In Search for Background Solar Wind Effects on Mercrury's Na Exosphere

Abstract:

Sodium plays a special role in Mercury’s exosphere: due to its strong resonance line it has been observed and monitored by Earth-based telescopes for decades. Different and highly variable patterns of Na-emission have been identified, the most common and recurrent being the high latitude double-peak pattern. It is clear that the exosphere is interacting with the surface and with the magnetosphere under the influence of solar wind and the interstellar medium, but the role and weight of the single processes are still under discussion.
The THEMIS solar telescope operating on Tenerife (Canary islands) has observed Mercury’s Na exosphere along several campaigns since 2007. A dataset of images taken between 2009 and 2013 are analysed here in relation with propagated solar wind data. A small subset of the images shows a low level of correlation between Na-emission and solar wind dynamic pressure. The amount of data at present is not sufficient to make a clear statement on whether the correlation is a coincidence or can be explained by other factors (position of Mercury and Earth, solar activity, etc.). Nevertheless, the authors present a comprehensive study taking into account several factors: the distance to the Sun, position in relation to the ecliptic plane, rotation of Mercury, solar UV and X-ray radiation, solar wind dynamic pressure and true anomaly angle (TAA) of Mercury.