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Title: Unraveling the excitation mechanisms of highly oblique lower-band chorus waves

Abstract

Excitation mechanisms of highly oblique, quasi-electrostatic lower band chorus waves are investigated using Van Allen Probes observations near the equator of the Earth's magnetosphere. Linear growth rates are evaluated based on in situ, measured electron velocity distributions and plasma conditions and compared with simultaneously observed wave frequency spectra and wave normal angles. Accordingly, two distinct excitation mechanisms of highly oblique lower band chorus have been clearly identified for the first time. The first mechanism relies on cyclotron resonance with electrons possessing both a realistic temperature anisotropy at keV energies and a plateau at 100–500 eV in the parallel velocity distribution. The second mechanism corresponds to Landau resonance with a 100–500 eV beam. In both cases, a small low-energy beam-like component is necessary for suppressing an otherwise dominating Landau damping. In conclusion, our new findings suggest that small variations in the electron distribution could have important impacts on energetic electron dynamics.

Authors:
 [1];  [2];  [3];  [4];  [4];  [5];  [5];  [5]; ORCiD logo [6]; ORCiD logo [7];  [8]
  1. Boston Univ., MA (United States). Center for Space Physics; Univ. of California, Los Angeles, CA (United States). Dept. of Atmospheric and Oceanic Sciences
  2. Alternative Energies and Atomic Energy Commission (CEA), Arpajon (France)
  3. Univ. of California, Los Angeles, CA (United States). Dept. of Earth, Planetary, and Space Sciences
  4. Univ. of California, Los Angeles, CA (United States). Dept. of Atmospheric and Oceanic Sciences
  5. Univ. of Iowa, Iowa City, IA (United States). Dept. of Physics and Astronomy
  6. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); New Mexico Consortium, Los Alamos, NM (United States). Space Sciences Division
  7. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  8. Univ. of New Hampshire, Durham, NH (United States). Inst. for the Study of Earth, Oceans, and Space
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:
1345934
Report Number(s):
LA-UR-16-26165
Journal ID: ISSN 0094-8276
Grant/Contract Number:
AC52-06NA25396; 967399; 921647; NNX15AF61G; NNX14AI18G; NNX13AI61G; FA9550-15-1-0158; PLR1341359; AGS1405054
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Geophysical Research Letters
Additional Journal Information:
Journal Volume: 43; Journal Issue: 17; Journal ID: ISSN 0094-8276
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; oblique chorus excitation; Beam instability; Temperature anisotropy; Lower band chorus; Heliospheric and Magnetospheric Physics

Citation Formats

Li, Wen, Mourenas, D., Artemyev, A. V., Bortnik, J., Thorne, R. M., Kletzing, C. A., Kurth, W. S., Hospodarsky, G. B., Reeves, Geoffrey D., Funsten, Herbert O., and Spence, H. E. Unraveling the excitation mechanisms of highly oblique lower-band chorus waves. United States: N. p., 2016. Web. doi:10.1002/2016GL070386.
Li, Wen, Mourenas, D., Artemyev, A. V., Bortnik, J., Thorne, R. M., Kletzing, C. A., Kurth, W. S., Hospodarsky, G. B., Reeves, Geoffrey D., Funsten, Herbert O., & Spence, H. E. Unraveling the excitation mechanisms of highly oblique lower-band chorus waves. United States. doi:10.1002/2016GL070386.
Li, Wen, Mourenas, D., Artemyev, A. V., Bortnik, J., Thorne, R. M., Kletzing, C. A., Kurth, W. S., Hospodarsky, G. B., Reeves, Geoffrey D., Funsten, Herbert O., and Spence, H. E. 2016. "Unraveling the excitation mechanisms of highly oblique lower-band chorus waves". United States. doi:10.1002/2016GL070386. https://www.osti.gov/servlets/purl/1345934.
@article{osti_1345934,
title = {Unraveling the excitation mechanisms of highly oblique lower-band chorus waves},
author = {Li, Wen and Mourenas, D. and Artemyev, A. V. and Bortnik, J. and Thorne, R. M. and Kletzing, C. A. and Kurth, W. S. and Hospodarsky, G. B. and Reeves, Geoffrey D. and Funsten, Herbert O. and Spence, H. E.},
abstractNote = {Excitation mechanisms of highly oblique, quasi-electrostatic lower band chorus waves are investigated using Van Allen Probes observations near the equator of the Earth's magnetosphere. Linear growth rates are evaluated based on in situ, measured electron velocity distributions and plasma conditions and compared with simultaneously observed wave frequency spectra and wave normal angles. Accordingly, two distinct excitation mechanisms of highly oblique lower band chorus have been clearly identified for the first time. The first mechanism relies on cyclotron resonance with electrons possessing both a realistic temperature anisotropy at keV energies and a plateau at 100–500 eV in the parallel velocity distribution. The second mechanism corresponds to Landau resonance with a 100–500 eV beam. In both cases, a small low-energy beam-like component is necessary for suppressing an otherwise dominating Landau damping. In conclusion, our new findings suggest that small variations in the electron distribution could have important impacts on energetic electron dynamics.},
doi = {10.1002/2016GL070386},
journal = {Geophysical Research Letters},
number = 17,
volume = 43,
place = {United States},
year = 2016,
month = 8
}

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  • Chorus in the inner magnetosphere has been observed frequently at geomagnetically active times, typically exhibiting a two-band structure with a quasi-parallel lower band and an upper band with a broad range of wave normal angles. But recent observations by Van Allen Probes confirm another type of lower band chorus, which has a large wave normal angle close to the resonance cone angle. It has been proposed that these waves could be generated by a low-energy beam-like electron component or by temperature anisotropy of keV electrons in the presence of a low-energy plateau-like electron component. This paper, however, presents an alternativemore » mechanism for generation of this highly oblique lower band chorus. Through a nonlinear three-wave resonance, a quasi-parallel lower band chorus wave can interact with a mildly oblique upper band chorus wave, producing a highly oblique quasi-electrostatic lower band chorus wave. This theoretical analysis is confirmed by 2-D electromagnetic particle-in-cell simulations. Furthermore, as the newly generated waves propagate away from the equator, their wave normal angle can further increase and they are able to scatter low-energy electrons to form a plateau-like structure in the parallel velocity distribution. As a result, the three-wave resonance mechanism may also explain the generation of quasi-parallel upper band chorus which has also been observed in the magnetosphere.« less
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