Characterization of the sources of VLF hiss and chorus observed on Geos 1

Abstract

Ray‐tracing techniques are used to locate the apparent source regions of natural electromagnetic emissions observed on the GEOS 1 satellite and to determine the wave normal directions at this apparent source region. They are applied to VLF hiss, whose analysis in terms of the wave distribution function (WDF) has established that they often consisted of a sum of two or more very distinct wave packets at the point of observation. An example of a two‐peaked WDF chorus is also examined. A two‐dimensional ray‐tracing program is used. For the cases analyzed here, GEOS 1 data indicate that the earth's magnetic field is to be considered as dipolar, and a diffusive equilibrium model is used to describe the medium. It is shown theoretically that even if the exact location of the source region is difficult to estimate accurately, more precise information can still be obtained on the wave normal distribution at the source region. In the examples considered, it is pointed out that the VLF hiss was generated in the vicinity of the equator at high wave normal angles.