High-resolution Observations of Downflows at One End of a Pre-eruption Filament

Abstract

Abstract Studying the dynamics of filaments at the pre-eruption phase can shed light on the precursor of eruptive events. Such high-resolution studies (of the order of 0.″1) are highly desirable yet very rare. In this work, we present a detailed observation of a pre-eruption evolution of a filament obtained by the 1.6 m New Solar Telescope (NST) at the Big Bear Solar Observatory (BBSO). One end of the filament is anchored at the sunspot in the NOAA active region (AR) 11515, which is well observed by NST Hα off-bands from four hours before to one hour after the filament eruption. A M1.6 flare is associated with the eruption. We observed persistent downflowing materials along the Hα multi-threaded component of the loop toward the AR end during the pre-eruption phase. We traced the trajectories of plasma blobs along the Hα threads and obtained a plane-of-sky velocity of 45 km s−1 on average. Furthermore, we estimated the real velocities of the downflows and the altitude of the filament by matching the observed Hα threads with magnetic field lines extrapolated from a nonlinear force-free field model. Observations of chromospheric brightenings at the footpoints of the falling plasma blobs are also presented. The lower limit of the kinetic energy per second of the downflows through the brightenings is found to be ∼1021 erg. Larger FOV observations from BBSO full-disk Hα images show that the AR end of the filament started ascending four hours before the flare. We attribute the observed downflows at the AR end of the filament to the draining effect of the filament rising prior to its eruption. During the slow-rise phase, the downflows continuously drained away ∼1015g mass from the filament over a few hours, which is believed to be essential for the instability, and could be an important precursor of eruptive events.