Inhomogeneous magnetic coupling in exoplanets: The stop and go of WASP-18 b’s atmospheric flows

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
² Institute for Theoretical Physics and Computational Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 November 2025
Accepted: 16 February 2026

Abstract

Context. Early studies of ionization in hot Jupiter atmospheres suggest that magnetic coupling may affect their dynamics, and hence their weather and climate states. These effects may be most pronounced in ultrahot gas giants, assuming they generate their own global magnetic field. WASP-18 b, one of the best studied ultrahot Jupiters, hosts a highly ionized dayside atmosphere extending deep enough to be strongly influenced by magnetic forces. Phase curve observations suggest an effective magnetic drag, yet its impact on the atmospheric circulation remains poorly constrained.

Aims. The aim is to explore the effect of magnetic drag in atmospheres with an inhomogeneous ionization on the local and global dynamics to ultimately provide a pathway to constrain the planet’s magnetic field strength.

Methods. An analytical parameterization for anisotropic magnetic drag, including both Pedersen and Hall drag components, and associated frictional heating in the globally neutral atmosphere, was implemented in the 3D general circulation model ExoRad to study WASP-18 b. Fundamental plasma parameters were analyzed to explore where magnetic coupling becomes important in the atmosphere, depending on the dipolar field geometry, the ionization fraction, and the collisional coupling between charged particles and neutrals. Climate characteristics were compared for different drag formulations, to assess whether anisotropic drag physics is required to accurately capture magnetic coupling effects.

Results. Anisotropic magnetic drag and frictional heating, both shaped by local ionization, strongly affect wind strength and direction in the upper atmosphere, modifying the day-night circulation and producing observable temperature asymmetries. Anisotropic drag enhances the evening-morning terminator temperature difference at 0.1 bar, and generates two off-equator hotspots with reduced eastward shift. The terminator regions are in particular susceptible to how magnetic drag is described in the model.

Conclusions. Anisotropic magnetic drag damps and redirects the dayside-to-nightside winds, partially decoupling the equatorial flow at the morning terminator while maintaining the nightside jet. Locally changing drag forces and frictional heating create asymmetric temperature patterns that manifest as primary and secondary hotspot regions.