Evolution Of Downflows In The Transition Region Above A Sunspot Over Short Time-Scales

TL;DR

This study investigates the short-term evolution of high-velocity downflows in the transition region above sunspots, revealing their dynamic nature, structure, and potential connection to larger-scale flows like coronal loops.

Contribution

It provides detailed analysis of super-sonic and sub-sonic downflows' temporal and spatial evolution, linking them to specific structures and formation mechanisms in the solar atmosphere.

Findings

Downflows evolve over minutes and can recur within an hour.

Super-sonic components are linked to larger-scale flows and structures.

Downflows are associated with bright regions and thin 'fingers' extending from foot-points.

Abstract

Downflows with high velocities occur in the transition region above many sunspots; however, how these signatures evolve over short time-scales in both spatial and spectral terms is still unknown. In this article, we investigate the evolution of downflows detected within the transition region on time-scales of the order minutes and search for clues as to their formation mechanisms in co-temporal imaging data. The high-resolution spectral and imaging data used to identify downflows here were sampled by IRIS on the 20th and 21st May 2015. Imaging data from the Hinode and SDO satellites provided context about the wider solar atmosphere. Four downflows were identified and analysed. The potential super-sonic components of these downflows had widths of around 2" and evolved over time-scales of the order minutes. The measured apparent downflow velocities were structured both in time and space, with the highest apparent velocities occurring above a bright region detected in Si IV 1400 {\AA} images. Downflows with apparent velocities below 50 km s^{-1} were observed to extend a few arcsecs away from the foot-points suggesting the potential super-sonic components are linked to larger-scale flows. The electron density and mass flux for these events were within the ranges 10^{9.6}-10^{10.2} cm^{-3} and 10^{-6.81}-10^{-7.48} g cm^{-2} s^{-1}, respectively. Each downflow formed at the foot-point of thin `fingers' extending out around 3-5" in Si IV 1400 {\AA} data with smaller widths (<1") than the super-sonic downflows. Downflows can appear, disappear, and recur within time-scales of less than one hour in sunspots. As potential super-sonic downflow signatures were detected at the foot-points of both extended fingers in Si IV 1400 {\AA} SJI data and sub-sonic downflows in Si IV 1394 \AA\ spectra, it is likely that these events are linked to flows within structures such as coronal loops.