@article{LiuLiuXuetal.2013, author = {Liu, Rui and Liu, Chang and Xu, Yan and Liu, Wei and Kliem, Bernhard and Wang, Haimin}, title = {Observation of a moretown wave and wave-filament interactions associated with the renowned X9 flare on 1990 May 24}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {773}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/773/2/166}, pages = {13}, year = {2013}, abstract = {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.}, language = {en} } @article{LiuKliemToeroeketal.2012, author = {Liu, Rui and Kliem, Bernhard and Toeroek, Tibor and Liu, Chang and Titov, Viacheslav S. and Lionello, Roberto and Linker, Jon A. and Wang, Haimin}, title = {Slow rise and partial eruption of a double-decker filament. I. observations and interpretation}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {756}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/756/1/59}, pages = {14}, year = {2012}, abstract = {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.}, language = {en} } @article{LiuKliemTitovetal.2016, author = {Liu, Rui and Kliem, Bernhard and Titov, Viacheslav S. and Chen, Jun and Wang, Yuming and Wang, Haimin and Liu, Chang and Xu, Yan and Wiegelmann, Thomas}, title = {STRUCTURE, STABILITY, AND EVOLUTION OF MAGNETIC FLUX ROPES FROM THE PERSPECTIVE OF MAGNETIC TWIST}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {818}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/0004-637X/818/2/148}, pages = {22}, year = {2016}, abstract = {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.}, language = {en} } @article{KliemToeroekTitovetal.2014, author = {Kliem, Bernhard and Toeroek, Tibor and Titov, Viacheslav S. and Lionello, Roberto and Linker, Jon A. and Liu, Rui and Liu, Chang and Wang, Haimin}, title = {Slow rise and partial eruption of a double-decker filament. II. A double flux rope model}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {792}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/792/2/107}, pages = {10}, year = {2014}, abstract = {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.}, language = {en} }