  Transits in the H I 1216 A (Lyman alpha), O I 1334 A, C II 1335 A, and Si III
1206.5 A lines constrain the properties of the upper atmosphere of HD209458b.
In addition to probing the temperature and density profiles in the
thermosphere, they have implications for the properties of the lower
atmosphere. Fits to the observations with a simple empirical model and a direct
comparison with a more complex hydrodynamic model constrain the mean
temperature and ionization state of the atmosphere, and imply that the optical
depth of the extended thermosphere of the planet in the atomic resonance lines
is significant. In particular, it is sufficient to explain the observed transit
depths in the H I 1216 A line. The detection of O at high altitudes implies
that the minimum mass loss rate from the planet is approximately 6e6 kg/s. The
mass loss rate based on our hydrodynamic model is higher than this and implies
that diffusive separation is prevented for neutral species with a mass lower
than about 130 amu by the escape of H. Heavy ions are transported to the upper
atmosphere by Coulomb collisions with H+ and their presence does not provide as
strong constraints on the mass loss rate as the detection of heavy neutral
atoms. Models of the upper atmosphere with solar composition and heating based
on the average solar X-ray and EUV flux agree broadly with the observations but
tend to underestimate the transit depths in the O I, C II, and Si III lines.
This suggests that the temperature and/or elemental abundances in the
thermosphere may be higher than expected from such models...The detection of
Si2+ in the thermosphere indicates that clouds of forsterite and enstatite do
not form in the lower atmosphere...
