A large fraction of exoplanets have semimajor axes much smaller than any planet in the Solar System and may therefore experience previously unseen types of stellar wind interaction. Understanding these interactions may be important both for future detection methods and for atmospheric mass loss estimates and implicitly for the evolution of exoplanetary atmospheres and for interpreting the observed population of exoplanets. In this work we try to investigate such close-in stellar wind interaction using primarily hybrid simulations that describe the time evolution of plasmas in a 3D box by modeling electrons as a fluid and ions as particles.
Two scenarios of stellar wind interaction with unmagnetized, Earth-sized, close-in exoplanets orbiting a Sun-like star are constructed and investigated: 1.) stellar wind interaction with an extremely hydrodynamically expanding atmosphere and therefore expanding ionosphere, and 2.) quasiparallel stellar wind interaction i.e. where the IMF is approximately parallel to the stellar wind. We observe expected features like bow shock, magnetic draping and ion composition boundary but also how an expanding atmosphere increases the size of the entire interaction region creating a significant wake behind the obstacle, and how a quasiparallel interaction leads to an unstable shock and to stellar wind increasingly penetrating the ionosphere.
An attempt is also made to analytically estimate the standoff distance for magnetospheres resulting from expanding ionospheres and compare these estimates with standoff distances obtained from simulations.