Van Allen Probes Observations of Second Harmonic Poloidal Standing Alfvén Waves

Kazue Takahashi, Satoshi Oimatsu, Masahito Nosé, Kyungguk Min, Seth G. Claudepierre, Anthony Chan, John Wygant, Hyomin Kim

Research output: Contribution to journalArticlepeer-review

17 Scopus citations

Abstract

Long-lasting second-harmonic poloidal standing Alfvén waves (P2 waves) were observed by the twin Van Allen Probes (Radiation Belt Storm Probes, or RBSP) spacecraft in the noon sector of the plasmasphere, when the spacecraft were close to the magnetic equator and had a small azimuthal separation. Oscillations of proton fluxes at the wave frequency (∼10 mHz) were also observed in the energy (W) range 50–300 keV. Using the unique RBSP orbital configuration, we determined the phase delay of magnetic field perturbations between the spacecraft with a 2nπ ambiguity. We then used finite gyroradius effects seen in the proton flux oscillations to remove the ambiguity and found that the waves were propagating westward with an azimuthal wave number (m) of ∼−200. The phase of the proton flux oscillations relative to the radial component of the wave magnetic field progresses with W, crossing 0 (northward moving protons) or 180° (southward moving protons) at W ∼ 120 keV. This feature is explained by drift-bounce resonance (mωd ∼ ωb) of ∼120 keV protons with the waves, where ωd and ωb are the proton drift and bounce frequencies. At lower energies, the proton phase space density (FH+) exhibits a bump-on-tail structure with ∂FH+/∂W >0 occurring in the 1–10 keV energy range. This FH+ is unstable and can excite P2 waves through bounce resonance (ω ∼ ωb), where ω is the wave frequency.

Original languageEnglish (US)
Pages (from-to)611-637
Number of pages27
JournalJournal of Geophysical Research: Space Physics
Volume123
Issue number1
DOIs
StatePublished - Jan 2018

Keywords

  • Van Allen Probes
  • bounce and drift-bounce resonances
  • energetic protons
  • plasmasphere
  • poloidal ULF waves
  • second harmonic

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