Oxygen cyclotron harmonic waves observed using Van Allen Probes

Fine structured multiple-harmonic electromagnetic emissions at frequencies around the equatorial oxygen cyclotron harmonics are observed by Van Allen Probe A outside the core plasmasphere (L~5) off the magnetic equator (MLAT~?7.5°) during a geomagnetic storm. We find that the multiple-harmonic emissions have power spectrum density (PSD) peaks during 2–8 equatorial oxygen gyroharmonics (f ~ n \(f_{O^{+}}\), n=2–8), while the fundamental mode (n=1) is absent, implying that the harmonic waves are generated near the equator and propagate into the observation region. Additionally, these electromagnetic emissions are linearly polarized. Different from the equatorial noise emission that propagates considerably obliquely, these emissions have moderate wave normal angles (approximately 40°–60°), which predominately increase as the harmonic number increases. Considering their frequency and wave normal angle characteristics, it is suggested that these multiple-harmonic emissions play an important role in the dynamic variation of radiation belt electrons.