A General Circulation Model for Gaseous Exoplanets with Double-Gray Radiative Transfer

TL;DR

This paper introduces a new atmospheric circulation model for gaseous exoplanets using a double-gray radiative transfer scheme, enabling self-consistent flux and heating calculations, and explores magnetic drag effects on hot Jupiter atmospheres.

Contribution

The paper presents a detailed implementation of a double-gray radiative transfer scheme in a circulation model for gaseous exoplanets, including magnetic drag effects and thermal phase curve predictions.

Findings

Magnetic drag reduces atmospheric flux offsets and flux ratios.

Drag-free models show ~15% kinetic energy loss, while strong-drag models show none.

Thermal phase curves reveal significant differences in hotspot offsets.

Abstract

We present a new version of our code for modeling the atmospheric circulation on gaseous exoplanets, now employing a "double-gray" radiative transfer scheme, which self-consistently solves for fluxes and heating throughout the atmosphere, including the emerging (observable) infrared flux. We separate the radiation into infrared and optical components, each with its own absorption coefficient, and solve standard two-stream radiative transfer equations. We use a constant optical absorption coefficient, while the infrared coefficient can scale as a powerlaw with pressure. Here we describe our new code in detail and demonstrate its utility by presenting a generic hot Jupiter model. We discuss issues related to modeling the deepest pressures of the atmosphere and describe our use of the diffusion approximation for radiative fluxes at high optical depths. In addition, we present new models using a simple form for magnetic drag on the atmosphere. We calculate emitted thermal phase curves and find that our drag-free model has the brightest region of the atmosphere offset by ~12 degrees from the substellar point and a minimum flux that is 17% of the maximum, while the model with the strongest magnetic drag has an offset of only ~2 degrees and a ratio of 13%. Finally, we calculate rates of numerical loss of kinetic energy at ~15% for every model except for our strong-drag model, where there is no measurable loss; we speculate that this is due to the much decreased wind speeds in that model.