Whistler Wave Bursts Upstream of the Uranian Bow Shock

C. W. Smith, M. L. Goldstein and H. K. Wong

Journal of Geophysical Research, A94, 17,035-17,048 (1989)

Abstract:

Large-amplitude magnetic field waves have been recorded by Voyager 2 upstream of the Uranian bow shock. Three distinct wave types have been observed during the 24 hours prior to the inbound shock crossing, with the bulk of the observations coming within 45 minutes of the inbound shock crossing. One wave type is seen at 20-40 mHz in the spacecraft frame of reference and exhibits right-hand eliptical polarization. The second waveform is seen at 0.1-0.4 Hz and is predominantly left-hand polarized in the spacecraft frame. Both of the above wave types are seen as nearly monochromatic oscillations with minimum variance directions at moderately oblique angles to the mean magnetic field. Both are seen only as sudden, intense bursts that last for at most 4 minutes. Both waveforms occur simultaneously with observations of energetic charged particle populations, although they are not seen during the period of greatest particle intensity, and both are observed only within and immediately upstream of the shock pedestal. We argue that both waveforms are obliquely propagating whistler waves driven unstable by gyrating proton populations. The third wave type is not monochromatic, exhibits spacecraft frame frequencies from 5 to 50 mHz, and is seen 10-17 hours prior to the inbound shock crossing. We argue that these fluctuations are also whistler waves and that the phase speeds and group velocities are sufficient for them to propagate against the solar wind.