Magnetic induction processes in hot Jupiters, application to KELT-9b

Abstract

The small semimajor axes of hot Jupiters lead to high atmospheric temperatures of up to several thousand Kelvin. Under these conditions, thermally ionized metals provide a rich source of charged particles and thus build up a sizeable electrical conductivity. Subsequent electromagnetic effects, such as the induction of electric currents, Ohmic heating, magnetic drag, or the weakening of zonal winds have thus far been considered mainly in the framework of a linear, steady-state model of induction. For hot Jupiters with an equilibrium temperature Teq > 1500 K, the induction of atmospheric magnetic fields is a runaway process that can only be stopped by non-linear feedback. For example, the back-reaction of the magnetic field on to the flow via the Lorentz force or the occurrence of magnetic instabilities. Moreover, we discuss the possibility of self-excited atmospheric dynamos. Our results suggest that the induced atmospheric magnetic fields and electric currents become independent of the electrical conductivity and the internal field, but instead are limited by the planetary rotation rate and wind speed. As an explicit example, we characterize the induction process for the hottest exoplanet, KELT-9b, by calculating the electrical conductivity along atmospheric P-T profiles for the dayside and nightside. Despite the temperature varying between 3000 and 4500 K, the resulting electrical conductivity attains an elevated value of roughly 1 S m-1 throughout the atmosphere. The induced magnetic fields are predominately horizontal and might reach up to a saturation field strength of 400 mT, exceeding the internal field by two orders of magnitude.