The origin and propagation of chorus in the outer magnetosphere

Abstract

Wave normals of chorus in the outer magnetosphere have been determined for the first time from data obtained with the Ogo 5 search coil magnetometer. These measurements combined with simultaneous information concerning geomagnetic field, plasma density, and the electron energy and pitch angle distributions provide a consistent picture of the generation, propagation, and subsequent damping of chorus in agreement with theory. Specifically, the data are consistent with chorus generation within 25° of the equatorial plane on the dayside and within 2° on the nightside. Chorus is generated by a Doppler-shifted cyclotron resonance with electrons between 5 and 150 keV but only when the pitch angle distribution is peaked at 90° to and the anisotropy exceeds a critical value. In the source, wave normals are contained within an unstable generation cone of 20° half angle about . Away from the source the data indicate that both unducted and ducted propagation of chorus can occur at different times. The ducted chorus wave normals are contained within a cone with a half angle of 40° about out to magnetic latitudes of 50°, the limit of the measurements. Such ducts must have relative density enhancements of about 50% above background. In the case of unducted chorus the size of the wave normal cone is approximately constant, and its axis remains close to the magnetic meridian plane. However, within that plane the axis direction varies continuously from one parallel to (θ = 0°)to one perpendicular to pointing away from the earth (θ = 90°). When θ > 25°, attenuation becomes strong and very little chorus was detected with wave normals beyond θ = 60°. Little evidence for chorus reflection, which can occur when θ = 90°, was found. This lack of evidence is probably due to the fact that the wave normal measurement was not successful with weak signals.