  Planetary magnetic fields can affect the predicted mass loss rate for
close-in planets that experience large amounts of UV irradiation. In this work,
we present a method to detect the magnetic fields of close-in exoplanets
undergoing atmospheric escape using transit spectroscopy at the 10830 Angstrom
line of helium. Motivated by previous work on hydrodynamic and
magneto-hydrodynamic photoevaporation, we suggest that planets with magnetic
fields that are too weak to control the outflow's topology lead to blue-shifted
transits due to day-to-night-side flows. In contrast, strong magnetic fields
prevent this day-to-night flow, as the gas is forced to follow the magnetic
field's roughly dipolar topology. We post-process existing 2D photoevaporation
simulations to test this concept, computing synthetic transit profiles in
helium. As expected, we find that hydrodynamically dominated outflows lead to
blue-shifted transits on the order of the sound speed of the gas. Strong
surface magnetic fields lead to unshifted or slightly red-shifted transit
profiles. High-resolution observations can distinguish between these profiles;
however, eccentricity uncertainties generally mean that we cannot conclusively
say velocity shifts are due to the outflow for individual planets. The majority
of helium observations are blue-shifted, which could be a tentative indication
that close-in planets generally have surface dipole magnetic field strengths
$\lesssim 0.1$ gauss. More 3D hydrodynamic and magneto-hydrodynamic are needed
to confirm this conclusion robustly.
