Determination of comet Halley Gas emission characteristics from mass loading of the solar wind

Abstract

We set out to fit the velocity profile of the solar wind during Giotto's approach to comet Halley with the mass loading produced by a simple model of the neutral particle distribution. The model assumes a constant gas production rate, Q, ionization rate, ν, and a radial expansion velocity, Ve, from the comet. It is used to calculate the implanted ion flux at Giotto for any given time and position along the spacecraft trajectory. Comparing the added flux with Giotto solar wind proton data from the inbound leg outside the bow shock, we compute the ratio of the total mass‐loaded ion flux to the solar wind flux at the spacecraft. Hence using equations given by Galeev et al. (1985) we infer values of the solar wind velocity, u∞, far upstream from the comet. Imposing the condition that u∞ should be as nearly as possible constant in time, we deduce values of Q and the ratio Ve/ν required to fit the model to our data. The values obtained are consistent with those derived by more direct methods. Finally, we use our Q, Ve and ν values to predict a possible innermost shock boundary.