Hot Jupiters provide an extreme laboratory for atmospheric dynamics. Their strongly irradiated atmospheres host fast winds, large day–night temperature contrasts, and — at sufficiently high temperatures — enough ionization for the flow to interact with magnetic fields. In this talk, I will present an MHD modelling framework for hot-Jupiter atmospheres that investigates the transition from purely hydrodynamic circulation to fully nonlinear magnetohydrodynamic behaviour.

With increasing temperature, magnetic effects such as the bending of internal field lines, Lorentz-force braking of the flow, and ohmic dissipation become increasingly important. Below about 1400 K, the models remain essentially hydrodynamic. Between roughly 1400 K and 1800 K, the atmospheric dynamics enter a linear induction regime, in which shear flows wrap internal magnetic field lines around the planet. At still higher temperatures, around 1900 K and above, the atmosphere transitions into a self-sustained dynamo regime. In this regime, atmospheric induction no longer depends on the imposed internal magnetic field, but instead maintains its own magnetic field through the nonlinear coupling between flow and field.

In the dynamo regime, we find a novel triple balance: the characteristic flow speed, Alfvén speed, and rotational speed become comparable. This result is particularly promising observationally, because the planetary rotation rate is often known or constrained. If this balance holds, it may provide a route to infer otherwise inaccessible atmospheric wind speeds and magnetic-field strengths from the rotation rate and observable atmospheric morphology.

More broadly, these results point to a regime-based view of hot-Jupiter atmospheres, in which MHD models may provide a unifying framework for connecting atmospheric dynamics, magnetic-field generation, and observable trends across the population.