  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.
