  The atmospheres of hot Jupiters and other strongly-forced exoplanets are
susceptible to a thermal instability in the presence of ohmic dissipation, weak
magnetic drag and strong winds. The instability occurs in radiatively-dominated
atmospheric regions when the ohmic dissipation rate increases with temperature
faster than the radiative (cooling) rate. The instability domain covers a
specific range of atmospheric pressures and temperatures, typically P ~ 3-300
mbar and T ~ 1500-2500K for hot Jupiters, which makes it a candidate mechanism
to explain the dayside thermal "inversions" inferred for a number of such
exoplanets. The instability is suppressed by high levels of non-thermal
photoionization, in possible agreement with a recently established
observational trend. We highlight several shortcomings of the instability
treatment presented here. Understanding the emergence and outcome of the
instability, which should result in locally hotter atmospheres with stronger
levels of drag, will require global non-linear atmospheric models with adequate
MHD prescriptions.
