In this paper, we present a multi-wavelength 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 EUV channels. Both X-class flares initiated from the core of the sigmoid sequentially within an interval of ∼3 hours 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 pre-flare phase which further intensified and spatially expanded during each event. The identification of a pre-existing 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 pre-flare 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 prior to both flares which are subsequently supported by the observations of the eruption of hot EUV channels. The second X-class flare diverged from the standard flare scenario in the evolution of two sets of flare ribbons, that are spatially well separated, providing firm evidence of magnetic reconnections at two coronal heights.

In this paper, we present a comprehensive study of the evolutionary phases of a major M6.6 long duration event with special emphasize on its pre-flare phase. The event occurred in NOAA 12371 on 2015 June 22. A remarkable aspect of the event was an active pre-flare phase lasting for about an hour during which a hot EUV coronal channel was in the build-up stage and displayed cospatial hard X-ray (HXR) emission up to energies of 25 keV. This is the first evidence of the HXR coronal channel. The coronal magnetic field configuration based on nonlinear-force-free-field modeling clearly exhibited a magnetic flux rope (MFR) oriented along the polarity inversion line (PIL) and cospatial with the coronal channel. We observed significant changes in the AR’s photospheric magnetic field during an extended period of ≈42 hr in the form of rotation of sunspots, moving magnetic features, and flux cancellation along the PIL. Prior to the flare onset, the MFR underwent a slow rise phase (≈14 km s−1) for ≈12 minutes, which we attribute to the faster build-up and activation of the MFR by tether-cutting reconnection occurring at multiple locations along the MFR itself. The sudden transition in the kinematic evolution of the MFR from the phase of slow to fast rise (≈109 km s−1 with acceleration ≈110 m s−2) precisely divides the pre-flare and impulsive phase of the flare, which points toward the feedback process between the early dynamics of the eruption and the strength of the flare magnetic reconnection.

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We present a multiwavelength analysis of two homologous, short-lived, impulsive flares of GOES class M1.4 and M7.3 that occurred from a very localized minisigmoid region within the active region NOAA 12673 on 2017 September 7. Both flares were associated with initial jetlike plasma ejection that for a brief amount of time moved toward the east in a collimated manner before drastically changing direction toward the southwest. Nonlinear force-free field extrapolation reveals the presence of a compact double-decker flux rope configuration in the minisigmoid region prior to the flares. A set of open field lines originating near the active region that were most likely responsible for the anomalous dynamics of the erupted plasma gave the earliest indication of an emerging coronal hole near the active region. The horizontal field distribution suggests a rapid decay of the field above the active region, implying high proneness of the flux rope system toward eruption. In view of the low coronal double-decker flux ropes and compact extreme ultraviolet brightening beneath the filament, along with associated photospheric magnetic field changes, our analysis supports the combination of initial tether-cutting reconnection and subsequent torus instability for driving the eruption.

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We present a multi-wavelength analysis of the eruption of a hot coronal channel associated with an X1.0 flare (SOL2013-10-28T02:03) from the active region NOAA 11875 by combining observations from AIA/SDO, HMI/SDO, RHESSI, and HiRAS. EUV images at high coronal temperatures indicated the presence of a hot channel at the core of the active region from the early pre-flare phase evidencing the pre-existence of a quasi-stable magnetic flux rope. The hot channel underwent an activation phase after a localized and prolonged pre-flare event occurring adjacent to one of its footpoints. Subsequently, the flux rope continued to rise slowly for ≈16 min during which soft X-ray flux gradually built-up characterizing a distinct precursor phase. The flux rope transitioned from the state of slow rise to the eruptive motion with the onset of the impulsive phase of the X1.0 flare. The eruptive expansion of the hot channel is accompanied by a series of type III radio bursts in association with impulsive rise of strong hard X-ray non-thermal emissions that included explicit hard X-ray sources of energies up to ≈50 keV from the coronal loops and ≈100 keV from their footpoint locations. Our study contains evidence that pre-flare activity occurring within the spatial extent of a stable flux rope can destabilize it toward eruption. Moreover, sudden transition of the flux rope from the state of slow rise to fast acceleration precisely bifurcated the precursor and the impulsive phases of the flare which points toward a feedback relationship between early CME dynamics and the strength of the large-scale magnetic reconnection.

We report multiwavelength study of a complex M-class solar eruptive flare that consists of three different sets of flare ribbons, viz. circular, parallel, and remote ribbons. Magnetic field modelling of source active region NOAA 12242 exhibits the presence of 3D null-point magnetic topology that encompasses an inner bipolar region. The event initiates with the faint signatures of the circular ribbon along with remote brightening right from the pre-flare phase that points toward the ongoing slow yet persistent null-point reconnection. We first detected flux cancellation and an associated brightening, which are likely signatures of tether-cutting reconnection that builds the flux rope near the polarity inversion line (PIL) of the inner bipolar region. In the next stage, with the onset of M8.7 flare, there is a substantial enhancement in the brightening of circular ribbon, which essentially suggests an increase in the rate of ongoing null-point reconnection. Finally, the eruption of underlying flux rope triggers ‘standard flare reconnection’ beneath it producing an abrupt rise in the intensity of the parallel ribbons as well as enhancing the rate of null-point reconnection by external forcing. We show that within the the fan dome, the region with magnetic decay index n > 1.5 borders the null-point QSL. Our analysis suggests that both the torus instability and the breakout model have played role toward the triggering mechanism for the eruptive flare. This event is a nice example of the dynamical evolution of a flux rope initially confined in a null-point topology that subsequently activates and erupts with the progression of the circular-cum-parallel ribbon flare.

In this paper, we present multiwavelength observations of the triggering of a failed-eruptive M-class flare from active region NOAA 11302 and investigate the possible reasons for the associated failed eruption. Photospheric observations and nonlinear force-free field extrapolated coronal magnetic field revealed that the flaring region had a complex quadrupolar configuration with a preexisting coronal nullpoint situated above the core field. Prior to the onset of the M-class flare, we observed multiple periods of small-scale flux enhancements in GOES and RHESSI soft X-ray observations from the location of the nullpoint. The preflare configuration and evolution reported here are similar to the configurations presented in the breakout model, but at much lower coronal heights. The core of the flaring region was characterized by the presence of two flux ropes in a double-decker configuration. During the impulsive phase of the flare, one of the two flux ropes initially started erupting, but resulted in a failed eruption. Calculation of the magnetic decay index revealed a saddle-like profile where the decay index initially increased to the torus-unstable limits within the heights of the flux ropes, but then decreased rapidly and reached negative values, which was most likely responsible for the failed eruption of the initially torus-unstable flux rope.