45603
2016
2016
eng
22
818
article
IOP Publ. Ltd.
Bristol
1
--
--
--
STRUCTURE, STABILITY, AND EVOLUTION OF MAGNETIC FLUX ROPES FROM THE PERSPECTIVE OF MAGNETIC TWIST
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.
The astrophysical journal : an international review of spectroscopy and astronomical physics
10.3847/0004-637X/818/2/148
0004-637X
1538-4357
wos2016:2019
148
WOS:000372302800047
Liu, R (reprint author), Univ Sci & Technol China, Dept Geophys & Planetary Sci, CAS Key Lab Geospace Environm, Hefei 230026, Peoples R China.; Liu, R (reprint author), Collaborat Innovat Ctr Astronaut Sci & Technol, Hefei 230026, Peoples R China., rliu@ustc.edu.cn
Thousand Young Talents Program of China, NSFC [41222031, 41474151]; NSFC [41131065]; CAS [KZZD-EW-01-4, 2012T1J0017]; fundamental research funds for the central universities; DFG; NSF SHINE program; NSF [AGS 1348513, 1408703]; NASA [NNX13AG13G, NNX13AF76G]
importub
2020-03-22T19:55:01+00:00
filename=package.tar
c286dd75d0f30aecbb7b257750b890ab
Rui Liu
Bernhard Kliem
Viacheslav S. Titov
Jun Chen
Yuming Wang
Haimin Wang
Chang Liu
Yan Xu
Thomas Wiegelmann
eng
uncontrolled
coronal mass ejections (CMEs)
eng
uncontrolled
Sun: corona
eng
uncontrolled
Sun: filaments, pominences
eng
uncontrolled
Sun: flares
eng
uncontrolled
Sun: magnetic fields
Institut für Physik und Astronomie
Referiert
Import
37556
2014
2014
eng
10
2
792
article
IOP Publ. Ltd.
Bristol
1
--
--
--
Slow rise and partial eruption of a double-decker filament. II. A double flux rope model
Force-free equilibria containing two vertically arranged magnetic flux ropes of like chirality and current direction are considered as a model for split filaments/prominences and filament-sigmoid systems. Such equilibria are constructed analytically through an extension of the methods developed in Titov & Demoulin and numerically through an evolutionary sequence including shear flows, flux emergence, and flux cancellation in the photospheric boundary. It is demonstrated that the analytical equilibria are stable if an external toroidal (shear) field component exceeding a threshold value is included. If this component decreases sufficiently, then both flux ropes turn unstable for conditions typical of solar active regions, with the lower rope typically becoming unstable first. Either both flux ropes erupt upward, or only the upper rope erupts while the lower rope reconnects with the ambient flux low in the corona and is destroyed. However, for shear field strengths staying somewhat above the threshold value, the configuration also admits evolutions which lead to partial eruptions with only the upper flux rope becoming unstable and the lower one remaining in place. This can be triggered by a transfer of flux and current from the lower to the upper rope, as suggested by the observations of a split filament in Paper I. It can also result from tether-cutting reconnection with the ambient flux at the X-type structure between the flux ropes, which similarly influences their stability properties in opposite ways. This is demonstrated for the numerically constructed equilibrium.
The astrophysical journal : an international review of spectroscopy and astronomical physics
10.1088/0004-637X/792/2/107
0004-637X
1538-4357
wos:2014
107
WOS:000341172200025
Kliem, B (reprint author), Univ Potsdam, Inst Phys & Astron, Karl Liebknecht Str 24, D-14476 Potsdam, Germany., bkliem@uni-potsdam.de
DFG; STFC; NSF [AGS-1249270, AGS-1153226, AGS-1348513]; Chinese Academy
Science and Technology Center); NASA [NNX13AG13G, NNX13AF76G]
Bernhard Kliem
Tibor Toeroek
Viacheslav S. Titov
Roberto Lionello
Jon A. Linker
Rui Liu
Chang Liu
Haimin Wang
eng
uncontrolled
instabilities
eng
uncontrolled
magnetohydrodynamics (MHD)
eng
uncontrolled
Sun: coronal mass ejections (CMEs)
eng
uncontrolled
Sun: filaments, prominences
eng
uncontrolled
Sun: flares
Institut für Physik und Astronomie
Referiert
35667
2012
2012
eng
14
1
756
article
IOP Publ. Ltd.
Bristol
1
--
--
--
Slow rise and partial eruption of a double-decker filament. I. observations and interpretation
We study an active-region dextral filament that was composed of two branches separated in height by about 13 Mm, as inferred from three-dimensional reconstruction by combining SDO and STEREO-B observations. This "double-decker" configuration sustained for days before the upper branch erupted with a GOES-class M1.0 flare on 2010 August 7. Analyzing this evolution, we obtain the following main results. (1) During the hours before the eruption, filament threads within the lower branch were observed to intermittently brighten up, lift upward, and then merge with the upper branch. The merging process contributed magnetic flux and current to the upper branch, resulting in its quasi-static ascent. (2) This transfer might serve as the key mechanism for the upper branch to lose equilibrium by reaching the limiting flux that can be stably held down by the overlying field or by reaching the threshold of the torus instability. (3) The erupting branch first straightened from a reverse S shape that followed the polarity inversion line and then writhed into a forward S shape. This shows a transfer of left-handed helicity in a sequence of writhe-twist-writhe. The fact that the initial writhe is converted into the twist of the flux rope excludes the helical kink instability as the trigger process of the eruption, but supports the occurrence of the instability in the main phase, which is indeed indicated by the very strong writhing motion. (4) A hard X-ray sigmoid, likely of coronal origin, formed in the gap between the two original filament branches in the impulsive phase of the associated flare. This supports a model of transient sigmoids forming in the vertical flare current sheet. (5) Left-handed magnetic helicity is inferred for both branches of the dextral filament. (6) Two types of force-free magnetic configurations are compatible with the data, a double flux rope equilibrium and a single flux rope situated above a loop arcade.
The astrophysical journal : an international review of spectroscopy and astronomical physics
10.1088/0004-637X/756/1/59
0004-637X
wos:2011-2013
59
WOS:000309044300059
Liu, R (reprint author), Univ Sci & Technol China, Dept Geophys & Planetary Sci, CAS Key Lab Geospace Environm, Hefei 230026, Peoples R China., rui.liu@njit.edu
Thousand Young Talents Program of China; NSFC [41131065, 41121003]; 973
key project [2011CB811403]; CAS Key Research Program [KZZD-EW-01-4];
fundamental research funds for the central universities [WK2080000031];
NASA [NNX08-AJ23G, NNX08-AQ90G]; NSF [ATM-0849453, ATM-0819662]; DFG;
STFC; NASA's HTP program; NASA's LWS program; NASA's SRT program; CISM
(an NSF Science and Technology Center); Key Laboratory of Solar
Activity, National Astronomical Observatories of Chinese Academy of
Sciences [KLSA201201]
Rui Liu
Bernhard Kliem
Tibor Toeroek
Chang Liu
Viacheslav S. Titov
Roberto Lionello
Jon A. Linker
Haimin Wang
eng
uncontrolled
Sun: coronal mass ejections (CMEs)
eng
uncontrolled
Sun: filaments, prominences
eng
uncontrolled
Sun: flares
Institut für Physik und Astronomie
Referiert
34802
2013
2013
eng
13
2
773
article
IOP Publ. Ltd.
Bristol
1
--
--
--
Observation of a moretown wave and wave-filament interactions associated with the renowned X9 flare on 1990 May 24
Using Big Bear Solar Observatory film data recently digitized at NJIT, we investigate a Moreton wave associated with an X9 flare on 1990 May 24, as well as its interactions with four filaments F1-F4 located close to the flaring region. The interaction yields interesting insight into physical properties of both the wave and the filaments. The first clear Moreton wavefront appears at the flaring-region periphery at approximately the same time as the peak of a microwave burst and the first of two gamma-ray peaks. The wavefront propagates at different speeds ranging from 1500-2600 km s(-1) in different directions, reaching as far as 600 Mm away from the flaring site. Sequential chromospheric brightenings are observed ahead of the Moreton wavefront. A slower diffuse front at 300-600 km s(-1) is observed to trail the fast Moreton wavefront about one minute after the onset. The Moreton wave decelerates to similar to 550 km s(-1) as it sweeps through F1. The wave passage results in F1's oscillation which is featured by similar to 1 mHz signals with coherent Fourier phases over the filament, the activation of F3 and F4 followed by gradual recovery, but no disturbance in F2. Different height and magnetic environment together may account for the distinct responses of the filaments to the wave passage. The wavefront bulges at F4, whose spine is oriented perpendicular to the upcoming wavefront. The deformation of the wavefront is suggested to be due to both the forward inclination of the wavefront and the enhancement of the local Alfven speed within the filament channel.
The astrophysical journal : an international review of spectroscopy and astronomical physics
10.1088/0004-637X/773/2/166
0004-637X
wos:2011-2013
166
WOS:000323426100085
Liu, R (reprint author), Univ Sci & Technol China, Dept Geophys & Planetary Sci, CAS Key Lab Geospace Environm, Hefei 230026, Peoples R China., rliu@ustc.edu.cn
NSF [AGS 0839216, AGS 0849453, AGS-1153226, AGS-1153424]; Thousand Young
Talents Program of China; NSFC [41222031, 41131065, 41121003]; 973 key
project [2011CB811403]; CAS [KZZD-EW-01-4]; fundamental research funds
for the central universities [WK2080000031]; DFG; STFC; Chinese Academy
of Sciences [2012T1J0017]
Rui Liu
Chang Liu
Yan Xu
Wei Liu
Bernhard Kliem
Haimin Wang
eng
uncontrolled
Sun: filaments, prominences
eng
uncontrolled
Sun: flares
eng
uncontrolled
Sun: oscillations
eng
uncontrolled
waves
Institut für Physik und Astronomie
Referiert
52753
2018
2018
eng
14
614
article
EDP Sciences
Les Ulis
1
2018-06-06
2018-06-06
--
High-resolution imaging and near-infrared spectroscopy of penumbral decay
Aims. Combining high-resolution spectropolarimetric and imaging data is key to understanding the decay process of sunspots as it allows us to scrutinize the velocity and magnetic fields of sunspots and their surroundings. Methods. Active region NOAA 12597 was observed on 2016 September 24 with the 1.5-meter GREGOR solar telescope using high-spatial-resolution imaging as well as imaging spectroscopy and near-infrared (NIR) spectropolarimetry. Horizontal proper motions were estimated with local correlation tracking, whereas line-of-sight (LOS) velocities were computed with spectral line fitting methods. The magnetic field properties were inferred with the "Stokes Inversions based on Response functions" (SIR) code for the Si I and Ca I NIR lines. Results. At the time of the GREGOR observations, the leading sunspot had two light bridges indicating the onset of its decay. One of the light bridges disappeared, and an elongated, dark umbral core at its edge appeared in a decaying penumbral sector facing the newly emerging flux. The flow and magnetic field properties of this penumbral sector exhibited weak Evershed flow, moat flow, and horizontal magnetic field. The penumbral gap adjacent to the elongated umbral core and the penumbra in that penumbral sector displayed LOS velocities similar to granulation. The separating polarities of a new flux system interacted with the leading and central part of the already established active region. As a consequence, the leading spot rotated 55 degrees clockwise over 12 h. Conclusions. In the high-resolution observations of a decaying sunspot, the penumbral filaments facing the flux emergence site contained a darkened area resembling an umbral core filled with umbral dots. This umbral core had velocity and magnetic field properties similar to the sunspot umbra. This implies that the horizontal magnetic fields in the decaying penumbra became vertical as observed in flare-induced rapid penumbral decay, but on a very different time-scale.
Astronomy and astrophysics : an international weekly journal
10.1051/0004-6361/201731801
1432-0746
wos:2018
A2
WOS:000434848200002
Verma, M (reprint author), Leibniz Inst Astrophys Potsdam AIP, Sternwarte 16, D-14482 Potsdam, Germany., mverma@aip.de
Czech Science FoundationGrant Agency of the Czech Republic [14-04338S]; European CommissionEuropean Commission Joint Research Centre [312495]; Spanish Ministry of Economy and Competitiveness [AYA2014-60476-P]; Leibniz Graduate School for Quantitative Spectroscopy in Astrophysics; AIP; Institute of Physics and Astronomy of the University of Potsdam; [VEGA 2/0004/16]
2021-11-19T14:49:34+00:00
sword
importub
filename=package.tar
dc2e73032814f2e029bc82442d4e4d68
false
true
Meetu Verma
Carsten Denker
H. Balthasar
Christoph Kuckein
R. Rezaei
Michal Sobotka
N. Deng
Haimin Wang
A. Tritschler
M. Collados
Andrea Diercke
Sergio Javier González Manrique
eng
uncontrolled
Sun: photosphere
eng
uncontrolled
sunspots
eng
uncontrolled
Sun: magnetic fields
eng
uncontrolled
Sun: infrared
eng
uncontrolled
techniques: imaging spectroscopy
eng
uncontrolled
techniques: spectroscopic
Astronomie und zugeordnete Wissenschaften
Institut für Physik und Astronomie
Referiert
Import
Bronze Open-Access