Van Allen Probes observations of oxygen cyclotron harmonic waves in the inner magnetosphere

English Abstract:

Waves with frequencies in the vicinity of the oxygen cyclotron frequency and its harmonics are often observed on the Van Allen Probes spacecraft in the inner equatorial magnetosphere during geomagnetically active times. We will present observations of these waves during the main phase of a geomagnetic storm on November 1, 2012. Oxygen cyclotron harmonics were observed simultaneously by fluxgate magnetometers onboard two satellites, at L~3 in the early morning magnetic local time sector, just outside the plasmapause (as monitored by the spacecraft potential). Analysis of the magnetic field data in field aligned coordinate system showed that these harmonic waves are heft-hand polarized and propagating obliquely to the background magnetic field. Observations of associated energetic particle distributions by the HOPE instrument suggest that anisotropic (peaked at 90-degree pitch angles) oxygen distributions with energy ~1-2 keV might have provided the energy source for these waves, however further theoretical analysis is warranted to establish the nature of this wave instability. Since polarization and propagation angle of oxygen cyclotron harmonic waves are very similar to electromagnetic ion cyclotron (EMIC) waves, which are believed to influence ring current and radiation belt dynamics, they may be also important for energetic particle dynamics in the inner magnetosphere.