LACK OF THERMAL-EQUILIBRIUM BETWEEN H+ AND O+ TEMPERATURES IN THE VENUS NIGHTSIDE IONOSPHERE

Analysis of orbiter retarding potential analyzer (ORPA) ion data recor ded in the nightside Venus hydrogen bulge region has revealed that the temperature of the H+ ions is substantially cooler by approximately 2 000 degrees K than that of the O+ ions above 200 km altitude. Below 20 0 km altitude the H+ and O+ temperatures cross, with the H+ temperatur e becoming the hotter temperature. Frequently, the velocity distributi on of the H+ ions appears to be non-Maxwellian with too large a fracti on of the ions having small velocities. We suggest that this may be th e result of H+ ions, created from charge exchange collisions, not havi ng completely thermalized with the ambient H+ gas. Charge exchange col lisions between cold neutral hydrogen atoms at a temperature of 100 de grees K and the hot O+ and H+ ion gases create H+ ions at a temperatur e of 100 degrees K. The temporal and spatial extent of the cooled H+ g as has not been investigated in this study. We have attempted to under stand the processes responsible for the observed temperature behavior by solving numerically the coupled electron, H+, and O+ energy equatio ns in one-dimensional form. Neglecting convective transport terms but including for the first time cooling processes resulting from charge e xchange between H+ and O+ ions with cold neutral hydrogen atoms and us ing separate H+, O+, and electron thermal conductivities corrected for collisions, we have obtained temperature profiles for the H+ and O+ g ases which are consistent with the observed profiles. The process prim arily responsible for the observed behavior of the H+ acid O+ temperat ures appears to be the difference in the thermal conductivity of the t wo ion gases with the thermal conductivity of H+ being several times l arger than that of O+ given the same temperature and density. This res ult must be tempered with the fact that several processes or condition s, which may be important, have been neglected.