Non-LTE effects on emission in the jovian upper atmosphere

Calculations of column intensities are performed for a number of infrared transitions of the H 3 + molecular ion, using a model atmosphere recently produced by Grodent et al. [2001. A self-consistent model of the jovian auroral thermal structure. J. Geophys. Res. 106, 12933–12952]. The line intensities integrated along the line of sight through the model atmosphere are first computed assuming that all of the emitting energy levels are in local thermodynamic equilibrium (LTE). These results are compared with those derived from a detailed balance calculation using a method recently proposed by Oka and Epp [2004. Non-thermal rotational distribution of H 3 + . Astrophys. J. In press]. It is shown that the population of excited vibrational levels starts to depart from that derived from LTE at altitudes higher than 500 km (above the jovian cloud tops). This effect has been noted previously by Kim et al. [1992. Densities and vibrational distribution of H 3 + in the jovian auroral atmosphere. J. Geophys. Res. 97, 6093–6101]. By 2000 km, all of the excited vibrational levels are populated at less than 10% of the expected LTE value. This has important implications for the jovian upper atmosphere. In particular, the H 3 + cooling effect will be greatly reduced high in the atmosphere. Modelled LTE line emission is greater than that derived from non-LTE modelling. Comparison of the non-LTE modelling with recent spectral measurements of the jovian auroral/polar regions in the L- and K-infrared windows shows that the Grodent et al. [2001. A self-consistent model of the jovian auroral thermal structure. J. Geophys. Res. 106, 12933–12952] profile overestimates the measured line intensity by ∼3. Allowing for this, the non-LTE modelling shows that the column densities derived from (quasi-)LTE treatment of the measured line intensities may underestimate the real H 3 + abundance by a factor of between 6 and 200. This means that attempts to derive important ionospheric properties, such as conductivity and related energy inputs due to magnetosphere–ionosphere coupling, from observed spectra will need to take this into account, if they are not to be seriously in error.