Mercury has thin atmosphere around the solid surface. Sodium (Na) is one of the abundant elements with strong emissions. There have been some observations reporting asymmetric distribution of the Na atmosphere between the dawn- and dusk-sides and between the high and low latitude regions, and its temporal variability of density distribution. The suggested source processes are photon-stimulated desorption by solar UV, solar wind ion sputtering, and micro-meteorite vaporization. It is suggested that interaction between solar wind and Mercuryfs magnetosphere causes the observed asymmetric distribution and temporal variation, however, even the dominant source process is still unclear. Hinode/SOT is only the instrument to get entire distribution of the atmosphere on the dayside with (1) spatial resolution enough to get latitudinal distribution of the Na atmosphere and (2) spectrum resolution to get emission of the Na line. The superior conjunction in May 2013 is a good opportunity to observe the dayside atmosphere of Mercury with Hinode/SOT because Mercury will lie exactly opposite to the Earth.
According to our estimation, we are able to catch signals from Mercury atmosphere by integrating more than ~1000 FG images with SOT. The images can be acquired if we run the observation with the highest temporal cadence for one hour during the passage of Mercury above the solar limb. The contrast of Mercury increases if we are able to capture emission of the Na D line from the Mercury atmosphere. This is why we run the wavelength scan with SOT/NFI to cover the expected emission wavelength.