THE ELECTRON-TEMPERATURE IN THE INNER COMA OF COMET-P/HALLEY

From the CH3OH2+/H3O+ and H3O+/H2O+ ratios measured by the Giotto Neutral Mass Spectrometer in the inner coma (distance < 20 000 km) of comet P/Halley we derive two independent electron temperature profiles. They agree very well and the electron temperature T(e) increases gradually from almost-equal-to 100 K at 2000 km to almost-equal-to 900 K at 8500 km followed by a sharp increase to above 20 000 K at 11 000 km. Then T(e) drops gradually to almost-equal-to 4000 K at 20 000 km. This profile is used for modeling the absolute H2O+, H3O+, and CH3OH2+ 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 10 000 and 20 000 km (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.