Wc. Knudsen et al., LACK OF THERMAL-EQUILIBRIUM BETWEEN H+ AND O+ TEMPERATURES IN THE VENUS NIGHTSIDE IONOSPHERE, J GEO R-S P, 102(A2), 1997, pp. 2185-2191
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.