Van Allen Probes observations of drift-bounce resonance and energy transfer between energetic ring current protons and poloidal Pc4 wave

TitleVan Allen Probes observations of drift-bounce resonance and energy transfer between energetic ring current protons and poloidal Pc4 wave
Publication TypeJournal Article
Year of Publication2018
AuthorsOimatsu, S, é, M, Takahashi, K, Yamamoto, K, Keika, K, Kletzing, CA, Smith, CW, MacDowall, RJ, Mitchell, DG
JournalJournal of Geophysical Research: Space Physics
Date Published04/2018
KeywordsVan Allen Probes
AbstractA poloidal Pc4 wave and proton flux oscillations are observed in the inner magnetosphere on the dayside near the magnetic equator by the Van Allen Probes spacecraft on 2 March 2014. The flux oscillations are observed in the energy range of 67.0 keV to 268.8 keV with the same frequency of the poloidal Pc4 wave. We find pitch angle and energy dispersion in the phase difference between the poloidal magnetic field and the proton flux oscillations, which are features of drift‐bounce resonance. We estimate the resonance energy to be ~120 keV for pitch angle (α) of 30° or 150°, and 170–180 keV for α = 50° or 130°. To examine the direction of energy flow between protons and the wave, we calculate the sign of the gradient of proton phase space density (df/dW) on both the inbound and outbound legs of the orbit. We find the gradient to be outward on both legs, which means that energy is transferred from the protons to the wave. During the poloidal Pc4 wave event, the Dst* index shows a measurable increase of ~6.7 nT. We estimate the total energy loss of the ring current from the recovery of the Dst* index and from the variation of proton flux by the drift‐bounce resonance. We suggest that energy transfer from the ring current protons to the poloidal Pc4 wave via the drift‐bounce resonance contributes to up to ~85 % of the increase of the Dst* index.
URLhttps://agupubs.onlinelibrary.wiley.com/doi/10.1029/2017JA025087
DOI10.1029/2017JA025087
Short TitleJ. Geophys. Res. Space Physics


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