Chromospheres as thermodynamic gatekeepers in Cool Stars

The chromosphere is key in regulating how energy and momentum are transferred from stellar interiors into interplanetary space. Yet it remains the most challenging layer to model in cool-star atmospheres due to the highly non-ideal physics involved. In both solar and stellar coronal models, chromospheric effects are currently absorbed into simplified boundary conditions, leading to homogeneous and unconstrained thermodynamics at the base of coronas. Using the radiative-MHD Bifrost code, we perform a parametric study of quiet-Sun magnetic flux emergence to quantify how chromospheric thermodynamics and atmospheric coupling respond to changing magnetic environments. We first show that increasing magnetic flux redistributes heating from shocks toward magnetic reconnection, producing a monotonic rise in chromospheric temperatures. However, the temperature at the base of the corona responds non-monotonically, reflecting a subtle competition between enhanced heating, increased radiative losses, and mass loading. These results highlight the importance of chromospheric diagnostics, the need to improve current chromospheric parametrisations, and inform us on how. We finally discuss advances we are currently making on this aspect in the context of space-weather environments, and subsequent perspectives for modelling exoplanetary systems and their host star evolution.