EGU2020-9575
https://doi.org/10.5194/egusphere-egu2020-9575
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation

Kazuhiro Yamamoto1, Masahito Nosé2, Kunihiro Keika3, David Hartley4, Charles Smith5, Robert MacDowall6, Louis Lanzerotti7, Donald Mitchell8, Harlan Spence9, Geoff Reeves10,11, John Wygant12, John Bonnell13, and Satoshi Oimatsu1
Kazuhiro Yamamoto et al.
  • 1Graduate School of Science, Kyoto University, Kyoto, Japan
  • 2Institute for Space‐Earth Environmental Research Nagoya University, Nagoya, Japan
  • 3Graduate School of Science, The University of Tokyo, Tokyo, Japan
  • 4Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA
  • 5Department of Physics and Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA
  • 6NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 7Center for Solar‐Terrestrial Research, New Jersey Institute of Technology, Newark, NJ, USA
  • 8Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
  • 9Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA
  • 10Space Science and Applications Group, Los Alamos National Laboratory, Los Alamos, NM, USA
  • 11Space Sciences Division, New Mexico Consortium, Los Alamos, NM, USA
  • 12School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA
  • 13Space Science Laboratory, University of California, Berkeley, Berkeley, CA, USA

Two wave packets of second harmonic poloidal Pc 4 waves with a wave frequency of ~7 mHz were detected by Van Allen Probe A at a radial distance of ~5.8 RE and magnetic local time of 13 hr near the magnetic equator, where plasmaspheric refilling was in progress. Proton butterfly distributions with energy dispersions were also measured at the same time; the proton fluxes at 10–30 keV oscillated with the same frequency as the Pc 4 waves. Using the ion sounding technique, we find that the Pc 4 waves propagated eastward with an azimuthal wave number (m number) of ~220 and ~260 for each wave packet, respectively. Such eastward propagating high‐m (m > 100) waves were seldom reported in previous studies. The condition of drift‐bounce resonance is well satisfied for the estimated m numbers in both events. Proton phase space density was also examined to understand the wave excitation mechanism. We obtained temporal variations of the energy and radial gradient of the proton phase space density and find that temporal intensification of the radial gradient can generate the two wave packets. The cold electron density around the spacecraft apogee was >100 cm−3 in the present events, and hence the eigenfrequency of the Pc 4 waves became lower. This causes the increase of the m number which satisfies the resonance condition of drift‐bounce resonance for 10–30 keV protons and meets the condition for destabilization due to gyrokinetic effect.

How to cite: Yamamoto, K., Nosé, M., Keika, K., Hartley, D., Smith, C., MacDowall, R., Lanzerotti, L., Mitchell, D., Spence, H., Reeves, G., Wygant, J., Bonnell, J., and Oimatsu, S.: Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9575, https://doi.org/10.5194/egusphere-egu2020-9575, 2020

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