Magnetohydrodynamic Simulations of Hot Jupiter Upper Atmospheres

TL;DR

This study uses 2D magnetohydrodynamic simulations to analyze how magnetic fields influence the upper atmospheres of hot Jupiters and their Ly$ ext{alpha}$ transmission spectra, revealing a strong dependence on magnetic field strength.

Contribution

It introduces detailed MHD simulations of hot Jupiter atmospheres including magnetic effects and compares results with observational data, highlighting the impact of magnetic fields on transit depths.

Findings

Transit depth increases with magnetic field strength for B0 > 10 G.

Equatorial dead zone contributes significantly to transit signal.

Simulations match observed spectra for specific magnetic field parameters.

Abstract

Two-dimensional simulations of hot Jupiter upper atmospheres including the planet's magnetic field are presented. The goal is to explore magnetic effects on the layer of the atmosphere that is ionized and heated by stellar EUV radiation, and the imprint of these effects on the Ly$\alpha$ transmission spectrum. The simulations are axisymmetric, isothermal, and include both rotation and azimuth-averaged stellar tides. Mass density is converted to atomic hydrogen density through the assumption of ionization equilibrium. The three-zone structure -- polar dead zone, mid-latitude wind zone, and equatorial dead zone -- found in previous analytic calculations is confirmed. For a magnetic field comparable to that of Jupiter, the equatorial dead zone, which is confined by the magnetic field and corotates with the planet, contributes at least half of the transit signal. For even stronger fields, the gas escaping in the mid-latitude wind zone is found to have a smaller contribution to the transit depth than the equatorial dead zone. Transmission spectra computed from the simulations are compared to HST STIS and ACS data for HD 209458b and HD 189733b, and the range of model parameters consistent with the data is found. The central result of this paper is that the transit depth increases strongly with magnetic field strength when the hydrogen ionization layer is magnetically dominated, for dipole magnetic field $B_0 > 10\ {\rm G}$. Hence transit depth is sensitive to magnetic field strength, in addition to standard quantities such as the ratio of thermal to gravitational binding energies.