Effect of the planet shine on the corona : Application to the Martian hot oxygen

Abstract : Systematic observations of the Martian hot oxygen corona by IUVS on MAVEN can be used to constrain estimates of the current neutral oxygen escape rate. In this paper, we investigate the effect of the photons emitted from the thermosphere and lower exosphere on the emissivity of the hot oxygen corona at 130.4 nm. We compare this source of illumination, generally neglected, to the direct solar illumination used to convert the O I 130.4 triplet brightness into line-of-sight column density. This study is performed using a radiative transfer model with two maxwellian oxygen populations, assuming spherical symmetry for the cold and hot oxygen densities. Contribution to the corona from the illumination of the exosphere by the sunlit atmosphere depends on the amount of cold oxygen, and varies with altitude and solar zenith angle. An analytic formulation to take into account variations of this effect with respect to the hot oxygen kinetic temperature is proposed. The effect of the atmosphere sunlit should be general and occur on other planets for other very optically thick resonance lines.