Studies of release and loss processes of Mercury's atmosphere investigated with ground based optical observations

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1) Development of the 2D imaging system for Mercury's atmosphericThe 2D imaging system for detection of the atmospheric weak emission layer surrounding Mercury is successfully developed based on the corona-graph technique. The observation wave length is set at the emissions of Na-D lines. The contrast between a weak emission surrounding a bright emission region (observation object) and the bright region is achieved to be 1:1000.2) Observations of Mercury's atmosphere: current statusThe observations were made for Dec. 2001 and April 2002. Due to a hard atmospheric smearing effect caused by a seasonal wind and low altitude of Mercury, reliable images have not been acquired successfully for the periods. However, we could establish observation and analysis methods which are required to derive 2D image of the weak emission layer of Mercury's atmosphere.3) Monte-Carlo simulation analysis for Mercury's atmosphereA Monte-Carlo simulation analysis for neutral and ionized particles has been made to investigate generation processes of Mercury's extended atmosphere, which is observed as the weak emission layer. As the result, it is suggested that plausible processes of the extended atmosphere are ion-sputtering or/and meteoritic vaporization, and the quantitative 2D imaging for the weak emission layer is quite important to determine the proper gas release process.