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

TL;DR

This paper investigates magnetic induction processes in Hot Jupiters like KELT-9b, revealing that high atmospheric temperatures lead to runaway magnetic field induction, which is limited by planetary rotation and wind speed, with potential for self-sustained dynamos.

Contribution

It introduces a nonlinear, runaway induction model for Hot Jupiters with T_{eq} > 1500 K, highlighting the limits set by planetary rotation and wind, and explores the possibility of atmospheric dynamos.

Findings

Induced magnetic fields can reach up to 400 mT, exceeding internal fields.

Induction becomes a runaway process at high temperatures, limited by nonlinear feedback.

Electrical conductivity remains high (~1 S/m) across the atmosphere despite temperature variations.

Abstract

The small semi-major axes of Hot Jupiters lead to high atmospheric temperatures of up to several thousand Kelvin. Under these conditions, thermally ionised 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 $T_{eq} > 1500$ 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 onto 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 characterise the induction process for the hottest exoplanet, KELT-9b by calculating the electrical conductivity along atmospheric $P-T$-profiles for the day- and nightside. Despite the temperature varying between 3000 K and 4500 K, the resulting electrical conductivity attains an elevated value of roughly 1 S/m 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.