Photoelectron excitation of Io's atmosphere

Abstract

A Monte Carlo model is developed to simulate the energy degradation of photoelectrons in the SO₂ atmosphere of Io. The photoelectron production rates are calculated for solar minimum condition. Due to the limitations in our knowledge about the Io's atmosphere, a pure SO₂ exponential atmosphere is considered in this study for 'thick' (surface number density 5×10¹²cm^-3) and 'thin' (5×10¹⁰cm^-3) atmosphere. The calculated brightness of the neutral S and O emissions are found to be three orders of magnitude less compared with the Hubble Space Telescope observed values. Taking all photoelectron of energy 100 eV, created via some heating process, also do not produce appreciable SI and OI intensities. We propose that most of the SI and OI emissions are likely to be produced through direct collision of electrons with atomic oxygen and sulfur in the upper atmosphere of Io.