Evidence from the Extreme-Ultraviolet Imaging Spectrometer for Axial Filament Rotation before a Large Flare

Abstract

Abstract In this article, we present observations made with the Extreme-ultraviolet Imaging Spectrometer on-board the Hinode solar satellite, of an active region filament in the HeII emission line at 256.32 $Å$. The host active region AR 10930 produces an X-class flare during these observations. We measure Doppler shifts with apparent velocities of up to 20 kms$^{-1}$, which are antisymmetric about the filament length and occur several minutes before the flare’s impulsive phase. This is indicative of a rotation of the filament, which is in turn consistent with expansion of a twisted flux rope due to the MHD helical kink instability. This is the first time that such an observation has been possible in this transition-region line, and we note that the signature observed occurs before the first indications of pre-flare activity in the GOES solar soft X-ray flux, suggesting that the filament begins to destabilise in tandem with a reorganization of the local magnetic field. We suggest that this expansion is triggered by the decrease of magnetic tension around, and/or total pressure above, the filament.