Lack of thermal equilibrium between H+ and O+ temperatures in the Venus nightside ionosphere

Abstract

Analysis of orbiter retarding potential analyzer (ORPA) ion data recorded in the nightside Venus hydrogen bulge region has revealed that the temperature of the H+ ions is substantially cooler by approximately 2000°K than that of the O+ ions above 200 km altitude. Below 200 km altitude the H+ and O+ temperatures cross, with the H+ temperature becoming the hotter temperature. Frequently, the velocity distribution of the H+ ions appears to be non‐Maxwellian with too large a fraction of the ions having small velocities. We suggest that this may be the result of H+ ions, created from charge exchange collisions, not having completely thermalized with the ambient H+ gas. Charge exchange collisions between cold neutral hydrogen atoms at a temperature of 100°K and the hot O+ and H+ ion gases create H+ ions at a temperature of 100°K. The temporal and spatial extent of the cooled H+ gas has not been investigated in this study. We have attempted to understand the processes responsible for the observed temperature behavior by solving numerically the coupled electron, H+, and O+ energy equations in one‐dimensional form. Neglecting convective transport terms but including for the first time cooling processes resulting from charge exchange between H+ and O+ ions with cold neutral hydrogen atoms and using separate H+, O+, and electron thermal conductivities corrected for collisions, we have obtained temperature profiles for the H+ and O+ gases which are consistent with the observed profiles. The process primarily responsible for the observed behavior of the H+ and O+ temperatures appears to be the difference in the thermal conductivity of the two ion gases with the thermal conductivity of H+ being several times larger than that of O+given the same temperature and density. This result must be tempered with the fact that several processes or conditions, which may be important, have been neglected.