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

Abstract

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 ∼100 eV. Otherwise, there should be some other source of the production of these emissions on Io.