Drifting Quasi-Periodic Modulation of the Fast Magnetosonic Mode: Van Allen Probe Observations

Abstract:

The fast magnetosonic mode is one of the dominant wave modes in the Earth's radiation belts. These waves influence the ring current by scattering ions in energy in the 10's of keV range, and are believed to be a heat source for radiation belt electrons. The fast magnetosonic mode observed around the Earth's inner equatorial magnetosphere sometimes exhibits quasi-periodic modulation as detected by the Van Allen probes. During each modulation the wave frequency exhibits a strong drifting (dispersive) signature characterized by a rising tone. Each tone is composed of harmonics with spacing close to the proton cyclotron frequency. The tones are band limited in frequency and mainly observed above the 20^th harmonic of the local proton cyclotron frequency. We observe this modulation mainly outside the plasmapause, but it has also been observed to penetrate down to 1.5 R_E. The modulation is observed up to magnetic latitudes of ±17º, at all magnetic local times, but its signatures are more pronounced on the dayside. For events where lower frequency ULF waves are detected, the period of the ULF wave is about twice the modulation period of the fast magnetosonic mode, suggesting strong wave-wave interactions. The modulation period varies from 50 to 200 s and its duration ranges from 0.2 to 3 h, with the maximum duration limited by the spacecraft orbit. We hypothesize that the rising tone is produced by changing Alfven velocities created by steepened density fluctuations due to plasma modification by an underlying ULF wave.