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

Abstract

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 nonparallel 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.

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“…To infer the magnetic connectivity, we used the NFFF extrapolation technique Hu et al , 2010, where the magnetic field is described by the double-curl Beltrami equation derived from a variational principle of the minimum energy dissipation rate (Bhattacharyya et al 2007). This method is well suited to the high plasma-β photospheric boundary (Gary 2001) and has been successfully used in many recent studies (Nayak et al 2019;Liu et al 2020;Yalim et al 2020;Prasad et al 2020). The extrapolation provides the magnetic field vector B(x, y, z), from which we derived the total (|J|), vertical (J z ), and horizontal (J hor ) current densities.…”

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“…Another relatively common low coronal signature of the eruption comes in the form of coronal dimmings, which are reflections of the expansion of coronal fields and mass depletion associated with CMEs (Hudson et al 1996; Thompson et al 1998;Dissauer et al 2018bDissauer et al , 2019Chikunova et al 2020). It has been shown that the localized core dimming regions tend to be rooted at the footpoints of the ejected flux rope (Sterling & Hudson 1997;Webb et al 2000;Temmer et al 2017;Veronig et al 2019) and that the spatio-temporal evolution of the dimming regions provides insight into the initiation and early evolution of the eruption (Miklenic et al 2011;Prasad et al 2020), as well as into the interaction of the flux rope footpoints with the surrounding magnetic field (e.g., Lörinčík et al 2021).…”

Multiwavelength Signatures of Episodic Null Point Reconnection in a Quadrupolar Magnetic Configuration and the Cause of Failed Flux Rope Eruption

“…However, numerical techniques used to calculate Q-values will always return finite values, thus we can expect a coronal null point to be associated with high degree Q-values (Pariat & Démoulin 2012). In a number of studies concerning coronal null points and their evolution, the coronal null points were found to be associated with a wide range of Q-values (∼10 4 -10 12 ; see, Yang et al 2015;Liu et al 2020;Qiu et al 2020;Prasad et al 2020). In the present study, we observed the null point to be associated with a value of Q ≈10 8.5 .…”