  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 ~3,500 km, corresponding to pressures between ~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 ~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 ~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.
