With the aim of understanding how the magnetic properties of active regions (ARs) control the eruptive character of solar flares, we analyze 719 flares of Geostationary Operational Environmental Satellite (GOES) class ≥C5.0 during 2010–2019. We carry out the first statistical study that investigates the flare-coronal mass ejection (CME) association rate as a function of the flare intensity and the AR characteristics that produce the flare, in terms of its total unsigned magnetic flux (ΦAR). Our results show that the slope of the flare–CME association rate with flare intensity reveals a steep monotonic decrease with ΦAR. This means that flares of the same GOES class but originating from an AR of larger ΦAR, are much more likely to be confined. Based on an AR flux as high as 1.0 × 1024 Mx for solar-type stars, we estimate that the CME association rate in X100-class “superflares” is no more than 50%. For a sample of 132 flares ≥M2.0 class, we measure three nonpotential parameters including the length of steep gradient polarity-inversion line (L SGPIL), the total photospheric free magnetic energy (E free), and the area with large shear angle (A Ψ). We find that confined flares tend to have larger values of L SGPIL, E free, and A Ψ compared to eruptive flares. Each nonpotential parameter shows a moderate positive correlation with ΦAR. Our results imply that ΦAR is a decisive quantity describing the eruptive character of a flare, as it provides a global parameter relating to the strength of the background field confinement.

With the aim of investigating how the magnetic field in solar active regions (ARs) controls flare activity, i.e., whether a confined or eruptive flare occurs, we analyze 106 flares of Geostationary Operational Environmental Satellite class ≥M1.0 during 2010–2019. We calculate mean characteristic twist parameters α FPIL within the “flaring polarity inversion line” region and α HFED within the area of high photospheric magnetic free energy density, which both provide measures of the nonpotentiality of the AR core region. Magnetic twist is thought to be related to the driving force of electric current-driven instabilities, such as the helical kink instability. We also calculate total unsigned magnetic flux (ΦAR) of ARs producing the flare, which describes the strength of the background field confinement. By considering both the constraining effect of background magnetic fields and the magnetic nonpotentiality of ARs, we propose a new parameter α/ΦAR to measure the probability for a large flare to be associated with a coronal mass ejection (CME). We find that in about 90% of eruptive flares, α FPIL/ΦAR and α HFED/ΦAR are beyond critical values (2.2 × 10−24 and 3.2 × 10−24 Mm−1 Mx−1), whereas they are less than critical values in ∼80% of confined flares. This indicates that the new parameter α/ΦAR is well able to distinguish eruptive flares from confined flares. Our investigation suggests that the relative measure of magnetic nonpotentiality within the AR core over the restriction of the background field largely controls the capability of ARs to produce eruptive flares.

round-the-clock solar observations with full-disk coverage of vector magnetograms and multi-wavelength images demonstrate that solar active regions (ARs) are ultimately connected with magnetic field. Often two or more ARs are clustered, creating a favorable magnetic environment for the onset of coronal mass ejections (CMEs). In this work, we describe a new type of magnetic complex: cluster of solar ARs. An AR cluster is referred to as the close connection of two or more ARs which are located in nearly the same latitude and a narrow span of longitude. We illustrate three examples of AR clusters, each of which has two ARs connected and formed a common dome of magnetic flux system. CME initiations were often tied to the instability of the magnetic structures connecting two partner ARs, in the form of inter-connecting loops and/or channeling filaments between the two ARs. We show the evidence that, at least, some of the flare/CMEs in an AR cluster are not a phenomenon of a single AR, but the result of magnetic interaction in the whole AR cluster. The observations shed new light on understanding the mechanism(s) of solar activity. Instead of the simple bipolar topology as suggested by the so-called standard flare model, a multi-bipolar magnetic topology is more common to host the violent solar activity in solar atmosphere. activity, coronal mass ejection, magnetic fields PACS number(s): 96.12.Hg, 96.12.Jt, 96.12.Kz Citation: Wang J X, Zhang Y Z, He H, et al. Cluster of solar active regions and onset of coronal mass ejections.

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