Exosphere

Abstract: Past studies addressing the thermal atmospheric escape of hydrogen from hot Jupiters have been based on the planet's effective temperature, which, as we show here, is not physically relevant for loss processes In consequence, these studies led to significant underestimations of the atmospheric escape rate (≤103 g s-1) and to the conclusion of long-term atmospheric stability From more realistic exospheric temperatures, determined from X-ray and extreme-ultraviolet (XUV) irradiation and thermal conduction in the thermosphere, we find that energy-limited escape and atmospheric expansion arise, leading to much higher estimations for the loss rates (≈1012 g s-1) These fluxes are in good agreement with recent determinations for HD 209458b based on observations of its extended exosphere We also show that for young solar-type stars, which emit stronger XUV fluxes, the inferred loss rates are significantly higher Thus, hydrogen-rich giant exoplanets under such strong XUV irradiances may evaporate down to their core sizes or shrink to levels at which heavier atmospheric constituents may prevent hydrodynamic escape These results could explain the apparent paucity of exoplanets so far detected at orbital distances less than 004 AU

Abstract: [1] The new Horizontal Wind Model (HWM07) provides a statistical representation of the horizontal wind fields of the Earth's atmosphere from the ground to the exosphere (0–500 km). It represents over 50 years of satellite, rocket, and ground-based wind measurements via a compact Fortran 90 subroutine. The computer model is a function of geographic location, altitude, day of the year, solar local time, and geomagnetic activity. It includes representations of the zonal mean circulation, stationary planetary waves, migrating tides, and the seasonal modulation thereof. HWM07 is composed of two components, a quiet time component for the background state described in this paper and a geomagnetic storm time component (DWM07) described in a companion paper.