Short-term variations of Mercury's Na exosphere observed with very high spectral resolution

Short time variations of Mercury's exosphere cannot be tracked easily from ground based observatories because of the difficulty of distinguishing them from Earth atmospheric effects. On July 13th 2008, using THEMIS solar telescope, we were able to simultaneously measure brightness, Doppler shift and width of the exospheric sodium D(2) emission line during half a day with a resolving power of similar to 370,000. Mercury's exosphere displayed an emission brightness peak in the Northern hemisphere which vanished in few hours and a more persistent Southern Hemispheric peak. The bulk Doppler shift of the exosphere suggests a period of strong escape from Mercury. The global changes of the Doppler shift and of the Doppler width suggest that a cloud of sodium atoms ejected before or at the beginning of our sequence of observations passed through THEMIS field of view moving anti-sunward. A preferentially southern ejection of sodium atoms leading to the observed persistent southern emission peak is consistent with the orientation of the Interplanetary Magnetic Field during that period. Citation: Leblanc, F., A. Doressoundiram, N. Schneider, S. Massetti, M. Wedlund, A. Lopez Ariste, C. Barbieri, V. Mangano, and G. Cremonese (2009), Short-term variations of Mercury'sNa exosphere observed with very high spectral resolution, Geophys. Res. Lett., 36, L07201, doi: 10.1029/2009GL038089.