Data-based MHD simulation of the origin of a coronal mass ejection: onset mechanism and twist buildup

Introduction Coronal mass ejections (CMEs) are major drivers of adverse space weather, and understanding their origin remains a fundamental problem in solar and space physics. Since the first detection of a CME more than four decades ago, theories and models concerning their origins have advanced substantially. However, the early-stage evolution of CMEs is still not fully understood, particularly regarding their initiation/eruption mechanisms and twist buildup. Methods Here, we perform an observationally based magnetohydrodynamic (MHD) simulation of a CME from active region 12371. This event is associated with a confined C3.9 precursor flare and an eruptive M6.5 main flare. Results Both the observed hot channel and the simulated flux rope exhibit two distinct stages, corresponding to the onset of the CME initiation and the subsequent main-acceleration phase. Our results suggest that the onset of these two stages are consistent with double-arc instability ( ( κ T c = 0.75 * > 0.1 ) ) and torus instability (n > 1.5), respectively, while fast magnetic reconnection develops later and further enhances the eruption. We further find that the twist number and magnetic fluxes of the CME flux rope are accumulated mainly before the eruption. Discussion These findings highlight the role of moderate magnetic reconnection before the eruption in the buildup of the CME precursor flux rope. We infer that the small confined flare preceding the main eruptive flare may reflect low-lying reconnection that forms double-arc loops, implying the onset of CME initiation. Such precursor activity may therefore provide a useful observational signature for CME forecasting.