Westward hotspot offset explained by subcritical dynamo action in an ultra-hot Jupiter atmosphere

Hot Jupiters are tidally-locked Jupiter-sized planets close to their host star. They have equilibrium temperatures above about 1000 K. Photometric observations find that the hotspot, the hottest location in the atmosphere, is shifted with respect to the substellar point. Some observations show eastward and some show westward hotspot offsets, while hydrodynamic simulations show an eastward offset due to advection by the characteristic eastward mean flow. In particular for ultra-hot Jupiters with equilibrium temperatures above 2000 Kelvin, electromagnetic effects must be considered since the ionization-driven significant electrical conductivity and the subsequent induction of magnetic fields likely result in substantial Lorentz forces. We here provide the first magnetohydrodynamic numerical simulation of an ultra-hot Jupiter atmosphere at an equilibrium temperature of about 2400 K that fully captures non-linear electromagnetic induction effects. We find a new turbulent flow regime, hitherto unknown for hot Jupiters. Its main characteristic is a break-down of the well-known laminar mean flows. This break-down is triggered by strong local magnetic fields. These fields are maintained by a subcritical dynamo process. It is initiated by a sufficiently strong background field from an assumed deep dynamo region at a realistic amplitude around 2.5 G. Our results show a zero or westward hotspot offset for the dynamo case, depending on atmospheric properties, while the hydrodynamic case has the usual eastward offset. Since our simulation has an eastward mean flow at the equator, radial flows must be important for producing the zero or westward hotspot offset. A subcritical dynamo offers a new scenario for explaining the diversity of observed hotspot offsets. In this scenario, the dynamo has been initiated by sufficiently strong fields at some time in the past only for a part of the population.