The transition region above sunspots

Samanta, Tanmoy; Zhang, Jingwen

doi:10.1186/s40562-018-0103-1

Cited by 8 publications

(6 citation statements)

References 61 publications

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“…The times and co-ordinates over-laid on each panel indicate the start time and centre position in the x−direction of the corresponding IRIS raster. temporal, and spatial resolution data since the launch of the Interface Region Imaging Spectrograph (IRIS; De Pontieu et al 2014) has allowed improved understanding of these events over recent years (for a recent review see Tian et al 2018).…”

Section: Introductionmentioning

confidence: 99%

Evolution of supersonic downflows in a sunspot

Nelson

Prasad

Mathioudakis

2020

A&A

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Context. Super-sonic downflows have been observed in transition region spectra above numerous sunspots; however, little research has been conducted to date into how persistent these signatures are within sunspots on time-scales longer than a few hours. Aims. Here, we aim to analyse the lead sunspot of AR 12526 to infer the properties and evolution of super-sonic downflows occurring within it using high-spatial and spectral resolution data. Methods. Sixteen large, dense raster scans sampled by the Interface Region Imaging Spectrograph are analysed. These rasters tracked the lead sunspot of AR 12526 across the solar disc at discrete times between the 27th March 2016 and the 2nd April 2016, providing spectral profiles from the Si iv, O iv, Mg ii, and C ii lines. Additionally, one sit-and-stare observation acquired on the 1st April 2016 centred on the sunspot is studied in order to analyse the evolution of super-sonic downflows on shorter time-scales. Results. Super-sonic downflows are variable within this sunspot both in terms of spatial structuring and velocities. 13 of the 16 raster scans display some evidence of super-sonic downflows in the Si iv 1394 Å line co-spatial to a sustained bright structure detected in the 1400 Å slit-jaw imaging channel, with a peak velocity of 112 km s −1 being recorded on the 29th March 2016. Evidence for super-sonic downflows in the O iv 1401 Å line was found in 14 of these rasters, with the spatial structuring in this line often differing from that inferred from the Si iv 1394 Å line. Only one example of a super-sonic downflow was detected in the C ii 1335 Å line, with no downflows being found in the Mg ii 2796 Å lines at these locations. In the sit-and-stare observations, no dual flow is initially detected, however, a super-sonic downflow does develop after around 60 minutes. This downflow accelerates from 73 km s −1 to close to 80 km s −1 in both the Si iv 1394 Å and O iv 1401 Å lines over the course of 20 minutes before the end of the observation. Conclusions. Super-sonic downflows were found in the Si iv 1394 Å line in 13 of the 16 rasters studied here. The morphology of these downflows evolved over the course of both hours and days and was often different in the Si iv 1394 Å and O iv 1401 Å lines. These events were found co-spatial to a bright region in the core of the Si iv 1394 Å line which appeared to form at the foot-points of coronal fan loops. Our results indicate that one raster is not enough to conclusively draw inferences about the properties of super-sonic downflows within a sunspot during its lifetime.

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Section: Introductionmentioning

confidence: 99%

Evolution of supersonic downflows in a sunspot

Nelson

Prasad

Mathioudakis

2020

A&A

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“…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).…”

Section: Introductionmentioning

confidence: 99%

Evolution of downflows in the transition region above a sunspot over short time-scales

Nelson

Prasad

Mathioudakis

2020

A&A

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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.

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“…Unprecedented observations of sunspots by IRIS have unveiled several new types of small-scale dynamic events in the TR above sunspots, e.g., penumbral bright dots (Tian et al 2014a;Vissers et al 2015;Alpert et al 2016;Samanta et al 2017), UV bursts around sunspots and light bridges (Toriumi et al 2015;Tian et al 2018a;Reid et al 2018). For an overview of these newly discovered phenomena in the TR above sunspots, we refer to Tian (2017) and Tian et al (2018b).…”

Section: Introductionmentioning

confidence: 99%

Statistical Investigation of Supersonic Downflows in the Transition Region above Sunspots

Samanta

Choudhary

2018

ApJ

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Downflows at supersonic speeds have been observed in the transition region (TR) above sunspots for more than three decades. These downflows are often seen in different TR spectral lines above sunspots. We have performed a statistical investigation of these downflows using a large sample which was missing earlier. The Interface Region Imaging Spectrograph (IRIS) has provided a wealth of observational data of sunspots at high spatial and spectral resolution in the past few years. We have identified sixty datasets obtained with IRIS raster scans. Using an automated code, we identified the locations of strong downflows within these sunspots. We found that around eighty percent of our sample show supersonic downflows in the Si IV 1403Å line. These downflows mostly appear in the penumbral regions, though some of them are found in the umbrae. We also found that almost half of these downflows show signatures in chromospheric lines. Furthermore, a detailed spectral analysis was performed by selecting a small spectral window containing the O IV 1400/1401Å and Si IV1403Å lines. Six Gaussian functions were simultaneously fitted to these three spectral lines and their satellite lines associated with the supersonic downflows. We calculated the intensity, Doppler velocity and line width for these lines. Using the O IV 1400/1401Å line ratio, we find that the downflow components are around one order of magnitude less dense than the regular components. Results from our statistical analysis suggest that these downflows may originate from the corona and that they are independent of the background TR plasma.

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The transition region above sunspots

Cited by 8 publications

References 61 publications

Evolution of supersonic downflows in a sunspot

Evolution of supersonic downflows in a sunspot

Evolution of downflows in the transition region above a sunspot over short time-scales

Statistical Investigation of Supersonic Downflows in the Transition Region above Sunspots

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