Authors: Cole Tamburri (CU Boulder, NSO, LASP); Adam F. Kowalski (CU Boulder, NSO, LASP), Maria D. Kazachenko (CU Boulder, NSO, LASP), Gianna Cauzzi (NSO), Alexandra Tritschler (NSO), Rahul Yadav (CU Boulder, NSO, LASP), Ryan French (LASP), Yuta Notsu (NSO, LASP), Kevin Reardon (NSO), Isaiah Tristan (CU Boulder, NSO, LASP)

Despite significant advances in solar and stellar flare modeling in recent years, there remain important discrepancies and unsolved mysteries in the broadening of chromospheric spectral lines during a flare. We use observations of the decay phase of a GOES class C6.7 flare occurring on 19 August 2022 at 20:42 UT from the Visible Spectropolarimeter (ViSP) at the National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) to analyze spectral evolution of the Ca II H 396.8 nm and Hε 397.0 nm lines. These are the first flare observations of this unique spectral window with the DKIST. We compare the ViSP observations to 1D radiative-hydrodynamic RADYN simulations and find that the model atmospheres, for an interval some 20 seconds after the energy injection, are able to generally reproduce the observed Ca II H andHε line profiles. This indicates that electron-beam-heated atmospheres are relevant to flare heating even in the gradual decay phase. We also place preliminary constraints on parameters relevant to the broadening of these lines, and find that a model with microturbulence parameter ξ = 5 km s^{-1} and multiplicative factor b = 20 on the standard quadratic Stark-Lo Surdo line width reproduces the intensity of Ca II H in the blue wing, although the simulation fails to reproduce the larger observed width in the red wing. We derive condensation electron densities ~4-5.3 x 10^{13} cm^{-3} at t = 43s in the simulation. Unlike lower-order Balmer-series lines, the width of Hε does not appear directly related to condensation electron density. We discuss the implications of this work and next steps in the context of RHD modeling and the present age of solar observations.