| Abstract: In this work we aim to investigate the onset mechanism of an X9.0 class solar flare hosted by AR 13842 on 2024 October 3 using a series of Non-force-free-field (NFFF) extrapolated photospheric vector magnetogram and observations. Notably, the extrapolated magnetic configuration before the onset of flare has depicted the presence of key magnetic features, magnetic flux rope (MFR), hyperbolic flux tube (HFT), shear arcades and quasi separatrix layers (QSLs) in the flaring region. Important is the appearance of sigmoidal brightening during the onset of flare, which nearly coincide with the sheared arcades. This suggests that the strong field-aligned currents associated with the sheared arcades lead to the Joule heating of plasma, potentially resulting in emissions corresponding to the sigmoid. Moreover, the sequence of extrapolation indicates a possible initiation of magnetic reconnection within the shear arcades, which enhances the magnetic flux in the MFR, thereby making the rope susceptible to eruption. Furthermore, we observe the movement of the foot points of the HFT in the QSLs regions. Interestingly, the foot points of the HFT nearly aligned with the observed flare brightening during the flare, highlighting the crucial role of slipping reconnection in generating the flare brightening. Additionally, these magnetic reconnection, when repeated over time, are anticipated to give rise to the observed flare ribbons as well as quasi-periodic pulsations (QPPs) with a dominant period of $> 1$ minutes. |
