Inhomogeneous magnetic coupling in exoplanets: the stop & go of WASP-18 b's atmospheric flows

TL;DR

This study models how inhomogeneous magnetic coupling influences atmospheric flows in ultra-hot Jupiters like WASP-18 b, revealing effects on wind patterns, temperature asymmetries, and observable features.

Contribution

It introduces an analytical parameterization of anisotropic magnetic drag in a 3D GCM to better understand magnetic effects on exoplanet atmospheres.

Findings

Magnetic drag affects wind strength and direction in the upper atmosphere.

Temperature asymmetries and hotspot shifts are influenced by magnetic coupling.

Terminator regions are highly sensitive to magnetic drag modeling.

Abstract

Early studies of ionization in hot Jupiter atmospheres suggest that magnetic coupling can shape their dynamics. These effects may be most pronounced in ultra-hot Jupiters that sustain global magnetic fields. WASP-18 b hosts an ionized dayside atmosphere extending deep enough to be strongly influenced by magnetic forces. Phase curve observations suggest effective magnetic drag, yet its impact on the atmospheric circulation remains poorly constrained. This work explores how magnetic drag in an inhomogeneously ionized atmosphere shapes local and global dynamics to provide a pathway to constrain the planet's magnetic field strength. An analytical parameterization for anisotropic magnetic drag, including both Pedersen and Hall drag components, and associated frictional heating in the globally neutral atmosphere, is implemented in the 3D General Circulation Model ExoRad to study WASP-18 b's atmosphere. Climate characteristics are compared for different drag formulations to assess whether anisotropic physics is required to capture magnetic coupling effects. Anisotropic magnetic drag and frictional heating, both set by local ionization, strongly affect wind strength and direction in the upper atmosphere, modify the day-night circulation, and produce observable temperature asymmetries. They enhance the evening-morning terminator temperature difference near 0.1 bar and generate two off-equator hotspots with reduced eastward shift. The terminator regions are particularly sensitive to how magnetic drag is modeled. Anisotropic magnetic drag damps and redirects dayside-to-nightside winds, partially decoupling the equatorial flow at the morning terminator while maintaining the nightside jet. Locally varying drag forces and frictional heating create asymmetric temperature patterns manifesting as primary and secondary hotspot regions.