  The magnetohydrodynamics of active region NOAA 11283 is simulated using an
initial non-force-free magnetic field extrapolated from its photospheric vector
magnetogram. We focus on the magnetic reconnections at a magnetic null point
that 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 ribbons
and coronal dimmings. The initial magnetic field from extrapolation displays a
three-dimensional (3D) null topology overlying a sheared arcade. Prior to the
flare, magnetic loops rise due to the initial Lorentz force, and reconnect at
the 3D null, leading to expansion and loss of confined plasma that produce the
observed pre-flare coronal dimmings. Further, the simulated dynamics documents
the transfer of twist from the arcade to the overlying loops through
reconnections, developing a flux rope. The non-parallel field lines comprising
the rope and lower-lying arcades form an X-type geometry. Importantly, the
simultaneous reconnections at the 3D null and the X-type geometry can explain
the observed circular and parallel flare ribbons. Reconnections at the 3D null
transform closed inner spine field lines into open field lines of the outer
spine. The footpoints of these open field lines correspond to a ring-shaped
coronal dimming region, tracing the dome. Further, the flux rope bifurcates
because of these reconnections which also results in the generation of open
magnetic field lines. The plasma loss along the open field lines can
potentially explain the observed coronal dimming.
