Out on a Limb: The Signatures of East–West Asymmetries in Transmission Spectra from General Circulation Models

Abstract In the era of JWST, observations of hot Jupiter atmospheres are becoming increasingly precise. As a result, the signatures of limb asymmetries due to temperature or abundance differences and the presence of aerosols can now be directly measured using transmission spectroscopy. Using a grid of general circulation models with varying irradiation temperatures (1500–4000 K) and prescriptions of cloud formation, we simulate 3D ingress/egress and morning-/evening-limb transmission spectra. We aim to assess the impact that clouds, 3D temperature structure, and nonuniform distribution of gases have on the observed spectra, and how these inhomogeneities can be identified. A second goal is to assess the relative merits of two separate methods (ingress/egress versus morning-/evening-limb spectroscopy) for isolating atmospheric asymmetries. From our models, it is evident that an east–west temperature difference is the leading-order effect for producing ingress/egress or morning-/evening-limb spectral differences. We additionally find that clouds contribute strongly to the observed limb asymmetry at moderate irradiation temperatures in our grid (∼2000 K < T irr < 3500 K). At lower temperatures clouds equally dominate the optical depth on both limbs, while at higher temperatures the entire terminator region remains cloud free. We develop limb asymmetry metrics that can be used to assess the degree of east–west asymmetry for a given planet and predict trends in these metrics with respect to irradiation temperature that are indicative of various physical processes. Our results are useful for predicting and diagnosing the signatures of limb asymmetries in JWST spectra.