How Anisotropic Magnetic Drag Shapes the Atmospheric Circulation of WASP-18 b

Magnetic coupling between weakly ionized atmospheres and planetary magnetic fields is expected to influence the circulation of ultra-hot Jupiters, where dayside thermal ionization becomes important. Similar coupling processes are well established in the upper atmospheres of the Solar System gas giants, where interactions between the charged particles and neutrals control momentum and energy exchange. WASP-18 b, one of the best-studied ultra-hot Jupiters, exhibits a highly ionized dayside atmosphere extending deep enough to be strongly influenced by magnetic forces, making it an ideal laboratory to study magnetic drag in exoplanet atmospheres. Previous studies have shown that magnetic fields can exert a drag on the neutral gas component, but their impact on the atmospheric circulation remains poorly constrained.

We investigate the effect of inhomogeneous ionization on atmospheric dynamics by implementing an analytically derived parametrization of anisotropic magnetic drag, including Pedersen and Hall drag components, together with the associated frictional heating, into the 3D General Circulation Model ExoRad. The drag coefficients are computed from the local ionization fraction, dipolar magnetic field geometry, and collisional coupling between charged particles and neutrals, following the framework used to describe collisional coupling in Solar System gas giant atmospheres and ionospheres.

Our simulations demonstrate that anisotropic magnetic drag significantly modifies wind strength and direction in the upper atmosphere, reshaping the day–night circulation and generating asymmetric temperature patterns. In particular, anisotropic drag enhances the morning–evening terminator temperature contrast near the 0.1 bar level and produces two off-equatorial hotspot regions with reduced eastward displacement. The terminator regions are especially sensitive to how magnetic drag is parametrized. These results emphasize the importance of anisotropic magnetic drag and frictional heating for interpreting phase-curve and high-resolution spectroscopic observations and for constraining planetary magnetic field strengths.