The Effects of Enhanced Fluxes of Soft Xrays on the Ionospheres and Thermospheres of Venus and Mars

We compare here the effects of enhanced fluxes of soft xrays, such as those measured by the SNOE satellite (e.g., Bailey et al., 1999) on models of the lower thermospheres/ionospheres of Venus and Mars. Electron density profiles measured by the Radio Science Experiment on Mars Global surveyor show a lower peak or shoulder below the main peak that is ascribed to photoionization by soft xrays and subsequent multiple ionizations by the high energy electrons that are produced. A lower peak was often observed in the electron density profiles measured by the Pioneer Venus Orbiter Radio Occultation experiment. Similar effects on the Earth's electron density profiles have been modeled (e.g., Solomon et al., 2001), although on Earth, one of the primary effects is on the neutral thermosphere. The densities of NO are observed to be enhanced in the region of soft xray absorption (e.g., Barth et al., 1996). On Venus and Mars, odd nitrogen, mostly in the form of atomic nitrogen, is also enhanced in the region of soft xray absorption, but increased fluxes of soft xrays cannot account for the NO densities measured in the Martian lower thermosphere by Viking.