Abstract
Context. Downflows with potentially super-sonic velocities have been reported to 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 and requires further research. Aims. In this article, we investigate the evolution of downflows detected within spectral lines sampling the transition region on time-scales of the order of minutes and we search for clues as to the formation mechanisms of these features in co-temporal imaging data. Methods. For the purposes of this article, we used high-resolution spectral and imaging data sampled by the Interface Region Imaging Spectrograph on the 20 and 21 May 2015 to identify and analyse downflows. Additionally, photospheric and coronal imaging data from the Hinode and Solar Dynamics Observatory satellites were studied to provide context about the wider solar atmosphere. Results. Four downflows were identified and analysed through time. The potential super-sonic components of these downflows had widths of around 2″ and were observed to evolve over time-scales of the order of 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 Å images. Downflows with apparent velocities below the super-sonic threshold that was assumed here were observed to extend a few arcseconds away from the foot-points, suggesting that the potential super-sonic components are linked to larger-scale flows. The electron density and mass flux for these events were found to be within the ranges of 109.6–1010.2 cm−3 and 10−6.81–10−7.48 g cm−2 s−1, respectively. Finally, each downflow formed at the foot-point of thin “fingers”, extending out around 3–5″ in Si IV 1400 Å data with smaller widths (< 1″) than the super-sonic downflow components. Conclusions. Downflows can appear, disappear, and recur within time-scales of less than one hour in sunspots. As the potential super-sonic downflow signatures were detected at the foot-points of both extended fingers in Si IV 1400 SJI data and sub-sonic downflows in Si IV 1394 Å spectra, it is likely that these events are linked to larger-scale flows within structures such as coronal loops.
Highlights
Sunspots are one of the most widely studied features in the zoo of solar phenomena
For a recent review of our understanding of the transition region above sunspots, see Tian et al (2018). The presence of both apparent sub-sonic and super-sonic downflows above sunspots in spectral lines forming at transition region temperatures was first reported nearly forty years ago using data from the High Resolution Telescope and Spectrograph (HRTS; see Dere 1982; Nicolas et al 1982)
Channel (Fig. 10) with the same orientation as a group of coronal fan loops observed in the SDO/AIA 171 Å filter. This potential alignment would agree with results presented in the previous literature and could support the hypothesis that flows within coronal loops are responsible for high velocity transition region downflows in the transition regions of sunspots
Summary
Sunspots are one of the most widely studied features in the zoo of solar phenomena. sunspots themselves can be (relatively) stable over the course of days, many transient events with lifetimes of the order of minutes to hours have been observed to occur within them. Higher in the atmosphere (the transition region and corona), bright umbral dots (Tian et al 2014a), shock wave behaviour (Tian et al 2014b), and, of most interest here, downflows which would be super-sonic at transition region temperatures (Dere 1982) have been detected in a large number of sunspots. For a recent review of our understanding of the transition region above sunspots, see Tian et al (2018) The presence of both apparent sub-sonic and super-sonic downflows above sunspots in spectral lines forming at transition region temperatures was first reported nearly forty years ago using data from the High Resolution Telescope and Spectrograph (HRTS; see Dere 1982; Nicolas et al 1982)
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