Updated Photon Scattering Coefficients (g_values) for Mercury’s Exospheric Species

Abstract

In an optically thin exosphere such as those of Mercury and the Moon, the optically stimulated emissions are directly proportional to the stimulating solar flux. An important quantity in relating measured emission to column abundance in remote-sensing observations is the g_value. The g_value is proportional to the f-value of the emission line and to the solar flux at the emission frequency. The solar flux varies inversely with the heliocentric distance from the Sun, but also with the heliocentric relative velocity of the emitting atom. The heliocentric relative velocity of the planet Mercury varies by ±10 km s−1, however, the atoms may additionally have high velocities relative to the planet owing to high-energy processes or through acceleration caused by radiation pressure. In this paper we extend g_values to heliocentric relative velocities ±50 km s−1. In addition to 12 species for which g_values have been published previously, we have computed g_values for two additional species, Mn and Al, that were discovered in Mercury’s exosphere in the course of the MESSENGER mission. We have not included updated g_values for C or OH, as these were not observed by MESSENGER. In this work, we employ the TSIS-1 hybrid reference solar spectrum for most calculations, with the SOLSTICE data being used in the EUV/far-UV. We note that the method employed here could be extended for application at extrasolar planets and comets, with the solar spectrum substituted by an appropriate stellar spectrum.