Properties of whistler mode wave packets at the leading edge of steepened magnetosonic waves: Comet Giacobini-Zinner

Abstract

We examine the physical characteristics of high frequency wave packets which are detected at the steepened edge of magnetosonic waves near comet Giacobini-Zinner. Peak-to-peak wave amplitudes (Δ B) can be large, with Δ B |B|≈ 1.5. The waves are left-hand polarized in the spacecraft frame and typically develop as a smooth progression in amplitude and phase from the leading edge of the low frequency magnetosonic waves. In over half of the cases examined, the wave oscillations decrease approximately linearly with time, ruling out electron Laundau damping as the dominant mechanism for the amplitude fall off. We surmise that the wave packets play an integral role in the reorientation and reduction in field magnitude from the steepened magnetosonic waves to the upstream ambient field. Observed wave packets have durations between 6 and 45 s with an average value near 18s. The number of oscillations within a packet can vary from one to over 20. In general, packets with more oscillations have the lowest wave periods. The wave period within any given packet is approximately constant, but it can vary from 1.3 to 11 s from event to event. Small frequency shifts are also observed within each packet. Generally, higher frequencies occur at the small amplitude (upstream) end. Although one might expect a relationship between the above wave properties and the field gradient across the steepened edge of the magnetosonic waves, no obvious correlations were found. Since the high frequency wave packets are observed to be attached to the long period magnetosonic waves, they too propagate upstream and are blown back by the solar wind across the ICE spacecraft. The observed properties of the wave packets are therefore consistent with anomalously Doppler shifted righthand polarized waves. The frequency in the solar wind frame is computed to be between 1 and 10 times the proton gyrofrequency. An empirical model for the structure of the wave packets is presented and potential theories for the generation mechanisms are discussed.