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

Zhenghua Huang   Submitted: 2016-09-29 00:43

Transition-region explosive events (EEs) are characterized by non-Gaussian line profiles with enhanced wings at Doppler velocities of 50-150 km s-1. They are believed to be the signature of solar phenomena that are one of the main contributors to coronal heating. The aim of this study is to investigate the link of EEs to dynamic phenomena in the transition region and chromosphere in an active region. We analyze observations simultaneously taken by the Interface Region Imaging Spectrograph (IRIS) in the Si IV 1394Å line and the slit-jaw (SJ) 1400Å images, and the Swedish 1-m Solar Telescope (SST) in the Hα line. In total 24 events were found. They are associated with small-scale loop brightenings in SJ 1400Å images. Only four events show a counterpart in the Hα -35 km s-1 and Hα +35 km s-1 images. Two of them represent brightenings in the conjunction region of several loops that are also related to a bright region (granular lane) in the Hα -35km s-1 and Hα +35 km s-1 images. Sixteen are general loop brightenings that do not show any discernible response in the Hα images. Six EEs appear as propagating loop brightenings, from which two are associated with dark jet-like features clearly seen in the Hα -35 km s-1 images. We found that chromospheric events with jet-like appearance seen in the wings of the Hα line can trigger EEs in the transition region and in this case the IRIS Si IV 1394Å line profiles are seeded with absorption components resulting from Fe II and Ni II. Our study indicates that EEs occurring in active regions have mostly upper-chromosphere/transition-region origin. We suggest that magnetic reconnection resulting from the braidings of small-scale transition region loops is one of the possible mechanisms of energy release that are responsible for the EEs reported in this paper.

Authors: Z. Huang, M. S. Madjarska, E. M. Scullion, L.-D. Xia, J. G. Doyle, T. Ray
Projects: IRIS, Swedish Solar telescope (SST)

Publication Status: accepted for publication in MNRAS
Last Modified: 2016-10-03 12:31
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Cool transition region loops observed by the Interface Region Imaging Spectrograph  

Zhenghua Huang   Submitted: 2015-07-29 09:46

We report on the first Interface Region Imaging Spectrograph (IRIS) study of cool transition region loops. This class of loops has received little attention in the literature. A cluster of such loops was observed on the solar disk in active region NOAA11934, in the Si IV 1402.8 ? spectral raster and 1400 Å slit-jaw (SJ) images. We divide the loops into three groups and study their dynamics and interaction. The first group comprises relatively stable loops, with 382-626 km cross-sections. Observed Doppler velocities are suggestive of siphon flows, gradually changing from -10 km s-1 at one end to 20 km s-1 at the other end of the loops. Nonthermal velocities from 15 to 25 km s-1 were determined. These physical properties suggest that these loops are impulsively heated by magnetic reconnection occurring at the blue-shifted footpoints where magnetic cancellation with a rate of 1015 Mx/s is found. The released magnetic energy is redistributed by the siphon flows. The second group corresponds to two footpoints rooted in mixed-magnetic-polarity regions, where magnetic cancellation occurred at a rate of 1015 Mx/s and line profiles with enhanced wings of up to 200 km s-1 were observed. These are suggestive of explosive-like events. The Doppler velocities combined with the SJ images suggest possible anti-parallel flows in finer loop strands. In the third group, interaction between two cool loop systems is observed. Evidence for magnetic reconnection between the two loop systems is reflected in the line profiles of explosive events, and a magnetic cancellation rate of 3x1015 Mx/s observed in the corresponding area. The IRIS observations have thus opened a new window of opportunity for in-depth investigations of cool transition region loops. Further numerical experiments are crucial for understanding their physics and their role in the coronal heating processes.

Authors: Zhenghua Huang, Lidong Xia, Bo Li, Maria Madjarska
Projects: IRIS

Publication Status: ApJ (accepted)
Last Modified: 2015-07-29 13:00
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Explosive events on subarcsecond scale in IRIS observations: a case study  

Zhenghua Huang   Submitted: 2014-11-17 20:22

We present study of a typical explosive event (EE) at subarcsecond scale witnessed by strong non-Gaussian profiles with blue- and red-shifted emission of up to 150 km s-1 seen in the transition-region Si iv 1402.8 Å, and the chromospheric Mg ii k 2796.4 Å and C ii 1334.5 Å observed by the Interface Region Imaging Spectrograph (IRIS) at unprecedented spatial and spectral resolution. For the first time an EE is found to be associated with very small-scale (∼120 km wide) plasma ejection followed by retraction in the chromosphere. These small-scale jets originate from a compact bright-point-like structure of ∼1.5'' size as seen in the IRIS 1330 Å images. SDO/AIA and SDO/HMI co-observations show that the EE lies in the footpoint of a complex loop-like brightening system. The EE is detected in the higher temperature channels of AIA 171 Å, 193 Å̊, and 131 Å suggesting that it reaches a higher temperature of log T = 5.36 ? 0.06 (K). Brightenings observed in the AIA channels with durations 90?120 s are probably caused by the plasma ejections seen in the chromosphere. The wings of the C ii line behave in a similar manner to the Si iv's, indicating close formation temperatures, while the Mg ii k wings show additional Doppler-shifted emission. Magnetic convergence or emergence followed by cancellation at a rate of 5 ?1014 Mx/s is associated with the EE region. The combined changes of the locations and the flux of different magnetic patches suggest that magnetic reconnection must have taken place. Our results challenge several theories put forward in the past to explain non-Gaussian line profiles, i.e., EEs. Our case study on its own, however, cannot reject these theories, thus further in-depth studies on the phenomena producing EEs are required.

Authors: Zhenghua Huang, Maria S. Madjarska, Lidong Xia, J. G. Doyle, Klaus Galsgaard, Hui Fu

Publication Status: ApJ in press
Last Modified: 2014-11-19 09:21
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Hα spectroscopy and multi-wavelength imaging of a solar flare caused by filament eruption  

Zhenghua Huang   Submitted: 2014-05-09 04:22

We study a sequence of eruptive events including filament eruption, a GOES C4.3 flare and a coronal mass ejection. We aim to identify the possible trigger(s) and precursor(s) of the filament destabilisation; investigate flare kernel characteristics; flare ribbons/kernels formation and evolution; study the interrelation of the filament-eruption/flare/coronal-mass-ejection phenomena as part of the integral active-region magnetic field configuration; determine Hα line profile evolution during the eruptive phenomena. Multi-instrument observations are analysed including Hα line profiles, speckle images at Hα -0.8 Å and Hα +0.8 Å from IBIS at DST/NSO, EUV images and magnetograms from the SDO, coronagraph images from STEREO and the X-ray flux observations from FERMI and GOES. We establish that the filament destabilisation and eruption are the main trigger for the flaring activity. A surge-like event with a circular ribbon in one of the filament footpoints is determined as the possible trigger of the filament destabilisation. Plasma draining in this footpoint is identified as the precursor for the filament eruption. A magnetic flux emergence prior to the filament destabilisation followed by a high rate of flux cancelation of 1.34x1016 Mx s-1 is found during the flare activity. The flare X-ray lightcurves reveal three phases that are found to be associated with three different ribbons occurring consecutively. A kernel from each ribbon is selected and analysed. The kernel lightcurves and Hα line profiles reveal that the emission increase in the line centre is stronger than that in the line wings. A delay of around 5-6 mins is found between the increase in the line centre and the occurrence of red asymmetry. Only red asymmetry is observed in the ribbons during the impulsive phases. Blue asymmetry is only associated with the dynamic filament.

Authors: Z. Huang, M. S. Madjarska, K. Koleva, J. G. Doyle, P. Duchlev, M. Dechev, and K. Reardon
Projects: GOES X-rays ,National Solar Observatory (Sac Peak),SDO-AIA,SDO-HMI,SoHO-LASCO,STEREO

Publication Status: accepted by A&A
Last Modified: 2014-05-09 17:06
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Coronal hole boundaries at small scales: IV. SOT view-Magnetic field properties of small-scale transient brightenings in coronal holes  

Zhenghua Huang   Submitted: 2012-10-05 15:41

We study the magnetic properties of small-scale transients in coronal holes and a few in the quiet Sun identified in X-ray observations in paper II and analysed in spectroscopic data in paper III. We aim to investigate the role of small-scale transients in the evolution of the magnetic field in an equatorial coronal hole. Two sets of observations of an equatorial coronal hole and another two in quiet-Sun regions were analysed using longitudinal magnetograms taken by the Solar Optical Telescope. An automatic feature tracking program, SWAMIS, was used to identify and track the magnetic features. Each event was then visually analysed in detail. In both coronal holes and quiet-Sun regions, all brightening events are associated with bipolar regions and caused by magnetic flux emergence followed by cancellation with the pre-existing and/or newly emerging magnetic flux. In the coronal hole, 19 of 22 events have a single stable polarity which does not change its position in time. In eleven cases this is the dominant polarity. The dominant flux of the coronal hole form the largest concentration of magnetic flux in terms of size while the opposite polarity is distributed in small concentrations. We found that in the coronal hole the number of magnetic elements of the dominant polarity is four times higher than the non-dominant one. The supergranulation configuration appears to preserve its general shape during approximately nine hours of observations although the large concentrations (the dominant polarity) in the network did evolve and/or were slightly displaced, and their strength either increased or decreased. The emission fluctuations/radiance oscillations seen in the X-ray bright points are associated with reoccurring magnetic cancellation in the footpoints. Unique observations of an X-ray jet reveal similar magnetic behaviour in the footpoints, i.e. cancellation of the opposite polarity magnetic flux. We found that the magnetic flux cancellation rate during the jet is much higher than in bright points. Not all magnetic cancellations result in an X-ray enhancement, suggesting that there is a threshold of the amount of magnetic flux involved in a cancellation above which brightening would occur at X-ray temperatures. Our study demonstrates that the magnetic flux in coronal holes is continuously "recycled" through magnetic reconnection which is responsible for the formation of numerous small-scale transient events. The open magnetic flux forming the coronal-hole phenomenon is largely involved in these transient features. The question on whether this open flux is transported as a result of the formation and evolution of these transient events, however, still remains open. (online materials of this paper is available at http://star.arm.ac.uk/highlights/2012/603/bpmag.html)

Authors: Z. Huang, M. S. Madjarska, J. G. Doyle, D. A. Lamb
Projects: Hinode/SOT,Hinode/XRT

Publication Status: Accepted by A&A
Last Modified: 2012-10-10 09:03
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Abstracts by Author
Explosive events in active region observed by IRIS and SST/CRISP
Cool transition region loops observed by the Interface Region Imaging Spectrograph
Explosive events on subarcsecond scale in IRIS observations: a case study
H alpha spectroscopy and multi-wavelength imaging of a solar flare caused by filament eruption
Coronal hole boundaries at small scales: IV. SOT view-Magnetic field properties of small-scale transient brightenings in coronal holes

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