IONIZATION OF EXTRASOLAR GIANT PLANET ATMOSPHERES

Many extrasolar planets orbit close in and are subject to intense ionizing radiation from their host stars. Therefore, we expect them to have strong, and extended, ionospheres. Ionospheres are important because they modulate escape in the upper atmosphere and can modify circulation, as well as leave their signatures, in the lower atmosphere. In this paper, we evaluate the vertical location Z(I) and extent D-I of the EUV ionization peak layer. We find that Z(I) approximate to 1-10 nbar-for a wide range of orbital distances (a = 0.047-1 AU) from the host star-and D-I/H-p greater than or similar to 15, where H-p is the pressure scale height. At Z(I), the plasma frequency is similar to 80-450 MHz, depending on a. We also study global ion transport, and its dependence on a, using a three-dimensional thermosphere-ionosphere model. On tidally synchronized planets with weak intrinsic magnetic fields, our model shows only a small, but discernible, difference in electron density from the dayside to the nightside (similar to 9 x 10(13) m(-3) to similar to 2 x 10(12) m(-3), respectively) at Z(I). On asynchronous planets, the distribution is essentially uniform. These results have consequences for hydrodynamic modeling of the atmospheres of close-in extrasolar giant planets.