Van Allen Probes Observations of Symmetric Stormtime Compressional ULF Waves

Previous spacecraft studies showed that stormtime poloidal ultralow-frequency (ULF) waves in the ring current region have an antisymmetric (second harmonic) mode structure about the magnetic equator. This paper reports Van Allen Probes observations of symmetric ULF waves in the postnoon sector during a moderate geomagnetic storm. The mode structure is determined from the presence of purely compressional magnetic field oscillations at the equator accompanied by strong transverse electric field perturbations. Antisymmetric waves were also detected but only very late in the recovery phase. The symmetric waves were detected outside the plasmasphere at L = 3.0-5.5 and had peak power at 4-10 mHz, lower than the frequency of the local fundamental toroidal standing Alfven wave. During the wave events, the flux of protons was enhanced at energies below similar to 5 keV, which appears to be a prerequisite for the waves. The protons may provide free energies to waves through drift resonance instability or drift compressional instability, which occur in the presence of radial gradients of plasma parameters.

Automated summary

This paper reports the observation of symmetric and antisymmetric ULF waves by Van Allen Probes. The symmetric waves are detected in the postnoon sector with lower frequency and peak power, and their presence appears to be related to enhanced proton flux.