Ion emission line profiles in cometary plasma tails

Abstract

We present a first attempt to model the profile of ion emission lines in a cometary plasma tail with the aid of a 3D-MHD model of the comet-solar wind interaction, and to compare the calculated line profiles to high-resolution measurements of the H_2_O^+^ doublet at 6159Å in comet 1P/Halley on January 6, 1986. The modelled line profiles are asymmetric and show a high velocity wing originating from fast ions in the outer parts of the tail along the line-of-sight. Comparison with the measured profiles shows that such high-velocity wings are indeed observed and are needed to fully approximate the observed profiles. We performed a quantitative analysis of the contribution of fast ions to the ion flow to examine if they are really sufficient to explain the discrepancy between measured H_2_O^+^ production rates and the rates expected from photoionization of water. It is found that the combined effect of fast ions and the limited field-of-view can indeed reduce the measured flux. However, the reduction seems not be sufficient to explain the whole discrepancy.