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Magnetic reconnection: from ``open'' EUV loops to closed post-flare ones observed by SDO  

Jun Zhang   Submitted: 2013-11-07 00:39

Employing the emph{Solar Dynamics Observatory} observations, we select three well observed events including two flares and one extreme-ultraviolet (EUV) brightening. During the three events, the EUV loops evidently changed. One event was related to a major solar flare which took place on 2012 July 12 in active region NOAA AR 11520. ``Open'' EUV loops rooted in a facula of the AR deflected to the post-flare loops and then merged with them while the flare ribbon approached the facula. Meanwhile, ``open'' EUV loops rooted in a pore disappeared from top to bottom as the flare ribbon swept the pore. From the low temperature channels (e.g., 171 {AA}) to the high temperature channels (e.g., 94 {AA}), the loop evolution was similar. The coronal magnetic fields extrapolated from the photospheric vector magnetograms also show that the fields apparently ``open'' prior to the flare become closed after it. The other two events were associated with a B1.1 flare on 2010 May 24 and an EUV brightening on 2013 January 03, respectively. During both of these two events, some ``open'' loops either disappeared or darkened before the formation of new closed loops. We suggest that the observations reproduce the image predicted by the standard magnetic reconnection model: ``open'' magnetic fields become closed due to reconnection, manifesting as a transformation from ``open'' EUV loops to closed post-flare ones.

Authors: Jun Zhang, Shuhong Yang, Ting Li, Yuzong Zhang, Leping Li and Chaowei Jiang
Projects: SDO-AIA

Publication Status: 2013, ApJ, 776, 57
Last Modified: 2013-11-07 10:19
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Two Types of Dynamic Cool Coronal Structures Observed with STEREO and HINODE  

Jun Zhang   Submitted: 2009-09-09 21:23

Solar coronal loops show significant plasma motions during their formation and eruption stages. Dynamic cool coronal structures, on the other hand, are often observed to propagate along coronal loops. In this paper, we report on the discovery of two types of dynamic cool coronal structures, and characterize their fundamental properties. Using the EUV 304 angstrom images from the Extreme UltraViolet Imager (EUVI) telescope on the Solar TErrestrial RElation Observatory (STEREO) and the Ca II filtergrams from the Solar Optical Telescope (SOT) instrument on HINODE, we study the evolution of an EUV arch and the kinematics of cool coronal structures. The EUV 304 angstrom observations show that a missile-like plasmoid moves along an arch-shaped trajectory, with an average velocity of 31 km s-1. About three hours later, a plasma arch forms along the trajectory, subsequently the top part of the arch fades away and disappears, meanwhile the plasma belonging to the two legs of the arch flows downward to the arch feet. During the arch formation and disappearance, SOT Ca II images explore dynamic cool coronal structures beneath the arch. By tracking these structures, we classify them into two types. Type I is threadlike in shape and flows downward with a greater average velocity of 72 km s-1, finally it combines a loop fibril at chromospheric altitude. Type II is shape-transformable and sometimes rolling as it flows downward with a smaller velocity of 37 km s-1, then disappears insularly in the chromosphere. It is suggested that the two types of structures are possibly controlled by different magnetic configurations.

Authors: J. Zhang and L. P. Li
Projects: Hinode/SOT,STEREO

Publication Status: accepted by RAA
Last Modified: 2009-09-10 08:14
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Magnetic reconnection: from ``open'' EUV loops to closed post-flare ones observed by SDO
Two Types of Dynamic Cool Coronal Structures Observed with STEREO and HINODE

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