Sodium D2 line profiles: clues to the temperature structure of Mercury's exosphere

The velocity distribution of atoms in an exosphere can be used to constrain source and loss processes. The velocity distribution itself can be constra ined by observing the line profile of an atmospheric emission line, The onl y atomic species known to exist in the Hermean exosphere that has a strong enough resonance transition in the visible spectrum that its line profile c an be observed from the ground with existing instruments is sodium. Line pr ofiles of the sodium D2 emission (5889 A) were obtained at two facilities. On 29 and 30 May 1997, we obtained line profiles with the 107 inch telescop e at The McDonald Observatory at coude focus. On 6 January 1998, we obtaine d line profiles at the Anglo-Australian telescope. The line profiles show t hat the sodium in Mercury's atmosphere is at least 700 It hotter than the s urface temperature throughout the atmosphere. This implies that the interac tion of the Na atoms with the surface is governed by energetic processes. ( C) 1999 Elsevier Science Ltd. All rights reserved.