The wave normals of ELF hiss emissions observed onboard GEOS 1 at the equatorial and off‐equatorial regions of the plasmasphere

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Abstract
The wave normal directions of plasmaspheric ELF hiss have been determined at the equatorial and off‐equatorial regions of the plasmasphere by using the data observed by GEOS 1 satellite. Two different methods of direction findings have been utilized: (1) the maximum entropy method for estimating the wave distribution functions and (2) the maximum likelihood method to estimate the propagation peaks. Three equatorial events have been analyzed, and it is found that just inside the plasmapause, the wave normal θ of ELF hiss is nearly aligned with the magnetic field, and when the observing position is 0.3–0.5 RE 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 the Gendrin angle. Then, off‐equatorial observations have been made at geomagnetic latitudes 30° to 24° for L = 4.66 to 2.90 inside the plasmasphere. The direction findings have yielded that the waves are always composed of two plane waves; both main and secondary peaks take very large wave normal angles, θ = 80°–85°. 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 Thorne et al. (1973), but majority of ELF hiss emissions are generated at large wave normals.