Narrow-line-width UV bursts in the transition region above Sunspots observed by IRIS
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Various small-scale structures abound in the solar atmosphere above active regions, playing an important role in the dynamics and evolution therein. We report on a new class of small-scale transition region structures in active regions, characterized by strong emissions but extremely narrow Si IV line profiles as found in observations taken with the Interface Region Imaging Spectrograph (IRIS).
Tentatively named as Narrow-line-width UV bursts (NUBs), these structures are located above sunspots and comprise of one or multiple compact bright cores at sub-arcsecond scales (see Fig. 2 a1-e1). We found six NUBs in two datasets (a raster in Fig.1 b and c, and a sit-and-stare dataset in Fig. 1 i). Among these, four events are short-living with a duration of ~10 mins while two last for more than 36 mins in Fig. 2 a2-e2. All NUBs have Doppler shifts of 15-18 km s-1, while the NUB found in sit-and-stare data possesses an additional component at ~50 km s-1 found only in the C II and Mg II lines.
Given that these events are found to play a role in the local dynamics, it is important to further investigate the physical mechanisms that generate these phenomena and their role in the mass transport in sunspots.
This work has been accepted for publication in ApJ Letter. We acknowledge the support from the 973 program, National Natural Science Foundation of China and Shandong provincial Natural Science Foundation.
The full article is available at http://adsabs.harvard.edu/abs/2016arXiv160804892H.
Figure 1. Identification of the events in the observed FOVs. Panel (a): two-dimensional histogram of the non-thermal velocities and intensities obtained from DATA1. The solid-line square marks the tail of the histogram that was used to make the identification of the events. Panels (b) and (c) present the Si iv radiance images of the two rasters of the same region with the identifications of the events superimposed (green symbols). Panel (d) displays the same region with the identified events observed in the continuum near 2832Å to give an overview of the photosphere of the region. Panels (e)–(g) give example spectra with single Gaussian fits (solid lines) averaged from event 4. The dashed vertical lines (in panels f and g) mark the wavelength range that shows absorption dips and are excluded from the Gaussian fits. Panels (f) and (i) present the results from DATA2.
Figure 2. Events 1–5 identified in DATA1 seen in IRIS SJ 1400 Å (Panels a1–e1). They are marked by the solid-line squares (yellow) and contoured by green lines. Their IRIS SJ 1400 and 1330 Å lightcurves are given as black and red lines in Panels a2–e2. The dashed lines denote the period of the IRIS spectral slit scan.