The electron temperature in the inner coma of comet P/Halley.

Abstract

From the CH_3_OH_2_^+^/H_3_O^+^ and H_3_O^+^/H_2_O^+^ ratios measured by the Giotto Neutral Mass Spectrometer in the inner coma (distance <20000km) of comet P/Halley we derive two independent electron temperature profiles. They agree very well and the electron temperature T_e_ increases gradually from =~100K at 2000km to =~900K at 8500km followed by a sharp increase to above 20000K at 11000km. Then T_e_ drops gradually to =~4000K at 20000km. This profile is used for modeling the absolute H_2_O^+^, H_3_O^+^, and CH_3_OH_2_^+^ densities. We obtain excellent agreement between modeled and measured ion densities provided we use the appropriate temperature dependences for the rate coefficients of ion-molecule reactions and for the dissociative recombination of ions with electrons. The up to tenfold ion density enhancement observed for some ion species for distances between 10000 and 20000km (pile-up region) is the result of the sharp T_e_ increase coupled with very low radial ion velocities in this distance range. Neither electron impact ionization nor dynamic processes play an important role.