Flux rope formation through flux cancellation of sheared coronal arcades in a 3D convectively-driven MHD simulation
Context. Space weather and its potential negative consequences for life on Earth has received increasing attention in recent decades. Particularly predicting CME onset has become important from a security perspective. To predict CMEs, one must first understand the dynamics leading to pre-eruptive magnetic field configurations such as flux ropes. Aims. In this study, we investigate the realistic formation of coronal flux ropes above the solar photosphere. The aim is to find if and how flux ropes can form there, and how the formation is related to flux cancellation at the photosphere. Methods. We run a convective non-symmetric 3D radiative MHD simulation with the code Bifrost. A linear force-free field with sheared coronal arcades is slowly inserted in the 24Mmx24Mmx30Mm simulation box. After this, the self-consistent stochastic plasma flows of the convection zone drive several small-scale flux cancellations and magnetic reconnection, without external influence. Lagrangian markers called corks are used to track the dynamic evolution of the magnetic field. Results. Over a period of 2.5 h, a flux rope is generated with photospheric footpoints separated by up to 12Mm. The flux rope forms gradually through several individual events, such as slipping reconnection, U-loop emergence, and thick-photosphere tether-cutting reconnection. Conclusions. Flux ropes can be formed in the solar atmosphere solely driven by convection and flux cancellations at the photosphere. However, not all flux cancellations contribute to the build-up of the flux rope, and some coronal reconnection events that do are not clearly related to flux cancellation. The formation process of flux ropes from coronal sheared arcades driven by convection is therefore more complex than in the original smooth flux cancellation model. But the end result is qualitatively the same. Flux cancellation works. A flux rope is formed.