IMAGE Observations of Sounder Stimulated and Naturally Occurring Fast Z mode Cavity Noise

Abstract:

We report first observations of sounder stimulated and naturally occurring fast Z mode (ZM) cavity noise detected by the Radio Plasma Imager (RPI) on the IMAGE satellite. The fast Z mode cavity noise is a banded, structure-less radio emission trapped inside fast Z mode cavities, which are characterized by a minimum (fz,min) in fast Z mode cut-off frequency (fz) along a geomagnetic field line [Gurnett et al., JGR, 1983]. Fast Z mode waves reflect at fz ~ f, where f is the wave frequency. Waves in the frequency range fz,min < f < fz,max, where fz,max is the maximum fz above fz,min altitude, are trapped within the cavity as they bounce back and forth between reflection altitudes (fz ~ f) above and below the fz,min altitude. These trapped waves will be observed by a satellite passing through the cavity. The observed cavity noise lower cutoff is at the local Z mode cut-off frequency (fz,Sat) and the upper cut-off is presumably close to fz,max. The cavity noise is observed typically inside the plasmasphere. Comparison of cavity noise as observed on the plasmagram obtained during active sounding with that observed on the dynamic spectra obtained from the interspersed passive wave measurements indicate that the cavity noise is either stimulated by transmissions from the sounder (RPI) or is of natural origin. The sounder stimulated noise is often accompanied by fast Z mode echoes. The naturally occurring cavity noise is observed on both the plasmagram and the dynamic spectra. We believe the stimulated cavity noise is generated due to scattering from small-scale irregularities of waves transmitted by RPI. One potential candidate for the source of naturally occurring Z mode cavity noise is the ring current electrons that can generate fast ZM waves via higher order cyclotron resonance [Nishimura et al., Earth Planets Space, 2007].