Wave normal directions and wave distribution functions for ground‐based transmitter signals observed on GEOS 1

Abstract

VLF whistler mode pulses of ∼1‐s duration emitted by the omega navigation system transmitter located in Norway have been observed on‐board the GEOS 1 satellite. The wave normal directions of 7 pulses have been determined by three different methods of analysis: the cross product method, Means' method, and the maximum entropy method of determination of the wave distribution function (WDF). The first method is based on the interpretation of measurements of the three magnetic wave field components in a 300‐Hz band, including the omega frequency and the second and the third method on the interpretation of the spectral matrix elements of the three magnetic wave field components at the Omega frequency. While the cross product method and Means' method assume that the electromagnetic wave field is that of a single plane wave propagated in a fixed direction with respect to the ambient magnetic field, the WDF method assumes the wave field to be random. The three methods and their solutions are compared and discussed in terms of the statistical properties of the signals (signal to noise ratio, stationarity in time, degree of polarization, ellipticity), and in terms of sensitivity to inconsistencies in the data. Some recommendations are made concerning the best choice of method in given situations. The wave normals are found to have angles ∼130° with the earth's magnetic field direction, pointing slightly eastward of the local magnetic meridian. They exhibit a time variation with a 0.2‐ to 0.4‐s periodicity.