  It has been observed that hot Jupiters located within 0.08 AU of their host
stars commonly display radii in excess of those expected based on models. A
number of theoretical explanations for this phenomenon have been suggested, but
the ability of any one mechanism to account for the full range of observations
remains to be rigorously proven. I identify an additional heating mechanism,
arising from the interaction of the interplanetary magnetic field and the
planetary magnetosphere, and show that this is capable of providing enough
energy to explain the observed planetary radii. Such a model predicts that the
degree of heating should be dependent on the stellar magnetic field, for which
stellar activity serves as a proxy. Accordingly, I examine populations of hot
Jupiters from the Kepler database and confirm that stellar activity (determined
using Kepler CDPP levels) is correlated with the presence of planetary radii
inflated beyond the basal level of R = 0.87 R_J identified by previous
researchers. I propose that the primary mechanism for transferring energy from
the magnetosphere to the planetary interior is Joule heating arising from
global electric circuits analogous to those seen in solar system objects.
