Two-stage Primary Acceleration in Filament Initial Eruption under a Fan–Spine Magnetic Configuration

Understanding the filament rising process is crucial for unveiling the triggering mechanisms of coronal mass ejections and forecasting space weather. In this paper, we present a detailed study of the filament initial eruption under a fan–spine structure. It was found that the filament underwent two distinct acceleration stages corresponding to class M1.0 and M4.6 flare events. The first acceleration stage commenced with the filament splitting, after which the upper portion was subsequently heated, being a hot channel, and slowly rose at an average speed of 22 km s−1. A set of hot reverse C-shaped loops appeared repeatedly during the filament splitting, and a hook structure was recognized at this phase, suggesting ongoing growth of the magnetic flux rope (MFR). When it reached a certain altitude, the hot channel appeared to get into a quasi-static phase with its upper edge seriously decelerated and its lower edge expanding downward. Approximately 30 minutes later, as a distinct annular ribbon appeared outside the hook structure, the hot channel rose again at a velocity of over 50 km s−1 accompanied by rapidly drifting footpoints and experienced a second acceleration stage with its axial flux increased to 1.1 × 1021 Mx. It is deduced that the filament initial eruption under a magnetic dome possesses multiple kinetic processes. We suggest that the magnetic reconnection taking place within and beneath the filament continues to trigger the growth of preeruptive MFR and the first acceleration, when the magnetic reconnection above the filament plays a key role in the second acceleration.