The wave distribution functions of plasmaspheric ELF hiss: GEOS 1 observation in the equatorial region

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The wave distribution functions of plasmaspheric ELF hiss have been determined at the equatorial region inside the plasmapause, by applying the maximum entropy method to the data observed by GEOS 1 satellite. Three equatorial events have been analyzed, and it is found that just inside the plasmapuse, the wave normal direction θ of ELF hiss is nearly aligned with the magnetic field, and when the observing position is 0.3-0.5 R_e (R_e : earth's radius) inside from the plasmapause, there are two different groups of wave normal angles; one is a medium wave normal angle ranging from 20°to 60° and the other is a large wave normal in a range from 70°to 80°, slightly smaller than the Gendrin angle. The present direction finding results of equatorial ELF hiss have been extensively compared with the previous theoretical model to discuss the generation mechanism of plasmaspheric ELF hiss. Then, it is concluded that some of the plasmaspheric hiss emissions are generated by the cyclotron instability with θ≅0°, initially proposed by R. M. THORNE et al. (J. Geophys. Res., 78,1581,1973), but a considerable number of ELF hiss emissions seem to be generated at large wave normals.
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