Oxygen Auger electrons observed in Mars' ionosphere

Over the course of 290 orbits, the Electron Reflectometer onboard Mars Glob al Surveyor consistently observed a plasma boundary at a median altitude of 380 km, where electron fluxes at energies greater than similar to 100 eV c hange abruptly by about an order of magnitude. Above the boundary, electron energy spectra are consistent with solar wind electrons that have been sho cked and then cooled by impact with exospheric neutrals. Below the boundary , electron energy spectra exhibit a broad feature from 20 to 50 eV, which l ikely results from a blend of unresolved. photoionization peaks that have b een predicted by published models of ionospheric photoelectrons at Mars. We attribute a second feature at similar to 500 eV to oxygen Auger electrons. The 500-eV flux level measured below the boundary responds to variations i n the solar soft x-ray flux and is consistent with a balance between photoi onization and loss by impact with atmospheric neutral atoms.