Formation and eruption of sigmoidal satructure from a weak field region of NOAA 11942

Abstract

Using observations from the Solar Dynamics Observatory, we studied an interesting example of a sigmoid formation and eruption from small-scale flux-canceling regions of active region (AR) 11942. Through an analysis of Helioseismic and Magnetic Imager and Atmospheric Imaging Assembly observations we infer that initially the AR is compact and bipolar in nature, evolved to a sheared configuration consisting of inverse J-shaped loops hosting a filament channel over a couple of days. By tracking the photospheric magnetic features, shearing and converging motions are observed to play a prime role in the development of S-shaped loops and further flux cancellation leads to tether-cutting reconnection of J loops. This phase is cotemporal with the filament rise motion, followed by sigmoid eruption at 21:32 UT on January 6. The flux rope rises in phases of slow (v avg = 26 km s−1) and fast (a avg = 55 m s−2) rise motion categorizing the coronal mass ejection (CME) as slow with an associated weak C1.0 class X-ray flare. The flare ribbon separation velocity peaks at around the peak time of the flare at which the maximum reconnection rate (2.14 V cm−1) occurs. Furthermore, the extreme ultraviolet light curves of 131, 171 Å have delayed peaks of 130 minutes compared to 94 Å and are explained by differential emission measure. Our analysis suggests that the energy release is proceeded by a much longer time duration, manifesting the onset of the filament rise and an eventual eruption driven by converging and canceling flux in the photosphere. Unlike strong eruption events, the observed slow CME and weak flare are indications of slow runway tether-cutting reconnection in which most of the sheared arcade is relaxed during the extended phase after the eruption.