Densities and vibrational distribution of H 3 + in the Jovian auroral ionosphere

Observations of the H 3 + infrared emission at 2 and 4 μm have suggested that H 3 + is in local thermodynamic equilibrium (LTE) in the region of the Jovian ionosphere from which the emissions originate. We have tested this assumption by calculating the vibrational distribution of H 3 + over the altitude range of 350 to 1500 km above the methane cloud tops (1 to 4 × 10 −3 μbar). We have constructed a model of the Jovian auroral ionosphere in which the neutral temperatures are enhanced over those of the mid‐latitude ionosphere, as suggested by observations and models of the auroral region. We have modeled the precipitation of 10‐keV electrons with an energy flux of 1 erg cm −2 s −1 . Both the energy and energy flux are less than those that are implicated in the production of the UV aurora. We have computed the densities and vibrational distribution of H 3 + and find that the distribution of the six lowest states of H 3 + can be determined fairly well in spite of uncertainties in the atomic and molecular data. Since the nearly resonant transfer of vibration from H 2 (υ=1) is an important process in populating the H 3 + (υ 1 =0,υ 2 =2) state, it is necessary to model the vibrational distribution of H 2 as well. The computed altitude profiles and vibrational distributions of H 3 + and H 2 are consistent with the observations of infrared emission in the 2‐and 4‐μm regions. The H 3 + is not in LTE near and above the H 3 + peak, since loss of the H 3 + (υ 1 =0,υ 2 =1) and H 3 + (υ 1 =0,υ 2 =2) states by radiation is approximately equal to the collisional loss rate.