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The magnetohydrodynamics of active region NOAA 11283 is simulated using aninitial non-force-free magnetic field extrapolated from its photospheric vectormagnetogram. We focus on the magnetic reconnections at a magnetic null pointthat participated in the X2.1 flare on 2011 September 6 around 22:21 UT(SOL2011-09-06T22:21X2.1) followed by the appearance of circular flare ribbonsand coronal dimmings. The initial magnetic field from extrapolation displays athree-dimensional (3D) null topology overlying a sheared arcade. Prior to theflare, magnetic loops rise due to the initial Lorentz force, and reconnect atthe 3D null, leading to expansion and loss of confined plasma that produce theobserved pre-flare coronal dimmings. Further, the simulated dynamics documentsthe transfer of twist from the arcade to the overlying loops throughreconnections, developing a flux rope. The non-parallel field lines comprisingthe rope and lower-lying arcades form an X-type geometry. Importantly, thesimultaneous reconnections at the 3D null and the X-type geometry can explainthe observed circular and parallel flare ribbons. Reconnections at the 3D nulltransform closed inner spine field lines into open field lines of the outerspine. The footpoints of these open field lines correspond to a ring-shapedcoronal dimming region, tracing the dome. Further, the flux rope bifurcatesbecause of these reconnections which also results in the generation of openmagnetic field lines. The plasma loss along the open field lines canpotentially explain the observed coronal dimming.

Magnetohydrodynamic Simulation of Magnetic Null-point Reconnections and Coronal dimmings during the X2.1 flare in NOAA AR 11283