Survey of the high resolution frequency structure of the fast magnetosonic mode and proton energy diffusion associated with these waves

Abstract:

The fast magnetosonic mode, also referred to as equatorial noise, occurs at frequencies mainly between the proton cyclotron frequency (f_cp) and the lower hybrid frequency. The wave properties of this mode are characterized by a strong magnetic compressional component. These waves are observed around the magnetic equator in the Earth's inner magnetosphere. Case studies of the spectra of these waves have found the emissions to be composed of 1) harmonics, usually with spacing near the local f_cp, 2) broad band hiss-like structure, or 3) a superposition of the two spectral types. No statistical studies of the frequency structure of these waves have been made. Using ~600,000 burst mode wave captures from the EMFISIS Wave Form Receiver and the EFW instrument on the Van Allen Probes spacecraft this mode will be identified in the high resolution frequency spectra and its frequency structure will be characterized. The variation of the frequency structure will be investigated as a function of normalized frequency, location, and geomagnetic conditions, and with spacecraft separation. The frequency structure will be compared with path integrated gain using proton ring distributions as the wave source.

Recently the modulation of the fast magnetosonic mode has been reported, with modulation periods in the range of 30s to 240s. It has been proposed that frequency drift observed during each modulation is due to strong inward diffusion in energy of the proton ring distributions that generate these waves. As the inner edge of the ring distribution diffuses towards lower energies the band of unstable harmonics increases in frequency. If in the source region, for modulations with periods greater than say 100s, the inward energy diffusion should be observable in the HOPE proton data which has a cycle time of 24s.