On the excitation of Io's atmosphere by the photoelectrons:: Application of the analytical yield spectral model of SO2

The analytical yield spectral model is applied to the calculation of photoelectron fluxes, excitation rates, and photoelectron-excited intensities of several neutral and ionized S and O UV emissions, and SO (240-265 nm) and SO2 (264-430) band emissions in the atmosphere of Io. Most of the OI emission intensity is produced in the e-O direct collisional-excitation rather than in the e-SO2 dissociative excitation process. Using the existing models of Io's atmosphere the calculated brightness of neutral O and S emissions on Io is found to be 2 to 3 orders of magnitude smaller compared to the HST-observed intensity. We have shown that larger intensities can be produced on increasing the densities of the atmospheric species, particularly the atomic species: a model where O dominates over SO2 above a few tens of km altitude can explain 10-30% of the HST brightness. Our model calculations have demonstrated that it is possible to explain the HST-observed brightness provided that (a) the Io's atmospheric densities are much larger (>factor of 10) and more extended in altitude than those predicted by the existing models, and (b) there is a mechanism to accelerate the ionospheric photoelectrons to energies similar to 100 eV. Otherwise, there should be some other source of the production of these emissions on Io.