A model of inner cometary ionospheres

A hydrodynamical model is presented to describe the movement of the thermalized charged components in the inner ionosphere of comet Halley. Photoelectrons are included via applying a two-stream-type approach. The numerical scheme is able to describe shock transitions in a natural way. Solutions are obtained for a number of different assumptions concerning electron heating rates but all show that the electron temperature increases sharply where the collisional electron-neutral coupling becomes unimportant. This temperature increase is accompanied by an increase in both the plasma pressure and in its associated polarization electric field and causes the plasma flow to go subsonic. In certain cases this transition occurs as an "inner shock" which can serve as a possible explanation for the observed ion pile-up in Halley's comet.