The configuration and failed eruption of a complex magnetic flux rope above a δ sunspot region^⋆

¹ Planetary Environmental and Astrobiological Research Laboratory (PEARL), School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, Guangdong 519082, PR China
e-mail: liulj8@mail.sysu.edu.cn
² CAS center for Excellence in Comparative Planetology, Hefei, PR China
³ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University, Belfast BT71NN, UK
⁴ CAS Key Laboratory of Geospace Environment, Department of Geophysics and Planetary Sciences, University of Science and Technology of China, Hefei, Anhui 230026, PR China
⁵ Institute of Space Science and Applied Technology, Harbin Institute of Technology, Shenzhen, PR China
⁶ CAS Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China

Received: 2 January 2021
Accepted: 10 February 2021

Abstract

Aims. We aim to investigate the configuration of a complex flux rope above the δ sunspot region in the National Oceanic and Atmospheric Administration AR 11515 and its eruptive expansion during a confined M5.3-class flare.

Methods. We studied the formation of the δ sunspot using the continuum intensity images and photospheric vector magnetograms provided by the Helioseismic and Magnetic Imager on-board the Solar Dynamics Observatory (SDO). We employed the extreme-ultraviolet and ultraviolet images provided by the Atmospheric Imaging Assembly on-board SDO and the hard X-ray emission recorded by the Reuven Ramaty High-Energy Solar Spectroscopic Imager to investigate the eruptive details. The coronal magnetic field is extrapolated from the photospheric field using a nonlinear force free field (NLFFF) method, based on which the flux rope is identified through calculating the twist number T_w and squashing factor Q. We searched the null point via a modified Powell hybrid method.

Results. The collision between two newly emerged spot groups form the δ sunspot. A bald patch (BP) configuration forms at the collision location between one umbra and the penumbra, above which a complex flux rope structure is identified. The flux rope has a multilayer configuration, with one compact end and the other end bifurcating into different branches. It has a non-uniform T_w profile, which decreases from the core to the boundary. The outmost layer is merely sheared. A null point is located above the flux rope. The eruptive process consists of precursor flarings at a v-shaped coronal structure, rise of the filament, and brightening below the filament, corresponding well with the topological structures deduced from the NLFFF, including a higher null point, a flux rope, and a BP and a hyperbolic flux tube (HFT) below the flux rope. Two sets of post-flare loops and three flare ribbons in the δ sunspot region further support the bifurcation configuration of the flux rope.

Conclusions. Combining the observations and magnetic field extrapolation, we conclude that 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 δ spot configuration presents an environment prone to the formation of complex magnetic configurations that work together to produce activities.