Sigmoid-to-Flux-Rope Transition Leading to A Loop-Like Coronal Mass Ejection

TL;DR

This study observes the detailed magnetic reconfiguration and flux rope formation in a solar sigmoid eruption, linking photospheric flows to coronal reconnection and CME initiation, providing new insights into eruption mechanisms.

Contribution

It provides the first clear observation of topological reconfiguration from J-shaped to S-shaped loops during a sigmoid eruption, linking photospheric flows to tether-cutting reconnection.

Findings

Reconnection forms a continuous S-shaped loop from J-shaped loops.

Persistent converging flows toward the PIL drive tether-cutting reconnection.

The flux rope eruption leads to a CME with a speed increase prior to flare onset.

Abstract

Sigmoids are one of the most important precursor structures for solar eruptions. In this Letter, we study a sigmoid eruption on 2010 August 1 with EUV data obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO). In AIA 94 \AA\ (Fe XVIII; 6 MK), topological reconfiguration due to tether-cutting reconnection is unambiguously observed for the first time, i.e., two opposite J-shaped loops reconnect to form a continuous S-shaped loop, whose central portion is dipped and aligned along the magnetic polarity inversion line (PIL), and a compact loop crossing the PIL. A causal relationship between photospheric flows and coronal tether-cutting reconnections is evidenced by the detection of persistent converging flows toward the PIL using line-of-sight magnetograms obtained by the Helioseismic and Magnetic Imager (HMI) on board SDO. The S-shaped loop remains in quasi-equilibrium in the lower corona for about 50 minutes, with the central dipped portion rising slowly at ~10 km s-1. The speed then increases to ~60 km s-1 about 10 minutes prior to the onset of a GOES-class C3.2 flare, as the S-shaped loop speeds up its transformation into an arch-shaped loop, which eventually leads to a loop-like coronal mass ejection (CME). The AIA observations combined with H? filtergrams as well as hard X-ray (HXR) imaging and spectroscopy are consistent with most flare loops being formed by reconnection of the stretched legs of less-sheared J-shaped loops that envelopes the rising flux rope, in agreement with the standard tether-cutting scenario.