probes after the passage of an interplanetary shock on 7 June 2014. The flux at different pitch angles is modulated by Pc5 waves, with equatorially mirroring electrons reaching the satellite first. For 90° pitch angle electrons, the phase change of the flux modulations across energy exceeds 180° and increasingly tilts with time. Using estimates of the arrival time of particles of different pitch angles at the spacecraft location, a scenario is investigated in which shock-induced ULF waves interact with electrons through the drift resonance mechanism in a localized region westward of the spacecraft. Numerical calculations on particle energy gain with the modified ULF wavefield reproduce the observed boomerang stripes and modulations in the electron energy spectrogram. The study of boomerang stripes and their relationship to drift resonance taking place at a location different from the observation point adds new understanding of the processes controlling the dynamics of the outer radiation belt.

Authors:
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [1];  [1]; ORCiD logo [1]; ORCiD logo [3]; ORCiD logo [4]; ORCiD logo [5]
  1. Peking Univ., Beijing (China)
  2. Univ. of Alberta, Edmonton, AB (Canada)
  3. Univ. of New Hampshire, Durham, NH (United States)
  4. The Aerospace Corp., Los Angeles, CA (United States)
  5. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
National Aeronautic and Space Administration (NASA); USDOE
OSTI Identifier:
1402655
Report Number(s):
LA-UR-17-28036
Journal ID: ISSN 0094-8276; TRN: US1702890
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Geophysical Research Letters
Additional Journal Information:
Journal Volume: 44; Journal Issue: 15; Journal ID: ISSN 0094-8276
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; Heliospheric and Magnetospheric Physics; ULF waves; drift resonance; wave-particle interaction; radiation belts; localized waves; interplanetary shock

Citation Formats

Hao, Y. X., Zong, Q. -G., Zhou, X. -Z., Rankin, R., Chen, X. R., Liu, Y., Fu, S. Y., Spence, H. E., Blake, J. B., and Reeves, Geoffrey D. Relativistic electron dynamics produced by azimuthally localized poloidal mode ULF waves: Boomerang-shaped pitch angle evolutions. United States: N. p., 2017. Web. doi:10.1002/2017GL074006.
Hao, Y. X., Zong, Q. -G., Zhou, X. -Z., Rankin, R., Chen, X. R., Liu, Y., Fu, S. Y., Spence, H. E., Blake, J. B., & Reeves, Geoffrey D. Relativistic electron dynamics produced by azimuthally localized poloidal mode ULF waves: Boomerang-shaped pitch angle evolutions. United States. doi:10.1002/2017GL074006.
Hao, Y. X., Zong, Q. -G., Zhou, X. -Z., Rankin, R., Chen, X. R., Liu, Y., Fu, S. Y., Spence, H. E., Blake, J. B., and Reeves, Geoffrey D. 2017. "Relativistic electron dynamics produced by azimuthally localized poloidal mode ULF waves: Boomerang-shaped pitch angle evolutions". United States. doi:10.1002/2017GL074006.
@article{osti_1402655,
title = {Relativistic electron dynamics produced by azimuthally localized poloidal mode ULF waves: Boomerang-shaped pitch angle evolutions},
author = {Hao, Y. X. and Zong, Q. -G. and Zhou, X. -Z. and Rankin, R. and Chen, X. R. and Liu, Y. and Fu, S. Y. and Spence, H. E. and Blake, J. B. and Reeves, Geoffrey D.},
abstractNote = {Here, we present an analysis of “boomerang-shaped” pitch angle evolutions of outer radiation belt relativistic electrons observed by the Van Allen Probes after the passage of an interplanetary shock on 7 June 2014. The flux at different pitch angles is modulated by Pc5 waves, with equatorially mirroring electrons reaching the satellite first. For 90° pitch angle electrons, the phase change of the flux modulations across energy exceeds 180° and increasingly tilts with time. Using estimates of the arrival time of particles of different pitch angles at the spacecraft location, a scenario is investigated in which shock-induced ULF waves interact with electrons through the drift resonance mechanism in a localized region westward of the spacecraft. Numerical calculations on particle energy gain with the modified ULF wavefield reproduce the observed boomerang stripes and modulations in the electron energy spectrogram. The study of boomerang stripes and their relationship to drift resonance taking place at a location different from the observation point adds new understanding of the processes controlling the dynamics of the outer radiation belt.},
doi = {10.1002/2017GL074006},
journal = {Geophysical Research Letters},
number = 15,
volume = 44,
place = {United States},
year = 2017,
month = 7
}

Journal Article:
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  • The authors look at the question of precipitating electrons from the radiation belts. The common argument has been that pitch angle scattering of trapped electrons on whistler mode waves was the source of the precipitating electrons. As part of this theory there has been a threshold energy E[sub C] = B[sup 2]/2[mu][sub 0]N, the magnetic energy per particle which has been a threshold for this process. This energy is typically 10 keV. The observation of diffuse aurorae, as distinguished from distinct aurorae, was made in the 1970's. This is a rather weak and structureless ion and electron precipitation which extendsmore » around the auroral oval. The intensities are not inconsistent with pitch angle diffusion of plasma sheet electrons into the loss cone as being the source. The authors argue that there is no fundamental energy threshold for the whistler mode interactions, and therefore such scattering can account for the diffuse aurorae. They show theoretically that there is no reason why the characteristic energy E[sub C] should limit the energy of electrons participating in pitch angle diffusion from whistler waves. An additional consequence of this work is that experimental releases of lithium in the outer magnetosphere to stimulate increased precipitation or initiate a magnetic substorm are not likely to succeed, consistent with CRRES lithium releases.« less
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  • As Voyager 1 traversed the inner radiation belt of Jupiter, wave-particle interactions involving energetic electrons and whistler mode turbulence were strongly affected by the presence of the Io plasma torus. Within the high density torus the resonant electron energy was low and the associated high index of refraction yielded high B-to-E ratios for the wave fields, leading to very strong pitch-angle scattering. We show that significant spatial and temporal variations in plasma conditions produced large fluctuations in local scattering times, and we discuss the problems associated with the evaluation of precipitation lifetimes.
  • The time averaged equations of motion derived by Inan and Tkalcevic (1982) are used in a test particle study of Landau resonant interactions of whistler mode waves and energetic particles in the magnetosphere. By computing individual particle trajectories it is shown that the pitch angle scattering and energy exchange is significantly different for the two classes of particles, trapped and untrapped. The trapped particles are characterized by a bounded phase variation, whereas the nontrapped particles exhibit unbounded phase variation. The threshold wave electric field intensities necessary for trapping are determined. Full distribution test particle simulation is carried out to determinemore » the precipitated electron flux that would be induced as a result of these interactions. It is shown that for typical parameters the resulting precipitation fluxes for Landau resonance interactions are much smaller than those induced in gyroresonance interactions, even for wave field intensities that are much higher than the trapping threshold.« less
  • A test particle approach is used to compare gyroresonant pitch angle scattering of energetic electrons by coherent versus incoherent whistler mode waves, for the case in which the coherent wave amplitude is below the nonlinear phase trapping threshold. Wave packets of 400 ms duration propagating along the magnetic field at L = 4 within the plasmasphere are considered, and the wave-induced pitch angle scattering along the propagation path from one hemisphere to the other and the resulting precipitation flux are computed. An incoherent wave spectrum is simulated by random modulation of the wave frequency at intervals of 1 ms, therebymore » generating signals with nearly constant power spectral density over a bandwidth of 2 kHz centered at 5.5 kHz. The associated pitch angle scattering is compared with that of a monochromatic 5.5-kHz signal of 400 ms duration. Results of the test particle analysis are compared with those expected on the basis of a classical diffusion treatment, and an expression is derived for an effective ''diffusion'' coefficient for pitch angle scattering by coherent waves. The trajectory followed by a particle when interacting with incoherent waves essentially represents a random walk in velocity space, while for coherent waves the pitch angle of the particle varies in a well-defined manner. In spite of the fact that individual particle scattering are typically larger for coherent waves, the peak precipitation fluxes induced by incoherent waves are found to be approximately the same as those for coherent waves having the same total power. This results from the fact that incoherent waves interact with particles over a wider range of energies. As a consequence, the energy spectrum and the temporal extent of transient precipitation pulses due to incoherent wave packets are broader than those for equivalent coherent ones.« less