Purely compressional Pc1 waves observed by the Van Allen Probes

Abstract:

Waves in the Pc 1 frequency range (0.2 to 5 Hz) generated in Earth's magnetosphere can serve as diagnostics of instabilities that are understood to thermalize energetic plasma populations, including ions in the ring current and possibly electrons in the radiation belts. We have compared observations of Pc1 waves detected by the EFW double probe electric field experiment and EMFISIS fluxgate magnetometer on the two Van Allen Probes spacecraft during its first 1-1/2 years of operation. In addition to the more common transverse EMIC waves in the Pc1 band, these spacecraft have observed several tens of purely compressional wave events (with no evident transverse magnetic field components). Such purely compressional waves could result as wave energy piles up at the ion-ion hybrid resonance where the wave mode converts from a fast wave to a field-aligned EMIC wave. The wave frequency of the resonant mode is sensitive to the heavy ion density, so mode conversion could explain observations of decreasing frequency with radial distance. These compressional waves exhibited a nearly uniform distribution in local time, occurred at L shells from 2.2 to 5.8 (the spacecraft apogee) and ranged in frequency from 0.6 to over 16 Hz. Wave occurrences had little dependence on the level of geomagnetic activity, and in contrast to transverse EMIC waves, they were observed in association with low, stable fluxes of ring current ions. Almost all of these waves were observed inside the plasmapause, regardless of its radial location, and had Poynting vectors directed radially inward and sunward.