Multi-line Spectropolarimetric Observation of Flare Ribbon Fine Structures with ViSP/DKIST

Abstract: We present an analysis of flare ribbon fine structure observed during a GOES C2-class flare using high spatial and spectral resolution multi-line spectropolarimetric observations from the ViSP instrument at the DKIST. The ViSP recorded full Stokes spectra in three lines: Fe I 630.2 nm line pair, Na I D1, and Ca II 854.2 nm. To infer the stratification of temperature and line-of-sight (LOS) velocity across the ribbon, we performed non-LTE multi-line inversions of ViSP spectra. In the red wing of the Ca II 854.2 nm line, we identified multiple compact, roundish, and quasi-equally spaced bright structures, referred to as ribbon blobs, embedded within the flare ribbon. The sizes of these blobs range from 320 to 455 km, and they are spaced roughly ~1100 km apart. These features exhibit complex spectral profiles with pronounced asymmetries and double peaks near the line core of the Ca II 854.2 nm line. The blob regions were found to be significantly hotter (by ~1 kK) at log {\tau} = -4 compared to surrounding ribbon areas (~7 kK). The LOS velocity maps revealed both upflows and downflows at log {\tau} = -4$ and log {\tau} = -3, respectively. We discuss the plausible origins of these fine structures in the chromosphere, which may be related to electron beam heating, plasma draining, or tearing-mode instabilities in the reconnecting current sheet.