  Atmospheric retrievals of exoplanet transmission spectra allow constraints on
the composition and structure of the day-night terminator region. Such
retrievals in the past have typically assumed one-dimensional temperature
structures which were adequate to explain extant observations. However, the
increasing data quality expected from exoplanet spectroscopy with JWST
motivates considerations of multidimensional atmospheric retrievals. We present
AURA-3D, a three-dimensional atmospheric retrieval framework for exoplanet
transmission spectra. AURA-3D includes a forward model that enables rapid
computation of transmission spectra in 3D geometry for a given atmospheric
structure and can, therefore, be used for atmospheric retrievals as well as for
computing spectra from General Circulation Models (GCMs). In order to
efficiently explore the space of possible 3D temperature structures in
retrievals, we develop a parametric 3D pressure-temperature profile which can
accurately represent azimuthally-averaged temperature structures of a range of
hot Jupiter GCMs. We apply our retrieval framework to simulated JWST
observations of hot Jupiter transmission spectra, obtaining accurate estimates
of the day-night temperature variation across the terminator as well as the
abundances of chemical species. We demonstrate an example of a model hot
Jupiter transmission spectrum for which a traditional 1D retrieval of
JWST-quality data returns biased abundance estimates, whereas a retrieval
including a day-night temperature gradient can accurately retrieve the true
abundances. Our forward model also has the capability to include inhomogeneous
chemistry as well as variable clouds/hazes. This new retrieval framework opens
the field to detailed multidimensional atmospheric characterisation using
transmission spectra of exoplanets in the JWST era.
