Detection of neutral sodium above Mercury during the transit on 2003 May 7

The extent of the exosphere of Mercury above the planet's limb could for the first time be observed by detecting an excess absorption in the solar sodium line D2 during the transit of Mercury across the solar disk on 2003 May 7. The observations were performed with a 2d Fabry-Perot spectrograph of the Vacuum Tower Telescope at Izaña, Tenerife. The absorption excess, blue-shifted by 13 pm relative to the solar line, is mainly concentrated near the polar regions. There, the absorption excess can be traced up to ≈700 km above the limb. Between the two polar regions, along the eastern limb, a weaker absorption excess can be seen. A possible streamer-like feature stretches more than 2000 km above the northern region. Assuming the density to decrease exponentially with height, we derive for the polar maxima vertical column densities of 3 $\times$ $10^{10}~{\rm cm}^{-2}$, volume densities at the surface of 2.5 $\times$ $10^3~{\rm cm}^{-3}$, and a density scale height of 150 km