On the Hardness of the Photoelectron Energy Spectrum Near Mars

Photoelectrons are an important population of the dayside Martian upper atmosphere that contributes substantially to the local ionization and heat balances. This study is focused on the broad shape of the photoelectron energy spectrum at 10-50 eV. Assuming a power law distribution, we derive the hardness of each photoelectron energy spectrum measured on the dayside of Mars by the Solar Wind Electron Analyzer on board the Mars Atmosphere and Volatile Evolution spacecraft. Our analysis reveals two distinctive features: (1) The hardness tends to increase with decreasing altitude down to 140 km and then decrease toward lower altitudes; (2) the hardness tends to decrease with increasing solar zenith angle. These variations are restricted to regions below 250 km whereas the hardness appears to be fairly constant at higher altitudes. In practice, the hardness is modulated by the ionization of atmospheric neutrals by solar Extreme Ultraviolet and X-ray photons, inelastic collisions with atmospheric neutrals, Coulomb collisions with ionospheric thermal electrons, and photoelectron transport. While inelastic collisions tend to harden the photoelectron energy spectrum, both primary production and Coulomb collisions tend to soften it instead. We argue that the observed variations of the hardness can be understood in terms of the relative contributions of different processes. Our analysis also suggests the presence of appreciable temporal variability in hardness with a typical timescale of 10 min.