Accurate characterization of the ionosphere is important because it can modulate escape and circulation in the upper atmosphere, and influence the way in which the atmosphere interacts with the stellar wind and the planetary magnetic field. We have studied the degree of ionization in the upper atmospheres of extrasolar giant planets exposed to different X-ray and EUV fluxes by their host stars. We find that the EUV ionization peak (EIP) layer for a planet such as HD209458b orbiting a Sun-like star is centered at ZI = 1 – 10 nbar at an altitude of 1.1 – 1.35 RP. The electron-neutral mixing ratio at this location varies from xe = 10-5 at a = 1.0 AU to xe = 10-2 at a = 0.047 AU. H2 is the dominant species of the dayside ionosphere below the exobase at a > 0.2 AU. At a < 0.2 AU, H2 is dissociated thermally and by photoionization, and H is the dominant species above z ~ 1.1 RP. The atmosphere is fully ionized above z ~ 2 – 3 RP. The horizontal distribution of the plasma at ZI depends on the rotation rate. However, even on a tidally locked planet, the difference in electron density between the dayside and the night side is only one or two orders of magnitude. According to conventional criteria, the ionosphere behaves as plasma at all altitudes above the 10 μbar level and possibly even below it. The altitude range in our model can be probed by UV occultation experiments, and we use our calculations to predict absorption features for such observations.
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