In this article, we present a multiwavelength analysis of two X-class solar eruptive flares of classes X2.2 and X9.3 that occurred in the sigmoidal active region NOAA 12673 on 2017 September 6, by combining observations of Atmospheric Imaging Assembly and Helioseismic Magnetic Imager instruments on board the Solar Dynamics Observatory. On the day of the reported activity, the photospheric structure of the active region displayed a very complex network of δ-sunspots that gave rise to the formation of a coronal sigmoid observed in the hot extreme-ultraviolet channels. Both X-class flares initiated from the core of the sigmoid sequentially within an interval of ∼3 hr and progressed as a single sigmoid-to-arcade event. Differential emission measure analysis reveals strong heating of plasma at the core of the active region right from the preflare phase, which further intensified and spatially expanded during each event. The identification of a preexisting magnetic null by non-force-free-field modeling of the coronal magnetic fields at the location of early flare brightenings and remote faint ribbon-like structures during the preflare phase, which were magnetically connected with the core region, provide support for the breakout model of solar eruption. The magnetic extrapolations also reveal flux rope structures before both flares, which are subsequently supported by the observations of the eruption of hot extreme-ultraviolet channels. The second X-class flare diverged from the standard flare scenario in the evolution of two sets of flare ribbons, which are spatially well separated, providing firm evidence of magnetic reconnections at two coronal heights.

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.

The magnetohydrodynamics of the solar corona is simulated numerically. The simulation is initialized with an extrapolated non-force-free magnetic field using the vector magnetogram of the active region NOAA 12192, which was obtained from the solar photosphere. Particularly, we focus on the magnetic reconnections (MRs) occurring close to a magnetic null point that resulted in the appearance of circular chromospheric flare ribbons on 2014 October 24 around 21:21 UT, after the peak of an X3.1 flare. The extrapolated field lines show the presence of the three-dimensional (3D) null near one of the polarity-inversion lines—where the flare was observed. In the subsequent numerical simulation, we find MRs occurring near the null point, where the magnetic field lines from the fan plane of the 3D null form a X-type configuration with underlying arcade field lines. The footpoints of the dome-shaped field lines, inherent to the 3D null, show high gradients of the squashing factor. We find slipping reconnections at these quasi-separatrix layers, which are co-located with the post-flare circular brightening observed at chromospheric heights. This demonstrates the viability of the initial non-force-free field, along with the dynamics it initiates. Moreover, the initial field and its simulated evolution are found to be devoid of any flux rope, which is congruent with the confined nature of the flare.

Magnetohydrodynamic simulation is carried out for the NOAA AR 12615 in the time span t ∈ {05:48, 06:18} UT on 2016 December 5; covering events of a blowout jet and a C1.2 flare. The events are selected based on the small interval between their occurrences, which provides us with an opportunity to explore two energetically different events of magnetic reconnection (MR) within the run time of a single magnetohydrodynamic (MHD) simulation. The simulation is initiated with magnetic field extrapolated from the vector magnetogram provided by the Helioseismic Magnetic Imager/Solar Dynamics Observatory, using a non-force-free approximation. The extrapolated Lorentz force is found to decay at a rate faster than the volume current density, making the higher corona to be effectively force-free while the Lorentz force affects only the lower corona and the photosphere—a desirable feature that agrees with the contemporary understanding. For the simulation, the plasma is idealized to be incompressible, thermally homogeneous and having perfect electrical conductivity. The results affirm MRs near a set of two three-dimensional (3D) magnetic nulls to be responsible for initiating the jet. Moreover, a flux rope located near the nulls contributes to the jet by changing its magnetic field lines from an anchored to an open topology. The scenario agrees with the standard mini-filament breakout model for blowout jets and provides its first demonstration from a 3D data-constrained MHD simulation where the computational output is reconciled with magnetogram(s) only once. The generation of flare ribbons is attributed to reconnections at a 3D null and a quasi-separatrix layer (QSL), highlighting the importance of topological complexity in flare initiation.

Coronal dimmings associated with coronal mass ejections (CMEs) from the Sun have gained much attention since the late 1990s when they were first observed in high-cadence imagery of the SOHO/EIT and Yohkoh/SXT instruments. They appear as localized sudden decreases of the coronal emission at extreme ultraviolet (EUV) and soft X-ray (SXR) wavelengths, that evolve impulsively during the lift-off and early expansion phase of a CME. Coronal dimmings have been interpreted as “footprints” of the erupting flux rope and also as indicators of the coronal mass loss by CMEs. However, these are only some aspects of coronal dimmings and how they relate to the overall CME/flare process. The goal of this review is to summarize our current understanding and observational findings on coronal dimmings, how they relate to CME simulations, and to discuss how they can be used to provide us with a deeper insight and diagnostics of the triggering of CMEs, the magnetic connectivities and coronal reconfigurations due to the CME as well as the replenishment of the corona after an eruption. In addition, we go beyond a pure review by introducing a new, physics-driven categorization of coronal dimmings based on the magnetic flux systems involved in the eruption process. Finally, we discuss the recent progress in studying coronal dimmings on solar-like and late-type stars, and how to use them as a diagnostics for stellar coronal mass ejections and their properties.