It is interesting to note that while the velocity structure of the escaping plasma can lead to broader absorption that helps to explain the transit depths, it is not necessarily detectable in the data. For instance, Fig. 7 shows the transit depths based on the SOL2 model that has a relatively high radial velocity reaching 11 km s−1 by 5Rp. The velocity structure is not detectable because the optical depth of the high velocity material is not sufficient, the LOS velocity at the limb of the planet is slower than the radial velocities in general, and because spectral line broadening within the COS instrument smooths the structure out of the line profiles. If the presence of velocity structure is confirmed in the data (Linsky et al., 2010), it probably implies that there is detached, optically thick plasma moving at large velocities around the planet. If this turns out to be the case, interaction with the stellar wind probably plays a role in giving rise to the observed absorption. Such interaction may also produce turbulence that can broaden the absorption further (e.g., Tian et al., 2005). However, we note that non-thermal broadening such as that proposed by Ben-Jaffel and Hosseini (2010) does not appear to be necessary to explain the current observations.
SOL2 model
radial velocity
11 km s−1
km s−1
5Rp
Rp