Extraordinary mode resonance cones in the Uranian magnetosphere?

Abstract

A theory is introduced for previously unidentified waveforms involving decreasing frequency with time that were picked up in the Voyager 2 flyby of Uranus. They are identified as a possible band of extraordinary waves detected on resonance cone surfaces. Because of the resonance cone phenomenon, certain frequencies are only observed at certain angles from the source, and the frequency decreases as the angle decreases. The homogeneous plasma theory is shown to predict an almost linear drop in frequency with time. In principle one can use the wave frequency with time to estimate the distance and angle of the source for the homogeneous case. Inhomogeneities in the plasma are shown to produce a substantial variation on that and give the waveforms an important fine structure. They give arches to the waveform and can explain sudden drops in the frequency and changes in the slope of the waveforms at certain times, as is observed in the data. Inhomogeneities from a plasma wave (possibly an Alfvén wave) near the spacecraft are shown to explain the modulation in one of the waveforms. A decreasing magnetic field of growing negative slope can independently account for the arching over of the waveform and the decreasing modulation frequency. The multiple peaks in the frequency observed for given times in the waveform may be caused by the multiple peaks from the resonance cone interference structure, which are predicted in the warm plasma theory of resonance cones.