Photoelectron fluxes in the Martian ionosphere

Abstract

The equilibrium photoelectron distribution in the upper atmosphere of Mars is calculated and compared with a typical theoretical model of the photoelectron distribution in the upper atmosphere of the earth. The neutral upper atmosphere and ionosphere models used in the calculations for Mars are based on the Viking 1 neutral mass spectrometer and retarding potential analyzer data. The corresponding models in the case of the earth are based on Arecibo incoherent scatter data. While the primary photoelectron spectra in Mars' and earth's ionospheres are remarkably similar in spectral shape and finer structure features, the Martian equilibrium photoelectron spectrum is more structured and much softer than the earth's. The equilibrium photoelectron spectrum is used to investigate the loss of the photoelectron kinetic energy to the various constituents of the upper atmosphere of Mars, including the heating of the ambient electron gas. The ionization rates by photoelectron impact are calculated and are found to contribute about 30% to the total ionization rate on Mars. Photoelectron impact excitation rates of the A ²Π u and B ²Σ u + states of CO 2 + and the a ³Π and A ¹Π states of CO are calculated and compared with the corresponding excitation rates by other mechanisms. Photoelectron impact excitation is found to contribute 20–30% to the CO 2 + and CO airglow of Mars.