TY  - JOUR
A1  - Liu, Rui
A1  - Kliem, Bernhard
A1  - Titov, Viacheslav S.
A1  - Chen, Jun
A1  - Wang, Yuming
A1  - Wang, Haimin
A1  - Liu, Chang
A1  - Xu, Yan
A1  - Wiegelmann, Thomas
T1  - STRUCTURE, STABILITY, AND EVOLUTION OF MAGNETIC FLUX ROPES FROM THE PERSPECTIVE OF MAGNETIC TWIST
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We investigate the evolution of NOAA Active Region (AR) 11817 during 2013 August 10–12, when it developed a complex field configuration and produced four confined, followed by two eruptive, flares. These C-and-above flares are all associated with a magnetic flux rope (MFR) located along the major polarity inversion line, where shearing and converging photospheric flows are present. Aided by the nonlinear force-free field modeling, we identify the MFR through mapping magnetic connectivities and computing the twist number ${{ \mathcal T }}_{w}$ for each individual field line. The MFR is moderately twisted ($| {{ \mathcal T }}_{w}| \lt 2$) and has a well-defined boundary of high squashing factor Q. We found that the field line with the extremum $| {{ \mathcal T }}_{w}| $ is a reliable proxy of the rope axis, and that the MFR's peak $| {{ \mathcal T }}_{w}| $ temporarily increases within half an hour before each flare while it decreases after the flare peak for both confined and eruptive flares. This pre-flare increase in $| {{ \mathcal T }}_{w}| $ has little effect on the AR's free magnetic energy or any other parameters derived for the whole region, due to its moderate amount and the MFR's relatively small volume, while its decrease after flares is clearly associated with the stepwise decrease in the whole region's free magnetic energy due to the flare. We suggest that ${{ \mathcal T }}_{w}$ may serve as a useful parameter in forewarning the onset of eruption, and therefore, the consequent space weather effects. The helical kink instability is identified as the prime candidate onset mechanism for the considered flares.
KW  - coronal mass ejections (CMEs)
KW  - Sun: corona
KW  - Sun: filaments, pominences
KW  - Sun: flares
KW  - Sun: magnetic fields
Y1  - 2016
U6  - https://doi.org/10.3847/0004-637X/818/2/148
SN  - 0004-637X
SN  - 1538-4357
VL  - 818
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Gou, Tingyu
A1  - Liu, Rui
A1  - Kliem, Bernhard
A1  - Wang, Yuming
A1  - Veronig, Astrid M.
T1  - The birth of a coronal mass ejection
JF  - Science Advances
N2  - The Sun’s atmosphere is frequently disrupted by coronal mass ejections (CMEs), coupled with flares and energetic particles. The coupling is usually attributed to magnetic reconnection at a vertical current sheet connecting the flare and CME, with the latter embedding a helical magnetic structure known as flux rope. However, both the origin of flux ropes and their nascent paths toward eruption remain elusive. Here, we present an observation of how a stellar-sized CME bubble evolves continuously from plasmoids, mini flux ropes that are barely resolved, within half an hour. The eruption initiates when plasmoids springing from a vertical current sheet merge into a leading plasmoid, which rises at increasing speeds and expands impulsively into the CME bubble, producing hard x-ray bursts simultaneously. This observation illuminates a complete CME evolutionary path capable of accommodating a wide variety of plasma phenomena by bridging the gap between microscale and macroscale dynamics.
Y1  - 2019
U6  - https://doi.org/10.1126/sciadv.aau7004
SN  - 2375-2548
VL  - 5
IS  - 3
PB  - American Assoc. for the Advancement of Science
CY  - Washington
ER  - 
TY  - JOUR
A1  - Chen, Jun
A1  - Liu, Rui
A1  - Liu, Kai
A1  - Awasthi, Arun Kumar
A1  - Zhang, Peijin
A1  - Wang, Yuming
A1  - Kliem, Bernhard
T1  - Extreme-ultraviolet late phase of solar flares
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - A second peak in the extreme ultraviolet sometimes appears during the gradual phase of solar flares, which is known as the EUV late phase (ELP). Stereotypically ELP is associated with two separated sets of flaring loops with distinct sizes, and it has been debated whether ELP is caused by additional heating or extended plasma cooling in the longer loop system. Here we carry out a survey of 55 M-and-above GOES-class flares with ELP during 2010-2014. Based on the flare-ribbon morphology, these flares are categorized as circular-ribbon (19 events), two-ribbon (23 events), and complex-ribbon (13 events) flares. Among them, 22 events (40%) are associated with coronal mass ejections, while the rest are confined. An extreme ELP, with the late-phase peak exceeding the main-phase peak, is found in 48% of two-ribbon flares, 37% of circular-ribbon flares, and 31% of complex-ribbon flares, suggesting that additional heating is more likely present during ELP in two-ribbon than in circular-ribbon flares. Overall, cooling may be the dominant factor causing the delay of the ELP peak relative to the main-phase peak, because the loop system responsible for the ELP emission is generally larger than, and well separated from, that responsible for the main-phase emission. All but one of the circular-ribbon flares can be well explained by a composite "dome-plate" quasi-separatrix layer (QSL). Only half of these show a magnetic null point, with its fan and spine embedded in the dome and plate, respectively. The dome-plate QSL, therefore, is a general and robust structure characterizing circular-ribbon flares.
Y1  - 2020
U6  - https://doi.org/10.3847/1538-4357/ab6def
SN  - 0004-637X
SN  - 1538-4357
VL  - 890
IS  - 2
PB  - Institute of Physics Publ.
CY  - London
ER  -