Solar active region (AR) 12673 produced 4 X-class, 27 M-class, and numerous lower class flares during its passage across the visible solar disk in September 2017. Our study is to answer the questions why this AR was so flare-productive and how the X9.3 flare, the largest one of the last decade, took place. We find that there was a sunspot in the initial several days, and then two bipolar regions emerged nearby it successively. Due to the standing of the pre-existing sunspot, the movement of the bipoles was blocked, while the pre-existing sunspot maintained its quasi-circular shaped umbra only with the disappearance of a part of penumbra. Thus, the bipolar patches were significantly distorted, and the opposite polarities formed two semi-circular shaped structures. After that, two sequences of new bipolar regions emerged within the narrow semi-circular zone, and the bipolar patches separated along the curved channel. The new bipoles sheared and interacted with the previous ones, forming a complex topological system, during which numerous flares occurred. At the highly sheared region, a great deal of free energy was accumulated. On September 6, one negative patch near the polarity inversion line began to rapidly rotate and shear with the surrounding positive fields, and consequently the X9.3 flare erupted. Our results reveal that the block-induced complex structures built the flareproductive AR and the X9.3 flare was triggered by an erupting filament due to the kink instability. To better illustrate this process, a block-induced eruption model is proposed for the first time.

We present SDO/AIA observations of the interaction of a global EUV wave on 2011 June 7 with active regions (ARs), coronal holes (CHs) and coronal bright structures. The primary global wave has a three-dimensional dome shape, with propagation speeds ranging from 430-780 km s −1 in different directions. The primary coronal wave runs in front of the expanding loops involved in the CME and its propagation speeds are approximately constant within 10-20 minutes. Upon arrival at an AR on its path, the primary EUV wave apparently disappears and a secondary wave rapidly reemerges 75 Mm within the AR boundary at a similar speed. When the EUV wave encounters a coronal bright structure, an additional wave front appears there and propagates in front of it at a velocity nearly a factor of 2 faster. Reflected waves from a polar CH and a coronal bright structure are observed and propagate fractionally slower than the primary waves. Some of these phenomena can be equally explained by either a wave or non-wave model alone. However, taken together, these observations provide new evidence for the multitudes of global EUV waves, in which a true MHD fast-mode wave or shock propagates in front of an expanding CME bubble.

Using the observations from the Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO), we investigate six X-class and twentynine M-class flares occurring in solar active region (AR) 12192 from October 18 to 29. Among them, thirty (including six X-and twenty-four M-class) flares originated from the AR core and the other five M-flares appeared at the AR periphery. Four of the X-flares exhibited similar flaring structures, indicating they were homologous flares with analogous triggering mechanism. The possible scenario is: photospheric motions of emerged magnetic fluxes lead to shearing of the associated coronal magnetic field, which then yields a tether-cutting favorable configuration. Among the five periphery M-flares, four were associated with jet activities. The HMI vertical magnetic field data show that the photospheric fluxes of opposite magnetic polarities emerged, converged and canceled with each other at the footpoints of the jets before the flares. Only one M-flare from the AR periphery was followed by a coronal mass ejection (CME). From October 20 to 26, the mean decay index of the horizontal background field within the height range of 40-105 Mm is below the typical threshold for torus instability onset. This suggests that a strong confinement from the overlying magnetic field might be responsible for the poor CME production of AR 12192.

Using the high tempo-spatial resolution Hα images observed with the New Vacuum Solar Telescope, we report the solid observational evidence of magnetic reconnection between two sets of small-scale anti-parallel loops with an X-shaped topology. The reconnection process contains two steps: a slow step with the duration of more than several tens of minutes, and a rapid step lasting for only about three minutes. During the slow reconnection, two sets of anti-parallel loops reconnect gradually, and new loops are formed and stacked together. During the rapid reconnection, the anti-parallel loops approach each other quickly, and then the rapid reconnection takes place, resulting in the disappearance of former loops. In the meantime, new loops are formed and separate. The region between the approaching loops is brightened, and the thickness and length of this region are determined to be about 420 km and 1.4 Mm, respectively. During the rapid reconnection process, obvious brightenings at the reconnection site and apparent material ejections outward along reconnected loops are observed. These observed signatures are consistent with predictions by reconnection models. We suggest that the successive slow reconnection changes the conditions around the reconnection site and triggers instabilities, thus leading to the rapid approach of the anti-parallel loops and resulting in the rapid reconnection.

With the high tempo-spatial Interface Region Imaging Spectrograph 1330Å images, we find that many bright structures are rooted in the light bridge of NOAA 12192, forming a light wall. The light wall is brighter than the surrounding areas, and the wall top is much brighter than the wall body. The New Vacuum Solar Telescope Hα and the Solar Dynamics Observatory 171Å and 131Å images are also used to study the light wall properties. In 1330Å, 171Å, and 131Å, the top of the wall has a higher emission, while in the Hα line, the wall top emission is very low. The wall body corresponds to bright areas in 1330Å and dark areas in the other lines. The top of the light wall moves upward and downward successively, performing oscillations in height. The deprojected mean height, amplitude, oscillation velocity, and the dominant period are determined to be 3.6 Mm, 0.9 Mm, 15.4 km s −1 , and 3.9 min, respectively. We interpret the oscillations of the light wall as the leakage of p-modes from below the photosphere. The constant brightness enhancement of the wall top implies the existence of some kind of atmospheric heating, e.g., via the persistent small-scale reconnection or the magnetoacoustic waves. In another series of 1330Å images, we find that the wall top in the upward motion phase is significantly brighter than in the downward phase. This kind of oscillations may be powered by the energy released due to intermittent impulsive magnetic reconnection.

We analyze a unique event with an M1.8 confined circular-ribbon flare on 2016 February 13, with successive formations of two circular ribbons at the same location. The flare had two distinct phases of UV and EUV emissions with an interval of about 270 s, of which the second peak was energetically more important. The first episode was accompanied by the eruption of a mini-filament and the fast elongation motion of a thin circular ribbon (CR1) along the counterclockwise direction at a speed of about 220 km s −1 . Two elongated spine-related ribbons were also observed, with the inner ribbon co-temporal with CR1 and the remote brightenings forming ∼ 20 s later. In the second episode, another mini-filament erupted and formed a blowout jet. The second circular ribbon and two spine-related ribbons showed similar elongation motions with that during the first episode. The extrapolated 3D coronal magnetic fields reveal the existence of a fan-spine topology, together with a quasi-separatrix layer (QSL) halo surrounding the fan plane and another QSL structure outlining the inner spine. We suggest that continuous null-point reconnection between the filament and ambient open field occurs in each episode, leading to the sequential opening of the filament and significant shifts of the fan plane footprint. For the first time, we propose a compound eruption model of circular-ribbon flares consisting of two sets of successively formed ribbons and eruptions of multiple filaments in a fan-spine-type magnetic configuration.

Using the multi-wavelength data from Atmospheric Imaging Assembly on board the Solar Dynamic Observatory, we investigated two successive solar flares, a C5.1 confined flare and an X4.9 ejective flare with a halo coronal mass ejection, in NOAA AR 11990 from 2014 Feb 24 to 25. Before the confined flare onset, EUV brightening beneath the filament was detected. As the flare began, a twisted helical flux rope (FR) wrapping around the filament moved upward and then stopped, and in the meantime an obvious X-ray source below it was observed. Prior to the ejective X4.9 flare, some pre-existing loop structures in the active region interacted with each other, which produced a brightening region beneath the filament. Meanwhile, a small flaring loop appeared below the interaction region and some new helical lines connecting the far ends of the loop structures was gradually formed and continually added into the former twisted FR. Then, due to the resulting imbalance between the magnetic pressure and tension, the new FR together with the filament erupted outward. Our observations coincide well with tether-cutting model, suggesting that the two flares probably have the same triggering mechanism, i.e., tether-cutting reconnection. To our knowledge, this is the first direct observation of tether-cutting reconnection occurring between the pre-existing loops in active region. In the ejective flare case, the erupting filament exhibited an Ω-like kinked structure and underwent an exponential rise after a slow-rise phase, indicating the kink instability might be also responsible for the eruption initiation.

We establish the largest eruptive/confined flare database to date and analyze 322 flares of Geostationary Operational Environmental Satellite class M1.0 and larger that occurred during 2010–2019, i.e., almost spanning all of solar cycle 24. We find that the total unsigned magnetic flux ( ) of active regions (ARs) is a key parameter governing the eruptive character of large flares, with the proportion of eruptive flares exhibiting a strong anticorrelation with . This means that an AR containing a large magnetic flux has a lower probability that the large flares it produces will be associated with a coronal mass ejection (CME). This finding is supported by the high positive correlation we obtained between the critical decay index height and , implying that ARs with a larger have a stronger magnetic confinement. Moreover, the confined flares originating from ARs larger than Mx have several characteristics in common: stable filament, slipping magnetic reconnection, and strongly sheared post-flare loops. Our findings reveal new relations between the magnetic flux of ARs and the occurrence of CMEs in association with large flares. The relations obtained here provide quantitative criteria for forecasting CMEs and adverse space weather, and have important implications for “superflares” on solar-type stars and stellar CMEs.