Explosive events on subarcsecond scale in IRIS observations: a case study |
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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
Projects: IRIS,SDO-AIA,SDO-HMI
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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 |
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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
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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 |
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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
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Publication Status: Accepted by A&A
Last Modified: 2012-10-10 09:03
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