Atmospheric escape in hot Jupiters under sub-Alfvénic interactions

Abstract: Hot Jupiters might reside inside the Alfv\'en surface of their host star wind, where the stellar wind is dominated by magnetic energy. The implications of such a sub-Alfv\'enic environment for atmospheric escape are not fully understood. Here, we employ 3-D radiation-magnetohydrodynamic simulations and Lyman-$\alpha$ transit calculations to investigate atmospheric escape properties of magnetised hot Jupiters. By varying the planetary magnetic field strength ($B_p$) and obliquity, we find that the structure of the outflowing atmosphere transitions from a magnetically unconfined regime, where a tail of material streams from the nightside of the planet, to a magnetically confined regime, where material escapes through the polar regions. Notably, we find an increase in the planet escape rate with $B_p$ in both regimes, with a local decrease when the planet transitions from the unconfined to the confined regime. Contrary to super-Alfv\'enic interactions, which predicted two polar outflows from the planet, our sub-Alfv\'enic models show only one significant polar outflow. In the opposing pole, the planetary field lines connect to the star. Finally, our synthetic Ly-$\alpha$ transits show that both the red-wing and blue-wing absorptions increase with $B_p$. Furthermore, there is a degeneracy between $B_p$ and the stellar wind mass-loss rate when considering absorption of individual Lyman-$\alpha$ wings. This degeneracy can be broken by considering the ratio between the blue-wing and the red-wing absorptions, as stronger stellar winds result in higher blue-to-red absorption ratios. We show that, by using the absorption ratios, Lyman-$\alpha$ transits can probe stellar wind properties and exoplanetary magnetic fields.