Atmospheric escape from close-in exoplanets is thought to be crucial in shaping observed planetary populations. Recently, significant progress has been made in observing this process in action through excess absorption in transit spectra and narrowband light curves. We present a 3D hydrodynamic simulation and radiative transfer post-processing method for modeling the interacting flows of escaping planetary atmosphere and stellar winds. We focus on synthetic transmission spectra of the helium 1083 nm line, and discuss a planetary outflow of fixed mass-loss rate that interacts with stellar winds of varying order of magnitude. The morphology of these outflows in differing stellar wind environments changes dramatically, from torii that completely encircle the star when the ram pressure of the stellar wind is low, to cometary tails of planetary outflow when the stellar wind ram pressure is high. Our results demonstrate that this interaction leaves important traces on line kinematics and spectral phase curves in the helium 1083 nm triplet. In particular, the confinement of outflows through wind--wind collisions leads to absorption that extends in phase and time well beyond the optical transit. We further demonstrate that these differences are reflected in light curves of He 1083 nm equivalent width as a function of transit phase. Our results suggest that combining high-resolution spectroscopy with narrowband photometry offers a path to observationally probe how stellar wind environments shape exoplanetary atmosphere escape.
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