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.
Bibliographical noteFunding Information:
Work at JHU/APL was supported by NASA grant NNX14AB97G. Work at Aerospace was supported by RBSP-ECT funding provided by JHU/APL contract 967399 under NASA’s prime contract NAS5-01072. The work at the New Jersey Institute of Technology was supported by the NASA Van Allen Probes RBSPICE instrument project provided by JHU/APL subcontract 131803 under NASA prime contract NNN06AA01C. K. Takahashi is grateful to Lynn Kistler for her comments on HOPE data. Data used in this work are publicly available from the following sources: magnetic field and electron density data from the RBSP EMFISIS experiment (http://emfisis.physics.uiowa.edu/); electric field data from the RBSP EFW experiment (http://www.space. umn.edu/missions/rbspefw-home- university-of-minnesota); energetic particle data from the RBSP MagEIS experiment (https://www. rbsp-ect.lanl.gov/data_pub/rbspa/ mageis/) and the RBSP RBSPICE experiment (http://rbspice.ftecs.com/ Data.html); Dst and AE indices generated at the World Data Center Kyoto University (http://wdc.kugi.kyoto-u.ac.jp/dstdir/).
- Van Allen Probes
- bounce and drift-bounce resonances
- energetic protons
- poloidal ULF waves
- second harmonic