An Optically Thin View of the Flaring Chromosphere: Nonthermal widths in a chromospheric condensation during an X-class solar flare

The flaring chromosphere is where the bulk of the energy released in a flare is deposited, and is the origin of the bulk of the enhanced radiative output. Flare ribbons and footpoints in the chromosphere therefore offer great diagnostic potential of the energy release and transport processes at work during solar flares. Much work has been done in recent years to exploit the high spatial, temporal and spectral observations from the IRIS spacecraft, which has transformed our view of the flaring chromosphere. However, the strong chromospheric lines observed by IRIS are optically thick, requiring forward modelling to fully appreciate and extract the information they carry. Reproducing certain aspects of the Mg II resonance lines remain frustratingly out of reach in state-of-the-art flare models. Primary among those is that models are unable to satisfactorily reproduce the very broad line profiles, with synthetic line widths that are much too narrow. A commonly proposed resolution to this is to assert that very large values of microturbulence is required. We asses the validity of that approach by analysing optically thin lines in the flare chromosphere, and using the observationally derived value of nonthermal width as inputs to our numerical models. A nonthermal width of the order 10 km/s was found within the red wing components of three chromospheric lines, with relatively narrow stationary components. Radiation hydrodynamic simulations of this flare were produced, and in the post-processing steps to synthesise the IRIS lines we include within the downflows a microturbulence of 10 km/s. It was found that while we can reproduce the O I 1355.598A line rather well with this set up, and can capture the general shape and properties of the Mg II line widths, the synthetic lines are still too narrow. We speculate as to what the missing ingredients from our models that may lead to the broader observed line wings might be.