TY - JOUR A1 - Liu, Rui A1 - Liu, Chang A1 - Xu, Yan A1 - Liu, Wei A1 - Kliem, Bernhard A1 - Wang, Haimin T1 - Observation of a moretown wave and wave-filament interactions associated with the renowned X9 flare on 1990 May 24 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - 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. KW - Sun: filaments, prominences KW - Sun: flares KW - Sun: oscillations KW - waves Y1 - 2013 U6 - https://doi.org/10.1088/0004-637X/773/2/166 SN - 0004-637X VL - 773 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Liu, Rui A1 - Kliem, Bernhard A1 - Toeroek, Tibor A1 - Liu, Chang A1 - Titov, Viacheslav S. A1 - Lionello, Roberto A1 - Linker, Jon A. A1 - Wang, Haimin T1 - Slow rise and partial eruption of a double-decker filament. I. observations and interpretation JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - 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. KW - Sun: coronal mass ejections (CMEs) KW - Sun: filaments, prominences KW - Sun: flares Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/756/1/59 SN - 0004-637X VL - 756 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - 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 - Lee, Jeongwoo A1 - White, Stephen M. A1 - Liu, Chang A1 - Kliem, Bernhard A1 - Masuda, Satoshi T1 - Magnetic Structure of a Composite Solar Microwave Burst JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - A composite flare consisting of an impulsive flare SOL2015-06-21T01:42 (GOES class M2.0) and a more gradual, long-duration flare SOL2015-06-21T02:36 (M2.6) from NOAA Active Region 12371, is studied using observations with the Nobeyama Radioheliograph (NoRH) and the Solar Dynamics Observatory (SDO). While composite flares are defined by their characteristic time profiles, in this paper we present imaging observations that demonstrate the spatial relationship of the two flares and allow us to address the nature of the evolution of a composite event. The NoRH maps show that the first flare is confined not only in time, but also in space, as evidenced by the stagnation of ribbon separation and the stationarity of the microwave source. The NoRH also detected another microwave source during the second flare, emerging from a different location where thermal plasma is so depleted that accelerated electrons could survive longer against Coulomb collisional loss. The AIA 131 angstrom images show that a sigmoidal EUV hot channel developed after the first flare and erupted before the second flare. We suggest that this eruption removed the high-lying flux to let the separatrix dome underneath reconnect with neighboring flux and the second microwave burst follow. This scenario explains how the first microwave burst is related to the much-delayed second microwave burst in this composite event. KW - Sun: activity KW - Sun: coronal mass ejections (CMEs) KW - Sun: flares KW - Sun: magnetic fields KW - Sun: radio radiation KW - Sun: UV radiation Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aaadbc SN - 0004-637X SN - 1538-4357 VL - 856 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Kliem, Bernhard A1 - Toeroek, Tibor A1 - Titov, Viacheslav S. A1 - Lionello, Roberto A1 - Linker, Jon A. A1 - Liu, Rui A1 - Liu, Chang A1 - Wang, Haimin T1 - Slow rise and partial eruption of a double-decker filament. II. A double flux rope model JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - 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. KW - instabilities KW - magnetohydrodynamics (MHD) KW - Sun: coronal mass ejections (CMEs) KW - Sun: filaments, prominences KW - Sun: flares Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/792/2/107 SN - 0004-637X SN - 1538-4357 VL - 792 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER -