Ion Temperature and Flow Profiles in Comet p/ Halley's Close Environment

Abstract

The HIS instrument of the ion mass spectrometer on board the Giotto spacecraft identified the contact surface at 4800 km distance from the comet nucleus. This boundary is clearly seen by a drastic drop in the temperatures of different ion species from about 2000 K outside to values as low as 300 K inside. Also, inside the contact surface an outflow speed of >1 km s^-1 was measured, in contrast to a value around zero right outside. We discuss how these numbers might be affected by a potential charge-up of the spacecraft. Outside the contact surface, the ion temperature rises gradually with increasing distance. Between 9000 and 10000 km distance the ion density increases by a factor of 4. This "ion pile-up" is not yet explained uniquely. Between 25000 and 28000 km distance there is again a rather abrupt jump to significantly higher temperatures and higher outflow speeds.