Ionization of Extrasolar Giant Planet Atmospheres

• Published in: The Astrophysical journal Vol. 722; no. 1; pp. 178 - 187
• Main Authors: Koskinen, Tommi T, Cho, James Y-K, Achilleos, Nicholas, Aylward, Alan D
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 10.10.2010; IOP
• Subjects: Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ATMOSPHERES; EARTH ATMOSPHERE; Earth, ocean, space; ELECTRON DENSITY; EVOLUTION; Exact sciences and technology; FLUID MECHANICS; HYDRODYNAMICS; IONIZATION; IONIZING RADIATIONS; IONOSPHERE; LANGMUIR FREQUENCY; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; MECHANICS; PLANETARY ATMOSPHERES; PLANETS; RADIATIONS; SIMULATION; STAR EVOLUTION; STARS; THERMOSPHERE; Plasma; Magnetohydrodynamics; Turbulence; plasmas; Ionizing radiations; Extrasolar planets; Giant planet; Orbits; hydrodynamics; planets and satellites: general; Ionization; Models; Modelling; Upper atmosphere; Magnetic fields; Electron density; magnetohydrodynamics (MHD)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/722/1/178

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{sub I} and extent D{sub I} of the EUV ionization peak layer. We find that Z{sub I{approx}}1-10 nbar-for a wide range of orbital distances (a = 0.047-1 AU) from the host star-and D{sub I}/H{sub p{approx}}>15, where H{sub p} is the pressure scale height. At Z{sub I}, the plasma frequency is {approx}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 ({approx}9 x 10{sup 13} m{sup -3} to {approx}2 x 10{sup 12} m{sup -3}, respectively) at Z{sub 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.