Magnetic Scaling Laws for the Atmospheres of Hot Giant Exoplanets

Abstract: We present scaling laws for advection, radiation, magnetic drag and ohmic dissipation in the atmospheres of hot giant exoplanets. In the limit of weak thermal ionization, ohmic dissipation increases with the planetary equilibrium temperature (T_eq >~ 1000 K) faster than the insolation power does, eventually reaching values >~ 1% of the insolation power, which may be sufficient to inflate the radii of hot Jupiters. At higher T_eq values still, magnetic drag rapidly brakes the atmospheric winds, which reduces the associated ohmic dissipation power. For example, for a planetary field strength B=10G, the fiducial scaling laws indicate that ohmic dissipation exceeds 1% of the insolation power over the equilibrium temperature range T_eq ~ 1300-2000 K, with a peak contribution at T_eq ~ 1600 K. Evidence for magnetically dragged winds at the planetary thermal photosphere could emerge in the form of reduced longitudinal offsets for the dayside infrared hotspot. This suggests the possibility of an anticorrelation between the amount of hotspot offset and the degree of radius inflation, linking the atmospheric and interior properties of hot giant exoplanets in an observationally testable way. While providing a useful framework to explore the magnetic scenario, the scaling laws also reveal strong parameter dependencies, in particular with respect to the unknown planetary magnetic field strength.