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R.M. Killen (SwRI), B. Giles (Goddard SFC), A.E. Potter (LPI), B.V. Jackson (UCSD), J. Linker, Z. Mikic (SAIC)
Earth-based observations of Mercury's sodium atmosphere have revealed large-scale spatial variations and rapid temporal variations (e.g. Potter and Morgan, Adv. Space Res. 19, 1571, 1997; Potter, A.E. et al., Planet. Space. Sci., In press, 1999). We show how the observed brightness variations may be related to the topology of Mercury's magnetosphere in response to solar wind variations at Mercury's orbit. Using the Toffoletto-Hill magnetosphere model modified for Mercury (JGR 98, 1339, 1993), we have calculated the structure of the Hermean magnetosphere for November, 1997, corresponding to dates of our sodium images obtained at the National Solar Observatory at Kitt Peak, Arizona. The sodium images are reduced to column abundance using a Chamberlain type atmosphere with optically thick radiative transfer. Inputs to the magnetosphere model are solar wind density and velocity, and interplanetary magnetic field (IMF). Solar wind density and velocity at the orbit of Mercury are inferred from heliospheric tomography using radio scintillation measurements (Jackson et al., Adv. Space Res. 20, 23, 1997; Kojima et al., JGR 103, 1981, 1998). The IMF at Mercury's position is obtained from a model of the inner heliosphere constrained with the solar magnetic field, in combination with density and temperature profiles at the sun's surface (Linker et al., JGR 104, 9809, 1999). We use the morphology of the magnetosphere along with solar wind parameters to infer the rates of ion sputtering of sodium. The total sodium column is the sum of ion-sputtered sodium and the more slowly varying sources, photon-stimulated desorption and meteoritic vaporization. We show that the variations in ion sputtering and subsequent loss via ioniation may be responsible for rapid changes in the observed brightness distribution in sodium emissions. Our goal is to show how solar activity and Mercury's sodium exosphere are related.