Non-equilibrium Flux Rope Formation by Confined Flares Preceding a Solar Coronal Mass Ejection

TL;DR

This study provides evidence that magnetic flux ropes can form through confined eruptions before a CME, highlighting a process where non-equilibrium flux ropes develop prior to large-scale eruptions in solar active regions.

Contribution

It demonstrates that confined eruptions can lead to the formation of stable flux ropes through tether-cutting reconnection, a process preceding many CMEs.

Findings

Confined eruptions produce sigmoidal flux ropes detectable in EUV images.

Flux feeding during confined flares can trigger torus instability leading to CMEs.

The formation of flux ropes occurs prior to and facilitates the subsequent eruption.

Abstract

We present evidence that a magnetic flux rope was formed before a coronal mass ejection (CME) and its associated long-duration flare during a pair of preceding confined eruptions and associated impulsive flares in a compound event in NOAA Active Region 12371. Extreme-ultraviolet images and the extrapolated nonlinear force-free field show that the first two, impulsive flares, SOL2015-06-21T01:42, result from the confined eruption of highly sheared low-lying flux, presumably a seed flux rope. The eruption spawns a vertical current sheet, where magnetic reconnection creates flare ribbons and loops, a nonthermal microwave source, and a sigmoidal hot channel which can only be interpreted as a magnetic flux rope. Until the subsequent long-duration flare, SOL2015-06-21T02:36, the sigmoid's elbows expand, while its center remains stationary, suggesting non-equilibrium but not yet instability. The "flare reconnection" during the confined eruptions acts like "tether-cutting reconnection" whose flux feeding of the rope leads to instability. The subsequent full eruption is seen as an accelerated rise of the entire hot channel, seamlessly evolving into the fast halo CME. Both the confined and ejective eruptions are consistent with the onset of the torus instability in the dipped decay index profile which results from the region's two-scale magnetic structure. We suggest that the formation or enhancement of a non-equilibrium but stable flux rope by confined eruptions is a generic process occurring prior to many CMEs.