Over the course of 290 orbits, the Electron Reflectometer onboard Mars Global Surveyor consistently observed a plasma boundary at a median altitude of 380 km, where electron fluxes at energies greater than ~100 eV change abruptly by about an order of magnitude. Above the boundary, electron energy spectra are consistent with solar wind electrons that have been shocked and then cooled by impact with exospheric neutrals. Below the boundary, electron energy spectra exhibit a broad feature from 20 to 50 eV, which likely results from a blend of unresolved photoionization peaks that have been predicted by published models of ionospheric photoelectrons at Mars. We attribute a second feature at ~500 eV to oxygen Auger electrons. The 500-eV flux level measured below the boundary responds to variations in the solar soft x-ray flux and is consistent with a balance between photoionization and loss by impact with atmospheric neutral atoms. ¿ 2000 American Geophysical Union