Whistler Waves Associated with the Uranian Bow Shock: Outbound Observations

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

Journal of Geophysical Research, A96, 15,841-15,852 (1991)

Abstract:

The encounter of the Voyager 2 spacecraft with the Uranian planetary system led to a series of outbound crossings of the Uranian bow shock between January 27 and January 30 of 1986. Examination of magnetic field data recorded in close proximity to the shock reveals a series of whistler wave events that appear to result from processes associated with the shock. Two wave events display two separate and simultaneous wave enhancements each, one at approximately 0.2 Hz and the second more nearly at 1 Hz in the spacecraft reference frame. The actual frequencies of the waves vary with the two events. Both wave enhancements of the two events are right-hand polarized in the spacecraft frame of reference. We have examined these wave events using high-resolution magnetic field data and conclude that they are analogous to those whistler waves upstream of the Earth's bow shock that are driven by beams of electrons. Using observations by the plasma wave science experiment on Voyager 2 to infer the likely presence of electron beams with modest beam speeds, we present an instability analysis and show that a single electron beam with reasonable parameters can generate both of the observed whistler wave forms simultaneously. A third wave event is also seen which occurs both upstream and downstream of a shock crossing. This event displays only one relatively broad spectral enhancement in the same frequency regime and is left-hand polarized in the spacecraft frame. We argue that this event is most likely the result of a gyrating proton distribution associated with the oblique shock. As such, it is analagous to whistler waves observed earlier during the inbound crossing of the Uranian shock.