Magnetohydrodynamics in hot Jupiters

Abstract

Hot Jupiters are Jupiter-like exoplanets found in close-in orbits. This subjects them to high levels of stellar irradiance and is believed to tidally-lock them to their host stars, causing extreme day-night temperature differentials which in-turn drive atmospheric dynamics. A ubiquitous feature of hydrodynamic models of hot Jupiter atmospheres is equatorial superrotation, which advects their hotspots (equatorial temperature maxima) eastwards (prograde). Observational studies generally find eastward hotspot/brightspot offsets. However, recent observations of westward hotspot/brightspot offsets suggest that this is not ubiquitous. Prior to these observations, three-dimensional magnetohydrodynamic simulations predicted that westward hotspots could result from magnetohydrodynamic effects in the hottest hot Jupiters, yet the mechanism driving such reversals is not well understood. We study the underlying physics of magnetically-driven hotspot reversals using a shallow-water magnetohydrodynamic model. This captures the leading order physics of hot Jupiter atmospheres, but with reduced mathematical complexity. The model’s hydrodynamic counterpart is well-established and has successfully been used to explain equatorial superrotation in hydrodynamic models of hot Jupiter atmospheres in terms of planetary scale equatorial wave interactions. However, until now, shallow-water magnetohydrodynamic models have not been applied to hot Jupiters. Firstly, we find that the model can indeed capture the physics of magnetically-driven hotspot reversals. We use non-linear numerical simulations to understand the dominant force balances that drive the reversals and use a linear analysis of the system’s planetary scale equatorial waves to understand the reversal mechanism in terms of wave interactions. We then use the developed theory to place physically motivated observational constraints on the magnetic field strengths of hot Jupiters exhibiting westward hotspot/brightspot offsets, finding that on the hottest of these the observations can be explained by moderate planetary magnetic field strengths. Finally, we identify candidates that are likely to exhibit magnetically-driven hotspot reversals to help guide future observational missions.