Hui Tian1 , Xiaoshuai Zhu2,3 , Hardi Peter3 , Jie Zhao4 , Tanmoy Samanta1 , Yajie Chen1
1. School of Earth and Space Sciences, Peking University, Beijing 100871, China
2. Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
3. Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
4. Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, CAS, Nanjing 210008, China
As one of the most important mechanisms of energy release, magnetic reconnection plays an important role in the energization of space and astrophysical plasma. In the past decade, due to high-resolution observations by advanced telescopes, small-scale reconnection in the partially ionized lower solar atmosphere has received a lot of attention. One type of such reconnection events is ultraviolet (UV) bursts (or IRIS bomb), which are revealed as compact intense brightenings in the transition region images obtained with the 1400 and 1330 Å filters of IRIS. IRIS observations show that UV bursts tend to occur in emerging active regions. However, almost all the existing IRIS of UV bursts were observed during relatively late stages of the flux emergences (more than ~2 hr after the start of a flux emergence).
Figure 1| IRIS/SJI 2832 Å images, 1400 Å images, and HMI line-of-sight magnetograms taken around 12:32 UT, 16:09 UT, and 19:11 UT. Blue/purple contours mark the UV bursts seen in the 1400 Å/1700 Å images.
On 2016 September 20, IRIS observed an AR during its earliest emerging phase for almost 7 hr, starting from the first sign of flux emergence. Such a unique observation allows us to study the detailed evolution of UV bursts during the earliest stage of flux emergence. At the beginning, the region observed by IRIS appears to be a typical quiet-Sun region, showing obvious network structures in the HMI magnetograms and IRIS1400 Å images. From the photospheric images, no sunspots can be identified in the region scanned by the IRIS slit. As time evolves, HMI observed continuous emergence of small-scale magnetic bipoles with a rate of ∼1016 Mx/s. The emergence of magnetic fluxes and interactions between different polarities lead to frequent occurrence of the UV bursts, which exhibit as intense transient brightenings in the IRIS 1400 Å and AIA 1700 Å images. In the meantime, discrete small patches with the same magnetic polarity tend to move together and merge, leading to the enhancement of the magnetic fields and thus the formation of pores at some locations (Figure 1).
Figure 2| IRIS spectra of a UV burst. (A)–(B) IRIS 1400 Å SJI image and spectral image of the Si IV 1393.755 Å line taken at 18:52:12 UT. (C) The Si IV 1393.755 Å line profiles at the locations marked by the four diamonds with different colors.
The spectra of these UV bursts are characterized by the superposition of several chromospheric absorption lines on the greatly broadened profiles of some emission lines formed at typical transition region temperatures, which suggests heating of local materials to a few tens of thousands of Kelvin in the lower atmosphere by magnetic reconnection. Some bursts reveal blue- and redshifts of ∼100 km/s at neighboring pixels, indicating the spatially resolved bidirectional reconnection outflows (Figure 2).
Figure 3| (A)–(C) IRIS/SJI 1400 Å image, HMI line-of-sight magnetogram and SDO/AIA 1700 Å image taken at 16:36 UT. (D)–(F) Images of the squashing factor at the heights of 0 Mm, 1.4 Mm, and 3.6 Mm above the photosphere. Blue contours outlining the compact brightenings in the 1400 Å image at 16:36 UT are overplotted in other images. The green dots indicate locations of bald patches.
Most of the UV bursts appear to result from interactions between magnetic fields of different polarities. Flux cancellation with a rate of the order of ∼1015 Mx/s can be identified for many bursts. By inputting the vector photospheric magnetograms measured by HMI to a magnetohydrostatic model, we reconstruct three-dimensional magnetic field structures for the UV bursts, and find that a small fraction of the bursts is associated with bald patches (magnetic dips). We also investigate the magnetic connectivities for the UV bursts, and find that almost all bursts are located in regions with large squashing factors around the height of z=1 Mm (Figure 3). Since large squashing factors are a reflection of strong gradients of magnetic connectivities and strong electric current, this result supports the suggestion that UV bursts are powered by small-scale magnetic reconnection in the lower solar atmosphere. Considering their coincidence with not only bald patches but also separatrices, we believe these reconnection events are similar to Ellerman bombs. Indeed, our recent investigation showed that roughly half of the UV bursts are likely related to Ellerman bombs.
For details of this work, please refer to our rencent publication:
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