SLAMS at comet 19P/Borrelly: DS1 observations

摘要

Comet 19P/Borrelly plasma waves associated with the ion pickup process are being studied for the first time. The compressive plasma waves (short, large-amplitude, magnetic structures, or SLAMS) have peak-to-background magnetic field magnitude amplitudes as large as ~9-l. A new method of analysis has been developed to study the properties these compressive waves (and the bow shock) and is applied in this study. The bow shock at the time of the DS1 crossing was determined to be quasiparallel in nature with θ_(Bn)~22°. Using this new technique and minimum variance analyses over single wave cycles, most of the waves were determined to be circularly polarized, but some were noted to have linear and sunglass polarizations. The waves propagated obliquely to the ambient magnetic field B_o, with over 75% of the cases with θ_(KBO) > 45°. The intrinsic wave polarization in the plasma frame was investigated by examining waves propagating obliquely to the solar wind direction, θ_(KX), > 75°, where x is the solar wind velocity vector. From this analysis, a mix of right-hand (RH) and left-hand (LH) polarization waves were found, with no particular order, either as a function of distance upstream or closeness to the bow shock. Most of the waves were detected when B_o was oblique to V_(SW). The equal mix of RH and LH waves is not well explained by current theoretical models. It is possible that the Dubouloz and Scholer (1995) scenario could work for this cometary case if both RH and LH waves were generated by ring-beam distributions.