EDP Sciences
Free Access
Volume 425, Number 3, October III 2004
Page(s) 1119 - 1124
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361:20040477

A&A 425, 1119-1124 (2004)
DOI: 10.1051/0004-6361:20040477

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

H. Schleicher1, G. Wiedemann2, H. Wöhl1, T. Berkefeld1 and D. Soltau1

1  Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
    e-mail: schleicher@kis.uni-freiburg.de
2  Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

(Received 19 March 2004 / Accepted 1 June 2004)

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 D 2 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 $\approx$ 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.

Key words: planets and satellites: individual: Mercury

© ESO 2004