Latitudinal distribution of sodium in Mercury's exosphere

Since the first detection of sodium in Mercury's exosphere, many spectroscopic observations have been performed. Potter et al. (1999) imaged Mercury in sodium D-line emissions and found enhancements at high latitudes and significant diurnal changes. On the other hand, Sprague et al. (1997) reported a significant difference in brightness between dawn-side and dusk-side. The suggested release mechanisms of sodium atoms are chemical sputtering, thermal desorption, photon-stimulated desorption (PSD), ion sputtering, and micro-meteoroid vaporization. However, a comprehensive description of Mercury's sodium exosphere is still not available. Mercury has a very eccentric orbit, with a perihelion at 0.31 AU and an aphelion at 0.47 AU. Two-dimensional images of Mercury's sodium exosphere have been accumulated by Potter and Morgan (1990, 1997), Potter et al. (1999,2002), and Schleicher et al. (2004), which showed enhancements at high latitudes, but no 2D images have been obtained when the Sun-Mercury distance was shorter than 0.33 AU. Killen et al. (1990), Sprague et al. (1997), and, Domingue et al. (1997) reported the results of slit spectroscopic observations, some of which were carried out when the Sun-Mercury distance was shorter than 0.33 AU. North-South enhancements and asymmetries in column abundance were observed only when Sun-Mercury distance was more than 0.33 AU. We carried out the observations of sodium in Mercury's exosphere with Fabry-Perot interferometer at Haleakala High Altitude Observatory in Hawaii in March, 2005. We found enhancements at low latitude when the Sun-Mercury distance was 0.31 AU. The enhancement at low latitude may be due to PSD and thermal desorption, and the enhancements at high latitude may be due to ion-sputtering, which suggests that PSD and thermal desorption is a dominant source process when the Sun-Mercury distance is less than 0.33 AU, and ion-sputtering is dominant when the Sun-Mercury distance is more than 0.33 AU. This may be due to dependence of PSD rate on the surface temperature or relatively higher orbital speed of Mercury at perihelion. In this presentation, we report the results and discuss this phenomenon in detail.