Authors: Ryan J. French (LASP), Alin R. Paraschiv (NSO), Maria Kazachenko (LASP/NSO), Thomas A. Schad (NSO)

Solar flares are powered by the conversion of coronal magnetic energy into the acceleration of particles, heating of plasma, and emission across the electromagnetic continuum. However, despite magnetism being the key driver of solar flare evolution, there are very few techniques allowing the measurement of coronal magnetic fields in flares. In this work, we present observations of the decay phase of a long-duration X-class solar flare from the U.S. National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) Cryo-NIRSP instrument, capturing spectropolarimetric measurements of the coronal infrared Fe XIII 1074.7 and 1079.7 nm lines. We successfully measure Fe XIII circular polarization (Stokes V) of flare loops for the first time (in both lines), tracking the spatial and temporal evolution of line-of-sight magnetic field component. At the flare looptops, the magnetic geometry allows us to assume that B_los ≈ B_tot. Combining these looptop magnetic field measurements with density maps determined from the density-sensitive Fe XIII 1074.7 / 1079.7 nm pair, we calculate the Alfvén velocity of stable regions of the flare loop arcade looptops. We compare the Alfvén velocity at these locations with observed looptop Doppler and non-thermal velocities, providing insights into the nature of bulk and unresolved plasma flows in flare loops.