Quasiparallel and parallel stellar wind interaction and the magnetospheres of close-in exoplanets

Venus‐like interactions between stellar wind and planets can be expected to change radically when the interplanetary magnetic field (IMF) is quasiparallel to the stellar wind velocity in the frame of a planet. There should exist close‐in orbits in which planets are ideally exposed to such (quasi)parallel stellar winds. The abundance of close‐in exoplanets make it likely that exoplanets in such orbits exist. We study how the Venus‐like interaction between a supersonic stellar wind and a terrestrial exoplanet with atmosphere but no intrinsic magnetic field orbiting a Sun‐like star at 0.2 AU, depends on the IMF‐stellar wind angle. We extend an earlier study by adding simulations for the very lowest IMF‐stellar wind angles. For this we use a hybrid simulation code, A.I.K.E.F., that models ions as macroparticles and electrons as a massless, charge‐neutralizing adiabatic fluid. We find that as the IMF becomes closer and closer to parallel to the stellar wind, the lack of shielding stellar wind‐induced magnetic fields leads to the collapse of the magnetosheath and dayside bow shock and stellar wind impacting the atmosphere in bulk, opening up for extensive direct interaction between stellar wind and atmosphere, e.g. ENA production, energy deposition and atmospheric erosion. Combined with an observed order‐of‐magnitude decrease in the rate of ionospheric ions being lost to the stellar wind, the result may prove relevant for atmospheric evolution. Model shortcomings potentially weaken the conclusions that can be drawn from the most extreme case, the perfectly parallel interaction (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)