  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) with configuration highly consistent with a filament
existing for a long period before the flare, and the eruption of the whole
filament indicates that the MFR erupted during the flare. However, quantitative
analysis shows that the pre-flare MFR resides in a height too low to trigger a
torus instability (TI). The filament experienced a slow rise before the flare
onset, for which we estimate 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 PIL, which suggests that a
vertical current sheet forms before the eruption. Meanwhile, there is
continuously shearing motion along the PIL under the main branch of the
filament, which can drive the coronal field to form such a current sheet. As
such, we suggest that the event follows a reconnection-based initiation
mechanism as recently established using a high-accuracy MHD simulation, in
which an eruption is initiated by reconnection in a current sheet that forms
gradually within continuously-sheared magnetic arcade. The eruption should be
further driven by TI as the filament quickly rises into the TI domain during
the eruption.
