  Magnetic fields connect an array of planetary processes, from atmospheric
escape to interior convection. Despite their importance, exoplanet magnetic
fields are largely unconstrained by both theory and observation. In this
Letter, we propose a novel method for constraining the B field strength of hot
gas giants: comparing the velocities of heavy ions and neutral gas with
high-resolution spectroscopy. The core concept of this method is that ions are
directly deflected by magnetic fields. While neutrals are also affected by B
fields via friction with field-accelerated ions, ionic gas should be more
strongly coupled to the underlying magnetic field than bulk neutral flow.
Hence, measuring the difference between the two velocities yields rough
constraints on the B field, provided an estimate of the stellar UV flux is
known. We demonstrate that heavy ions are particularly well suited for this
technique, because they are less likely to be entrained in complex hydrodynamic
outflows than their lighter counterparts. We perform a proof-of-concept
calculation with Ba II, an ion whose velocity has been repeatedly measured at
high confidence with high-resolution spectroscopy. Our work shows that a 10G
magnetic field would produce ~ km/s ion--neutral velocity differences at a
microbar, whereas a 50G magnetic field would produce ~20km/s velocity
difference. With new leverage on magnetic fields, we will be able to
investigate magnetic field generation in the extreme edge cases of hot gas
giants, with wide-ranging consequences for planetary interior structure, dynamo
theory, and habitability.
