Small-scale swirl events in the quiet Sun chromosphere
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.
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