The configuration and failed eruption of a complex magnetic flux rope above a {\delta} sunspot region

TL;DR

This study analyzes the complex magnetic flux rope above a delta sunspot, revealing its layered structure, null point, and eruption process, which supports the breakout model of solar eruptions.

Contribution

It provides detailed 3D magnetic field analysis of a flux rope and its eruption mechanism in a delta sunspot region, highlighting the role of null points and topological features.

Findings

Flux rope has multiple layers with decreasing twist from core to boundary.

Eruption involves precursor reconnection at a null point and main reconnection at a bald patch or HFT.

The delta sunspot environment favors complex magnetic configurations leading to activity.

Abstract

Aims. We investigate the configuration of a complex flux rope above a {\delta} sunspot region in NOAA AR 11515, and its eruptive expansion during a confined M5.3-class flare. Methods. We study the formation of the {\delta} sunspot using continuum intensity images and photospheric vector magnetograms provided by SDO/HMI. We use EUV and UV images provided by SDO/AIA, and hard X-ray emission recorded by RHESSI to investigate the eruptive details. The coronal magnetic field is extrapolated with a non-linear force free field (NLFFF) method, based on which the flux rope is identified by calculating the twist number Tw and squashing factor Q. We search the null point via a modified Powell hybrid method. Results. The collision between two emerging spot groups form the {\delta} sunspot. A bald patch (BP) forms at the collision location, above which a complex flux rope is identified. The flux rope has multiple layers, with one compact end and one bifurcated end, having Tw decreasing from the core to the boundary. A null point is located above the flux rope. The eruptive process consists of precursor flaring at a 'v'-shaped coronal structure, rise of the filament, and flaring below the filament, corresponding well with the NLFFF topological structures, including the null point and the flux rope with BP and hyperbolic flux tube (HFT). Two sets of post-flare loops and three flare ribbons support the bifurcation configuration of the flux rope. Conclusions. The precursor reconnection, which occurs at the null point, weakens the overlying confinement to allow the flux rope to rise, fitting the breakout model. The main phase reconnection, which may occur at the BP or HFT, facilitates the flux rope rising. The results suggest that the {\delta} spot configuration presents an environment prone to the formation of complex magnetic configurations which will work together to produce activities.