Untwisting motion and bidirectional outflows of a filament

Context. Filament consists of cold and dense plasma that is suspended in the corona. Observations with a high spatial resolution indicate that its fine structure is highly dynamic during its evolution and eruption. Aims. We wish to understand the mechanism that produces twisting motions and bidirectional outflows in filaments by combining observation and simulation. Methods. The filament evolution was observed in multiple wavelengths by the New Vacuum Solar Telescope (NVST), the Solar Upper Transition Region Imager (SUTRI), the Chinese Hα Solar Explorer (CHASE), the Atmospheric Imaging Assembly (AIA), and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). A data-constrained magnetohydrodynamic (MHD) 3D simulation was performed to explain the physical phenomena behind the observations. Results. We report untwisting motions in a filament and its bidirectional outflows before and after a B8.4 X-ray flare. Before the flare, magnetic reconnection between threads occurred in the middle part of the filament, which resulted in the rise and oscillations of the filament. Brightenings in AIA 304 Å and AIA 171 Å in the interaction region were registered. After about 40 minutes, the right branch of the filament showed untwisting motions and bidirectional outflows in Hα as measured by time-distance diagrams and Dopplergrams of NVST. the MHD simulations showed that during the rising phase of the right branch of the flux rope (FR), magnetic reconnection of the sheared arcades below the flux rope occured that increased the FR twist. Magnetic reconnection induced bidirectional outflows and increased the plasma density in the FR by levitation. Conclusions. The 3D data-based MHD simulations confirmed that the bidirectional outflows observed in the NVST Hα filament were caused by magnetic reconnection. During the reconnection, plasma is injected, which leads to the dense observed filament with a high opacity in CHASE observations.