High-resolution observational analysis of flare ribbon fine structures

Abstract

Since the mechanism of energy release from solar flares is still not fully understood, the study of fine-scale features developing during flares becomes important for progressing towards a consistent picture of the essential physical mechanisms. Solar flares release most of their luminous energy in the chromosphere. These luminous signatures, known as flare ribbons, act as the footpoints of the released energy and are crucial for the interpretation of reconnection processes causing these events. We aim to probe the fine structures in flare ribbons at the chromospheric level using high-resolution observations with imaging and spectral techniques. We present a GOES C2.4 class solar flare (SOL2022-06-26T08:12) observed with the Swedish 1-m Solar Telescope (SST), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA). Utilising imaging data from SST, IRIS, and AIA, we detail both the global and fine-structure evolution of the flare. The high-resolution SST observations offer spectroscopic data in the Halpha Ca II and Hbeta lines, which we use to analyse the flare ribbon. The flare was associated with a filament eruption. Fibrils and coronal loops were connected from a negative polarity region to two positive polarity regions. Within the eastern flare ribbon, chromospheric bright blobs were detected and analysed in Ca II Halpha , and Hbeta wavelengths. A comparison of blobs in Hbeta observations and Si IV has also been performed. These blobs are observed as almost circular structures having widths from 140 km--200 km. The intensity profiles of the blobs show a red wing asymmetry. From the high spatial and temporal resolution Hbeta observations, we conclude that the periodicity of the blobs in the flare ribbon, which are near-equally spaced in the range 330--550 km, is likely due to fragmented reconnection processes within a flare current sheet. This supports the theory of a direct link between fine-structure flare ribbons and current sheet tearing. We believe our observations represent the highest resolution evidence of fine-structure flare ribbons to date.