A Transient Coronal Sigmoid in Active Region NOAA 11909: Build-up Phase,M-class Eruptive Flare,and Associated Fast Coronal Mass Ejection

摘要

In this article,we investigate the formation and disruption of a coronal sigmoid from the active region (AR) NOAA 11909 on 07 December 2013,by analyzing multi-wavelength and multi-instrument observations.Our analysis suggests that the formation of the sigmoid initiated ≈1 hour before its eruption through a coupling between two twisted coronal loop systems.This sigmoid can be well regarded as of 'transient' class due to its short lifetime as the eruptive activities started just after ≈20 min of its formation.A comparison between coronal and photospheric images suggests that the coronal sigmoid was formed over a simple β-type AR which also possessed dispersed magnetic field structure in the photosphere. The line-of-sight photospheric magnetograms also reveal small-scale flux cancellation events near the polarity inversion line,and overall flux cancellation during the extended pre-eruption phase which suggest the role of tether-cutting reconnection toward the build-up of the flux rope.The disruption of the sigmoid proceeded with a two-ribbon eruptive M1.2 flare (SOL2013-12-07T07:29).In radio frequencies,we observe type III and type II bursts in meter wavelengths during the impulsive phase of the flare.The successful eruption of the flux rope leads to a fast coronal mass ejection (with a linear speed of ≈1085 km s~(?1)) in SOHO/LASCO field-of-view.During the evolution of the flare,we clearly observe typical "sigmoid-to-arcade" transformation.Prior to the onset of the impulsive phase of the flare, flux rope undergoes a slow rise (≈15 km s~(?1)) which subsequently transitions into a fast eruption (≈110 km s~(?1)).The two-phase evolution of the flux rope shows temporal associations with the soft X-ray precursor and impulsive phase emissions of the M-class flare, respectively,thus pointing toward a feedback relationship between magnetic reconnection and early CME dynamics.