Magnetic interactions around cool stars: grazing comets and hotspot formation

Cool stars can have strong magnetic activity compared to the Sun, with flares and eruptions significantly more energetic than what is observed in our solar system. The presence of close-in exoplanets has been shown to impact the stellar flaring rate (Ilin et al. 2025), thought to be mainly caused by star-planet magnetic interactions (SPMI). In these interactions, Alfvén waves can propagate from the exoplanet back towards the star and deposit energy in the stellar atmosphere, forming localized hotspots (Strugarek et al. 2025) or potentially triggering eruptions. Paul et al. (2025) investigated the efficiency of such SPMI energy transfer by considering the reflection of Alfvén waves at the stellar transition region, but in a simpler 1D magnetohydrodynamic (MHD) model.

In this talk, I will present the follow up of this study, moving towards a more realistic 3D scenario. We investigate for the first time the propagation of Alfvén waves triggered by an afar exoplanet and propagating across the different layers of the solar atmosphere. To do so, we use the radiative MHD code Bifrost, designed to model in detail a localized region of the Sun from the convection zone up to the corona. By injecting Alfvén waves at the top of the simulation domain, we quantify the fraction of waves able to cross the transition region and how much energy is deposited at each height in the atmosphere. I will follow up by explaining how these results apply to an observational case study of the first tentative detection of SPMI in our solar system. Indeed, potential magnetic interactions between the Sun and comet Lovejoy, traveling through the solar corona during its perihelion, seem to have triggered solar eruptions, which has never been studied before. I will thus present the energetics of such interactions during this event and explain how it helps us better understand SPMI in stellar and exoplanetary systems.