Magnetically-driven hotspot reversals in ultra-hot Jupiter atmospheres

Recent westward hotspot/brightspot offset measurements present significant exceptions from the norm of eastward hotspots found in the general hot Jupiter population. If these signals are to be trusted, they cannot be explained by hydrodynamic theory of synchronously rotating hot Jupiters, which predicts hotspots are always recirculated eastward of the substellar point. However, three-dimensional magnetohydrodynamic (MHD) simulations predicted westward hotspot offsets would be possible in the hottest hot Jupiters (ultra-hot Jupiters). This is consistent with the emerging observational evidence as four out of the five westward offset measurements have been made for ultra-hot Jupiters, including one using TESS. Until now, however, the MHD mechanism that can drive such reversals has not been understood. We highlight the findings of two recent papers regarding the magnetic reversal mechanism. First, we identify MHD mechanism responsible for hotspot reversals. In the second part, we highlight physical consequences of the mechanism and apply a physically-motivated reversal criterion to a hot Jupiter dataset. Consequences include observational constraints of magnetic fields strengths and testable predictions that TESS can probe.