GENERATION MECHANISM OF PLASMASPHERIC ELF/VLF HISS - A STATISTICAL STUDY FROM GEOS-1 DATA

ELF/VLF hiss is whistler mode waves that interact with energetic electrons in the magnetosphere. One of the consequences of these interactions is the precipitation of energetic electrons into the atmosphere due to pitch angle diffusion. One related question that is still controversial is how these waves can reach the intensities that are measured. A study of ELF/VLF hiss in the plasmasphere (Solomon et al., 1988) has shown that the observed plasmaspheric hiss intensities can be explained by the presence of a relatively high level of anisotropic energetic electron fluxes, with the wave growth rate y being directly proportional to the flux level. Sonwalkar and Inan (1989) have argued, however, that this result was based on a limited number of cases only and that lightning-generated whistlers may be a more important source for plasmaspheric hiss. In order to clear up this problem we have performed a statistical study using 75 orbits of GEOS 1, providing 48 events. All of them occurred on the day side at 4 < L < 5, where L is the McIlwain parameter, in the vicinity of the geomagnetic equator during relatively quiet geomagnetic conditions. These events have the usual characteristics of plasmaspheric hiss (frequency range and intensity). Simultaneous measurements of hiss and electron fluxes show on a statistical basis that (1) there is some trend for the strongest wave events to correspond to the largest electron fluxes, (2) the observed fluxes for the 48 events are generally much above the ''typical'' values used in previous theoretical studies, and (3) in agreement with the suggestion of Solomon et al. (1988) the statistical results are consistent with wave generation in the equatorial region, the plasmaspheric hiss being amplified during a single crossing of the waves through this region.