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Register a new userWhy torus-unstable solar filaments experience failed eruption?
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To investigate the factors that control the success and/or failure of solar eruptions, we study the magnetic field and 3-Dimensional (3D) configuration of 16 filament eruptions during 2010 July - 2013 February. All these events, i.e., erupted but failed to be ejected to become a coronal mass ejection (CME), are failed eruptions with the filament maximum height exceeding $100 Mm$. The magnetic field of filament source regions is approximated by a potential field extrapolation method. The filament 3D configuration is reconstructed from three vantage points by the observations of STEREO Ahead/Behind and SDO spacecraft. We calculate the decay index at the apex of these failed filaments and find that in 7 cases, their apex decay indexes exceed the theoretical threshold ($n_{crit} = 1.5$) of the torus instability. We further determine the orientation change or rotation angle of each filament top during the eruption. Finally, the distribution of these events in the parameter space of rotation angle versus decay index is established. Four distinct regimes in the parameter space are empirically identified. We find that, all the torus-unstable cases (decay index $n > 1.5$), have a large rotation angles ranging from $50^circ - 130^circ$. The possible mechanisms leading to the rotation and failed eruption are discussed. These results imply that, besides the torus instability, the rotation motion during the eruption may also play a significant role in solar eruptions.
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Multi-wavelength observations of prominence eruptions provide an opportunity to uncover the physical mechanism of the triggering and the evolution process of the eruption. In this paper, we investigated an erupting prominence on October 14, 2012, recorded in H{alpha}, EUV, and X-ray wavelengths. The process of the eruption gives evidences on the existence of a helical magnetic structure and showing the twist being converting to writhe. The estimated twist is ~6{pi} (3 turns), exceeding the threshold of the kink instability. The rising plasma then reached a high speed, estimated at 228 km s-1, followed by a sudden rapid acceleration at 2715 m s-2, and synchronous with a solar are. Co-spatial cusp shaped structures were observed in both AIA 131{AA} and 94{AA} images, signifying the location of the magnetic reconnection. The erupted flux rope finally undergone a deceleration with a maximum value of 391 m s-2, which is even larger than the free-fall acceleration on the Sun (273 m s-2) , suggesting that the eruption finally failed, possibly due to an inward magnetic tension force.
Solar filaments are cold and dense materials situated in magnetic dips, which show distinct radiation characteristics compared to the surrounding coronal plasma. They are associated with coronal sheared and twisted magnetic field lines. However, the exact magnetic configuration supporting a filament material is not easy to be ascertained because of the absence of routine observations of the magnetic field inside filaments. Since many filaments lie above weak-field regions, it is nearly impossible to extrapolate their coronal magnetic structures by applying the traditional methods to noisy photospheric magnetograms, in particular the horizontal components. In this paper, we construct magnetic structures for some filaments with the regularized Biot--Savart laws and calculate their magnetic twists. Moreover, we make a parameter survey for the flux ropes of the Titov-Demoulin-modified model to explore the factors affecting the twist of a force-free magnetic flux rope. It is found that the twist of a force-free flux rope is proportional to its axial length to minor radius ratio, and is basically independent of the overlying background magnetic field strength. Thus, we infer that long quiescent filaments are likely to be supported by more twisted flux ropes than short active-region filaments, which is consistent with observations.
Solar eruptions are spectacular magnetic explosions in the Suns corona, and how they are initiated remains unclear. Prevailing theories often rely on special magnetic topologies that may not generally exist in the pre-eruption source region of corona. Here, using fully three-dimensional magnetohydrodynamic simulations with high accuracy, we show that solar eruptions can be initiated in a single bipolar configuration with no additional special topology. Through photospheric shearing motion alone, an electric current sheet forms in the highly sheared core field of the magnetic arcade during its quasi-static evolution. Once magnetic reconnection sets in, the whole arcade is expelled impulsively, forming a fast-expanding twisted flux rope with a highly turbulent reconnecting region underneath. The simplicity and efficacy of this scenario argue strongly for its fundamental importance in the initiation of solar eruptions.
Employing Solar Dynamic Observatory/Atmosphertic Imaging Assembly (AIA) multi-wavelength images, we study an eruption of two crossing filaments, and firstly report the current sheets (CSs) connecting the lower flare ribbons and the upper erupting filaments. On July 8, 2014, two crossing filaments are observed in the NOAA active region (AR) 12113. The lower-lying filament rises first, and then meets the higher-lying one. Thereafter, both of them erupt together. The filament eruption draws the overlying magnetic field lines upward, leading to the approach of two legs, with opposite magnetic polarities, of the overlying field lines. Two sets of bright CSs form at the interface of these two legs, and magnetic reconnection takes place in the CSs producing the underneath flare ribbons and post-flare loops. Several bright plasmoids appear in the CSs, and propagate along the CSs bi-directionally. The CSs and plasmoids are observed in AIA multi-wavelength channels, indicating that both of them have been heated during the reconnection process, with hot and warm components. Employing the differential emission measure (EM) analysis, we find that both the temperature and EM of the CSs decrease from the flare arcades outward to the erupting filaments, and those of the plasmoids are significantly larger than the regions where no plasmoid is detected.
Aims. We investigate the configuration of a complex flux rope above a {delta} sunspot region in NOAA AR 11515, and its eruptive expansion during a confined M5.3-class flare. Methods. We study the formation of the {delta} sunspot using continuum intensity images and photospheric vector magnetograms provided by SDO/HMI. We use EUV and UV images provided by SDO/AIA, and hard X-ray emission recorded by RHESSI to investigate the eruptive details. The coronal magnetic field is extrapolated with a non-linear force free field (NLFFF) method, based on which the flux rope is identified by calculating the twist number Tw and squashing factor Q. We search the null point via a modified Powell hybrid method. Results. The collision between two emerging spot groups form the {delta} sunspot. A bald patch (BP) forms at the collision location, above which a complex flux rope is identified. The flux rope has multiple layers, with one compact end and one bifurcated end, having Tw decreasing from the core to the boundary. A null point is located above the flux rope. The eruptive process consists of precursor flaring at a v-shaped coronal structure, rise of the filament, and flaring below the filament, corresponding well with the NLFFF topological structures, including the null point and the flux rope with BP and hyperbolic flux tube (HFT). Two sets of post-flare loops and three flare ribbons support the bifurcation configuration of the flux rope. Conclusions. The precursor reconnection, which occurs at the null point, weakens the overlying confinement to allow the flux rope to rise, fitting the breakout model. The main phase reconnection, which may occur at the BP or HFT, facilitates the flux rope rising. The results suggest that the {delta} spot configuration presents an environment prone to the formation of complex magnetic configurations which will work together to produce activities.
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