Inhomogeneous magnetic coupling in exoplanets: the stop & go of WASP-18 b's atmospheric flows
arXiv:2602.18101 · doi:10.1051/0004-6361/202558210
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.
References in corpus (29)
- petitRADTRANS: a Python radiative transfer package for exoplanet characterization and retrieval
- ExoMol molecular line lists XXXV: a rotation-vibration line list for hot ammonia
- ExoMol molecular line lists - XVI: The rotation-vibration spectrum of hot HS
- The 2020 release of the ExoMol database: molecular line lists for exoplanet and other hot atmospheres
- ExoMol line lists XVIII. The high temperature spectrum of VO
- ExoMol line lists -- XXXIX. Ro-vibrational molecular line list for CO
- Retrieving scattering clouds and disequilibrium chemistry in the atmosphere of HR 8799e
- Constraints on Deep-seated Zonal Winds Inside Jupiter and Saturn
- Early UV Ingress in WASP-12b: Measuring Planetary Magnetic Fields
- A hybrid line list for CH and hot methane continuum
- The Atmospheric Circulation of the Hot Jupiter WASP-43b: Comparing Three-Dimensional Models to Spectrophotometric Data
- The Atmospheric Circulation of Ultra-hot Jupiters
- ExoMol molecular line lists V: The ro-vibrational spectra of NaCl and KCl
- Magnetic field strengths of hot Jupiters from signals of star-planet interactions
- Fluid description of multi-component solar partially ionized plasma
- Magnetic Effects in Hot Jupiter Atmospheres
- Climate of an Ultra hot Jupiter: Spectroscopic phase curve of WASP-18b with HST/WFC3
- Idealised simulations of the deep atmosphere of hot jupiters: Deep, hot, adiabats as a robust solution to the radius inflation problem
- Sparkling nights and very hot days on WASP-18b: the formation of clouds and the emergence of an ionosphere
- THOR: A New and Flexible Global Circulation Model to Explore Planetary Atmospheres
- Dust in Brown Dwarfs IV. Dust formation and driven turbulence on mesoscopic scales
- Electrodynamics on extrasolar giant planets
- Magnetic Effects and 3D Structure in Theoretical High-Resolution Transmission Spectra of Ultrahot Jupiters: the Case of WASP-76b
- No evidence for radius inflation in hot Jupiters from vertical advection of heat
- Magnetic induction processes in Hot Jupiters, application to KELT-9b
- Understanding the atmospheric properties and chemical composition of the ultra-hot Jupiter HAT-P-7b: III. Changing ionisation and the emergence of an ionosphere
- A strong H- opacity signal in the near-infrared emission spectrum of the ultra-hot Jupiter KELT-9b
- Magnetic Field Evolution of Hot Exoplanets
- Accelerating exoplanet climate modelling: A machine learning approach to complement 3D GCM grid simulations