NON-LTE EFFECTS ON THE H3+ROVIBRATIONAL POPULATION IN THE JOVIAN IONOSPHERE

Abstract

We investigate non-LTE effects on the H₃? level populations to help the analysis of the observed 2 and 3.5 micron H₃? emissions from the Jovian ionosphere. We begin by constructing a simple three-level model, in order to compute the intensity ratio of the R(3,4) line in the hot band to the Q(1,0) line in the fundamental band, which have been observed in the Jovian auroral regions. We find that non-LTE effects produce only small changes in the intensity ratios for ambient H₂ densities less than or equal to 5×10 11 ㎝ ?3 . We then construct two comprehensive models by including all the collisional and radiative transitions between pairs of more than a thousand known H₃? rovibrational levels with energies less than 10000 ㎝ ?1 . By employing these models, we find that the intensity ratios of the lines in the hot and fundamental bands are affected greatly by non-LTE effects, but the details depend sensitively on the number of collisional and radiative transitions included in the models. Non-LTE effects on the rovibrational population become evident at about the same ambient H₂ densities in the comprehensive models as in the three-level model. However, the models show that rotational temperatures derived from the intensities of rotational lines in the v₂ and 2v₂ bands may differ significantly from the ambient temperatures in the non-LTE regime. We find that significant non-LTE effects appear near and above the H₃? peak, and that the kinetic temperatures in the Jovian thermospheric temperatures derived from the observed line ratios in the 2 and 3.5 micron H₃? emissions are highly model dependent.