Small-scale swirl events in the quiet Sun chromosphere

TL;DR

This study uses high-resolution solar observations to identify and analyze small-scale, dynamic swirl events in the quiet Sun chromosphere, revealing their structure, motion, and possible magnetic origins.

Contribution

It provides new detailed observations of small-scale swirl phenomena in the quiet Sun chromosphere, highlighting their morphology, dynamics, and potential magnetic field interactions.

Findings

Swirl events feature dark and bright rotating patches with spiral or ring shapes.

Swirls have diameters around 2 inches and exhibit fast upflows of up to -7 km/s.

Bright points near swirls suggest magnetic field twisting and wave-guided plasma spiraling.

Abstract

Recent progress in instrumentation enables solar observations with high resolution simultaneously in the spatial, temporal, and spectral domains. We use such high-resolution observations to study small-scale structures and dynamics in the chromosphere of the quiet Sun. We analyze time series of spectral scans through the Ca II 854.2nm spectral line obtained with the CRISP instrument at the Swedish 1-m Solar Telescope. The targets are quiet Sun regions inside coronal holes close to disc-centre. The line core maps exhibit relatively few fibrils compared to what is normally observed in quiet Sun regions outside coronal holes. The time series show a chaotic and dynamic scene that includes spatially confined "swirl" events. These events feature dark and bright rotating patches, which can consist of arcs, spiral arms, rings or ring fragments. The width of the fragments typically appears to be on the order of only 0.2", which is close to the effective spatial resolution. They exhibit Doppler shifts of -2 to -4 km/s but sometimes up to -7 km/s, indicating fast upflows. The diameter of a swirl is usually of the order of 2". At the location of these swirls, the line wing and wide-band maps show close groups of photospheric bright points that move with respect to each other. A likely explanation is that the relative motion of the bright points twists the associated magnetic field in the chromosphere above. Plasma or propagating waves may then spiral upwards guided by the magnetic flux structure, thereby producing the observed intensity signature of Doppler-shifted ring fragments.