Presenter: Naoki TERADA
Abstract:
Slow hydrodynamic escape from Earth-like exoplanets subject to an intense EUV radiation is simulated by a full-particle upper thermosphere-exosphere model using the DSMC method to understand the long-term evolution of planetary atmospheres.
Hydrodynamic escape is an efficient removal process of planetary atmospheres that would have operated in the early stage of their evolutions. With a decreasing EUV radiation from a central star, planetary atmospheres have experienced a subsonic hydrodynamic escape regime, called slow hydrodynamic escape in this study, followed by a hydrostatic escape regime, called Jeans escape. While hydrodynamic and Jeans escapes are relatively well understood, there is an incomplete understanding of the transition from hydrodynamic to Jeans escapes.
A previous study of slow hydrodynamic escape using a fluid model showed that the exobase temperature decreases with increasing incident EUV energy due to adiabatic cooling when the incident energy exceeds a critical value. But we find that the adiabatic cooling works less efficiently in a full-particle model. We discuss the transition from the collisional adiabatic expansion to the collisionless adiabatic free expansion around the exobase, and show calculated slow hydrodynamic escape rates.