We present a theoretical model fit to the HST /STIS optical transit transmission spectrum of HD209458b .
In our fit , we use the sodium absorption line profile along with the Rayleigh scattering by H _ { 2 } to help determine the average temperature-pressure profile at the planetary terminator , and infer the abundances of atomic and molecular species .
The observed sodium line profile spans an altitude range of \sim 3,500 km , corresponding to pressures between \sim 0.001 and 50 mbar in our atmospheric models .
We find that the sodium line profile requires either condensation into sodium sulfide or ionization , necessary to deplete atomic sodium only at high altitudes below pressures of \sim 3 mbar .
The depletion of sodium is supported by an observed sudden abundance change , from 2 times solar abundance in the lower atmosphere to 0.2 solar or lower in the upper atmosphere .
Our findings also indicate the presence of a hot atmosphere near stratospheric altitudes corresponding to pressures of 33 mbar , consistent with that of the observed dayside temperature inversion .
In addition , we find a separate higher altitude temperature rise is necessary within both the condensation and ionization models , corresponding to pressures below \sim 0.01 mbar .
This hot higher altitude temperature indicates that absorption by atomic sodium can potentially probe the bottom of the thermosphere , and is possibly sensitive to the temperature rise linked to atmospheric escape .