Ionizing stellar photons heat the upper regions of planetary atmospheres , driving atmospheric mass loss .
Gas escaping from several hot , hydrogen-rich planets has been detected using UV and X-ray transmission spectroscopy .
Because these planets are tidally locked , and thus asymmetrically irradiated , escaping gas is unlikely to be spherically symmetric .
In this paper , we focus on the effects of asymmetric heating on local outflow structure .
We use the Athena code for hydrodynamics to produce 3D simulations of hot Jupiter mass loss that jointly model wind launching and stellar heating via photoionization .
Our fiducial planet is an inflated , hot Jupiter with radius R _ { p } = 2.14 R _ { Jup } and mass M _ { p } = 0.53 M _ { Jup } .
We irradiate the initially neutral , atomic hydrogen atmosphere with 13.6 eV photons and compute the outflow ’ s ionization structure .
There are clear asymmetries in the atmospheric outflow , including a neutral shadow on the planet ’ s nightside .
Given an incident ionizing UV flux comparable to that of the Sun , we find a steady-state mass loss rate of \sim 2 \times 10 ^ { 10 } g s ^ { -1 } .
The total mass loss rate and the outflow substructure along the substellar ray show good agreement with earlier 1D models , for two different fluxes .
Our 3D data cube can be used to generate the outflow ’ s extinction spectrum during transit .
As a proof of concept , we find absorption of stellar Ly \alpha at Doppler-shifted velocities of up to \pm 50 km s ^ { -1 } .
Our work provides a starting point for further 3D models that can be used to predict observable signatures of hot Jupiter mass loss .