Consequences of expanding exoplanetary atmospheres for magnetospheres

Abstract

The atmospheres of close-in extrasolar planets are expected to both undergo hydrodynamic expansion due to strong heating and produce strong expanding ionospheres due to intense photoionization, while at the same time being exposed to strong stellar winds. This scenario can be expected to lead to new types of magnetospheres and previously unseen interactions between stellar wind plasma and ionospheres. Our aim is to start looking at these kinds of scenarios for close-in terrestrial planets using hybrid simulations. For this purpose we used a hybrid code, treating electrons as a massless, charge-neutralizing, adiabatic fluid and ions as macroparticles, to study the influence of a strongly expanding ionosphere on the stellar wind interaction for an unmagnetized close-in extrasolar terrestrial planet. For both with and without expansion, we can identify bow shock, magnetopause, and ion-composition boundary. The expanding ionosphere pushes the bow shock, magnetic draping, and ion composition boundary upstream and increases the size of the entire interaction region, creating a large wake behind the planet, largely void of electromagnetic fields and dominated only by the expanding ionosphere. On the dayside, little ionospheric radial bulk flow is actually observed since the ions are quickly thermalized after being added to the ionosphere.