Hydrodynamic planetary thermosphere model: 1. Response of the Earth's thermosphere to extreme solar EUV conditions and the significance of adiabatic cooling

It has been suggested that the exobase temperature of early terrestrial planetary atmosphere could have reached over 10,000 K. Although such high exobase temperatures should have caused the major gases at the exobase to experience fast Jeans escape, and the entire thermosphere should have experienced hydrodynamic flow, hydrostatic equilibrium was assumed to be valid in this earlier model. In this paper we develop a multicomponent hydrodynamic thermosphere model to self-consistently study the Earth's thermosphere under extreme solar EUV conditions. The model is validated against observations and other models for the present Earth's thermosphere. Simulations show that if forced in hydrostatic equilibrium and maintaining the current composition, the Earth's thermosphere could experience a fast transition to an atmospheric blowoff state when exposed to solar EUV radiation stronger than certain critical flux. When hydrodynamic flow and its associated adiabatic cooling are included, atmospheric blowoff is prevented and Earth's exobase temperature decreases with increasing solar EUV beyond the critical solar EUV flux. Simulations show that the transition of the thermosphere from the hydrostatic equilibrium regime to the hydrodynamic regime occurs when the exobase temperature reaches 7000 to 8000 K if atomic O and N dominate the upper thermosphere. The fast variations of the bulk motion velocities under different exobase temperatures suggest that the adiabatic cooling effect could have kept the exobase temperature lower than ∼1000 K if light gases such as atomic hydrogen were the dominant species in the Earth's thermosphere. We propose that hydrodynamic flow and associated adiabatic cooling should exist in the thermospheres of a broad range of early and/or close-in terrestrial type planets and that the adiabatic cooling effect must be included in the energy balance in order to correctly estimate their thermospheric structures and their evolutionary paths.