Direction finding of ELF hiss emissions in a detached plasma region of the magnetosphere
Résumé
Wave normal directions and wave distribution functions of ELF hiss emissions in a detached plasma region of the magnetosphere have been determined, using data from the geostationary satellite GEOS 2 located in the equatorial plane at L = 6.6. Three different methods of direction finding have been utilized: (1) Means's method based on the hypothesis of a single plane wave, (2) the maximum likelihood method, assuming a few plane waves, and (3) the maximum entropy method of determining the wave distribution function. An intercomparison of the results from those methods has proven that they provide very reliable and definite information on the wave normals. From analyses of two equatorial ELF hiss events it is found that the wave distribution function is composed of a single peak (i.e., the waves have a single propagation direction) and that the wave normals of the ELF hiss originating in the detached plasma region make very small angles with the Earth's magnetic field; also, we were able to make an experimental estimate of the angular width of the unstable cone of the emissions. Both our initial direction finding results in the equatorial plane and the corresponding previous results at higher geomagnetic latitudes are compared with theoretical predictions, assuming that the waves are generated by the electron cyclotron instability due to medium-energy (3-30 keV) electrons. It is concluded that all of the characteristics (morphology and direction-finding results) are consistent with the electron cyclotron instability.
Domaines
Planète et Univers [physics]
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