SST/CRISP Observations of Convective Flows in a Sunspot Penumbra

TL;DR

This study uses high-resolution spectropolarimetric data from SST/CRISP to confirm the presence of convective downflows in sunspot penumbrae, supporting recent simulations and revealing their ubiquity across penumbral structures.

Contribution

First detection of convective downflows in both core and wing of Fe I line in sunspot penumbrae at high spatial resolution, confirming overturning convection.

Findings

Convective downflows are evident in both 538.03 nm and 630.15 nm lines.

Downflows are present in both spines and inter-spines.

Small-scale intensity patterns are strongly related to convective flow patterns.

Abstract

Context. Recent discoveries of intensity correlated downflows in the interior of a sunspot penumbra provide direct evidence for overturning convection, adding to earlier strong indications of convection from filament dynamics observed far from solar disk center, and supporting recent simulations of sunspots. Aims. Using spectropolarimetric observations obtained at a spatial resolution approaching 0'.'1 with the Swedish 1-m Solar Telescope (SST) and its spectropolarimeter CRISP, we investigate whether the convective downflows recently discovered in the C i line at 538.03 nm can also be detected in the wings of the Fe i line at 630.15 nm Methods. We make azimuthal fits of the measured LOS velocities in the core and wings of the 538 nm and 630 nm lines to disentangle the vertical and horizontal flows. To investigate how these depend on the continuum intensity, the azimuthal fits are made separately for each intensity bin. By using spatially high-pass filtered measurements of the LOS component of the magnetic field, the flow properties are determined separately for magnetic spines (relatively strong and vertical field) and inter-spines (weaker and more horizontal field). Results. The dark convective downflows discovered recently in the 538.03 nm line are evident also in the 630.15 nm line, and have similar strength. This convective signature is the same in spines and inter-spines. However, the strong radial (Evershed) outflows are found only in the inter-spines. Conclusions. At the spatial resolution of the present SST/CRISP data, the small-scale intensity pattern seen in continuum images is strongly related to a convective up/down flow pattern that exists everywhere in the penumbra. Earlier failures to detect the dark convective downflows in the interior penumbra can be explained by inadequate spatial resolution in the observed data.