Mercury's sodium exosphere and interplanetary dust distribution

The interplanetary dust (IPD) distribution in the inner solar system is not yet well understood because of lack of direct dust measurements in the inner solar system and so one needs to rely on zodiacal light observations that are difficult to interpret. Mercury has a thin and unstable atmosphere, and the source processes of Na in its atmosphere are unclear. Results of past observations have revealed that the atmospheric Na density has no or low correlation with the solar flux, sunspot number, heliocentric distance, or solar radiation pressure. We show that the variability of Mercury’s atmospheric Na density depends strongly on the IPD distribution. That is, Na density is low (high) when Mercury is far away from (close to) the symmetry plane of IPD, and so one can infer the IPD distribution near Mercury orbit from the temporal variability of Na density in Mercury’s atmosphere. In this presentation, we report the new result of observation performed from 2008 to 2009, and the correlation between sodium density in Mercury's exosphere and interplanetary dust distribution near Mercury. Additionally, we plan to observe the emission from Mercury's exosphere at Okayama Astrophysical Observatory in Japan in the Messenger 3rd flyby. We will also report preliminary results (if we would succeed in the observation.)