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

TL;DR

This study uses high-resolution spectropolarimetric data from DKIST to analyze fine structures within a solar flare ribbon, revealing small, bright blobs with complex spectral profiles and temperature variations, shedding light on flare chromospheric dynamics.

Contribution

First high-resolution multi-line spectropolarimetric analysis of flare ribbon fine structures, revealing detailed properties and potential formation mechanisms of ribbon blobs.

Findings

Identified compact ribbon blobs with sizes 320-455 km and ~1100 km spacing.

Observed temperature enhancements of ~1 kK in blob regions.

Detected complex spectral asymmetries and velocity flows in the chromosphere.

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.