Double-decker Filament Configuration Revealed by Mass Motions

Abstract

It is often envisaged that dense filament material lies in the dips of magnetic field lines belonging to either a sheared arcade or a magnetic flux rope. But it is also debated which configuration correctly depicts filaments' magnetic structure, due to our incapacity to measure the coronal magnetic field. In this paper, we address this issue by employing mass motions in an active-region filament to diagnose its magnetic structure. The disturbance in the filament was driven by a surge initiated at the filament's eastern end in the NOAA active region 12685, which was observed by the 1-m New Vacuum Solar Telescope (NVST) in the Hα line center and line wing (±0.4Å). Filament material predominately exhibits two kinds of motions, namely, rotation about the spine and longitudinal oscillation along the spine. The former is evidenced by antisymmetric Doppler shifts about the spine; the latter features a dynamic barb with mass extending away from the Hα spine until the transversal edge of the EUV filament channel. The longitudinal oscillation in the eastern section of the filament is distinct from that in the west, implying that the underlying field lines have different lengths and curvature radii. The composite motions of filament material suggest a double-decker host structure with mixed signs of helicity, comprising a flux rope atop a sheared-arcade system.

Transverse oscillations of a double-structured solar filament

“…12 in Liu et al (2012) and the numerical simulations in Kliem et al (2014). The filament configuration with the twisted flux rope above the sheared magnetic arcade has also been proposed by Awasthi et al (2019), who analysed mass motions inside the filament. We note that in these models the magnetic field in the arcade and the rope at their interaction region, as well as between two ropes, are oppositely orientated.…”

“…This model differs in some aspects from the other models mentioned in the Introduction. The most similar model is that in which a single flux rope is on top of a sheared arcade, such as in the model shown in Awasthi et al (2019). However, it should be emphasised that in this model the magnetic field changes its sign at the bottom boundary of the magnetic flux ropes.…”

“…Longitudinal oscillations are excited, for example by jets or shocks at footpoints of the filament propagating along the filament. Transverse oscillations can be generated, for example by a shock wave propagating from the lower parts of the solar atmosphere and colliding with the filament (Awasthi et al 2019) and references therein.…”

Recent research progress of solar physics in China

“…利用该方法, Ouyang等人 [103] 发现磁绳不是CME爆发的 必要条件, 在CME爆发的前身结构中, 磁绳约占90%, 剪切磁拱约占10%. 暗条物质需要借助磁凹陷向上的磁张力才能悬浮 在日冕中, 故而是指示磁场结构的重要"示踪"物; 暗条 物质的动力学演化尤其是振荡 [66,67,104] 常常能提供磁场 结构的线索. Liu等人 [105] 基于多个卫星不同视角的观 测, 证认出一个双层暗条.…”

“…由此提出两种可能的位形: 一种为双磁绳, 另一种为剪切磁拱上方再附着一个磁绳. 双层暗条有 可能因任一层中缺失暗条物质 [28] 或两层紧密相连 [104] 而表现为普通的暗条. Zhang等人 [106] 研究了色球纤维 结构与上方暗条融合导致的暗条爆发, 表明磁通/物质 输运也是暗条爆发的可能机制.…”