Modeling the Dynamics and Thermochemistry for the Outer Atmospheres of the Ultrahot Jupiter WASP-121b

We present three-dimensional simulations of the ultrahot Jupiter (UHJ) WASP-121b from the planetary surface to extended outflows, coupling hydrodynamics with consistent nonequilibrium thermochemistry, ray-tracing radiative transfer, and hydrodynamics using the GPU-accelerated Kratos framework. The fiducial model exhibits several atmospheric layers, including the lower atmospheres controlled by day–night circulation, and transonic photoevaporative outflows at higher altitudes shaped into two spiral arms by the stellar gravity and orbital motion effects. Different species could trace different regions; Fe probes rotation-dominated inner layers, Na maps dense spiral arms where recombination balances photoionization, and H α and He 10830 Å features trace progressively more extended, ionized gas. With spiral arm velocities reaching ∼40 km s −1 projected along the line of sight, this morphology naturally reproduces the velocity pattern of observed high-velocity Na and H α absorption features without requiring significant superrotation jet streams, although the absolute absorption amplitudes could carry uncertainties from stellar UV luminosity and trace elemental abundances. Parametric studies reveal complex dependencies on stellar irradiation; enhanced far-ultraviolet intensifies outflows and extends spiral arms spatially and kinematically, while EUV and X-ray expand spiral structures into attenuated, ionized regions. Stellar wind confinement compresses the dayside outflow and enhances metastable helium absorption. This work demonstrates that current and future transmission spectral observations that probe multiple species can provide important constraints on astrophysical environments of UHJs by comparing state-of-the-art simulations.