Initiation mechanism of the first on-disk X-class flare of solar cycle 25

Abstract

In this paper we study the initiation mechanism of the first on-disk X-class eruptive flare in solar cycle 25. Coronal magnetic field reconstructions reveal a magnetic flux rope (MFR) whose configuration is highly consistent with a filament existing for a long period before the flare; the eruption of the whole filament indicates that the MFR erupted during the flare. However, quantitative analysis shows that the pre-flare MFR resides too low to trigger a torus instability (TI). The filament experienced a slow rise before the flare onset, for which we estimated evolution of the filament height using a triangulation method by combining the SDO and STEREO observations, and find it is also much lower than the critical height for triggering TI. On the other hand, the pre-flare evolution of the current density shows progressive thinning of a vertical current layer on top of the flare polarity inversion line (PIL), which suggests that a vertical current sheet forms before the eruption. Meanwhile, there is continuous shearing motion along the PIL under the main branch of the filament, which can drive the coronal field to form such a current sheet. We thus suggest that the event follows a reconnection-based initiation mechanism as recently established using a high-accuracy magnetohydrodynamics simulation, in which an eruption is initiated by reconnection in a current sheet that forms gradually within a continuously sheared magnetic arcade. The eruption should be further driven by TI as the filament quickly rises into the TI domain during the eruption.