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Explosive events in active region observed by IRIS and SST/CRISP

Z. Huang,1M. S. Madjarska,2 ,1 ,3E. M. Scullion,4L.-D. Xia,1J. G. Doyle2and T. Ray5

1Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai,264209 Shandong, China

2Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK

3Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Gottingen, Germany

4Department of Mathematics and Information Sciences, Northumbria University, Newcastle Upon Tyne NE1 8ST, UK

5Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland

 2017, MNRAS, 464, 1753

(ADS link: http://adsabs.harvard.edu/abs/2017MNRAS.464.1753H)



Transition-region explosive events (EEs) are characterized by non-Gaussian line profiles withenhanced wings at Doppler velocities of 50–150 km/s. They are believed to be the signatureof solar phenomena that are one of the main contributors to coronal heating. The aim ofthis study is to investigate the link of EEs to dynamic phenomena in the transition regionand chromosphere in an active region. We analyse observations simultaneously taken by theInterface Region Imaging Spectrograph (IRIS) in the Si IV 1394Å line and the slit-jaw (SJ)1400Å images, and the Swedish 1-m Solar Telescope in the Hα line. In total 24 events werefound. They are associated with small-scale loop brightenings in SJ 1400Å images. Only fourevents show a counterpart in the Hα− 35 km/sand Hα+ 35 km/simages. Two of themrepresent brightenings in the conjunction region of several loops that are also related to abright region (granular lane) in the Hα− 35 km/s and Hα+ 35 km/simages. 16 are generalloop brightenings that do not show any discernible response in the Hα images. Six EEs appearas propagating loop brightenings, from which two are associated with dark jet-like featuresclearly seen in the Hα− 35 km/simages. We found that chromospheric events with jet-likeappearance seen in the wings of the Hα line can trigger EEs in the transition region and in thiscase the IRIS Si IV  1394Å line profiles are seeded with absorption components resulting fromFe II and Ni II. Our study indicates that EEs occurring in active regions have mostly upperchromosphere/transition-region origin.We suggest that magnetic reconnection resulting fromthe braidings of small-scale transition region loops is one of the possiblemechanisms of energyrelease that are responsible for the EEs reported in this paper.


Figure 5. Si IV 1394Å EE spectra found in a loop brightening (event no. 20). From top to bottom, first three rows show the evolution of the loop region seen inthe IRIS SJ 1400 Å, Hα−35 km/s(HαB), and Hα+35 km/s(HαR), respectively. The dotted lines on the Hα images show the locations of the spectrometerslit. The plus signs (in red and green) are the locations of the identified EE spectra. The black box on the SJ image at 08:03:29 UT denotes the region, fromwhich the light curve (bottom row of this figure) is obtained. Bottom row: the left-hand panel displays the Si IV radiance image (top) and RB asymmetry at45–55 km/s(bottom) of the region, in which the event is outlined by a contour line (black solid line). The spectra shown in the middle panel are taken fromthe pixels marked with diamond symbols in the left-hand panel. The dashed line denotes the time (08:03:29 UT) when the Si IV spectra turn to single-Gaussian.The middle panel displays the variation of the Si IV spectra taken from the pixels shown in the left-hand panel (time is denoted by colours). The right-handpanel shows an SJ 1400Å light curve taken from the box region marked in the SJ 1400Å image, where the dashed line marks the time at 08:03:29 UT.

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