Yumi Bamba¹ & Kanya Kusano²
1. Institute of Space and Astronautical Science (ISAS)/Japan Aerospace Exploration Agency (JAXA)
, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
2. Institute for Space-Earth Environmental Research (ISEE)/Nagoya University
, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan

We studied geometric structures leading to 32 flare events with an aim to evaluate the applicability of flare-trigger condition of our numerical model and observations^[1]. We analyzed the events based on their photospheric magnetic field configurations, presence of precursor brightenings, and shapes of the initial flare ribbons. Consequently, we concluded that our flare-trigger model, proposed by Kusano et al.(henceforth “K12”)^[2] has proposed important conditions for flare-triggering.

Figure 1| Chart of the classification procedure for the selected events.

We so far proposed a flare-trigger model “K12” based on MHD simulations that prescribes flare-trigger conditions by two geometrical parameters: sheared angle θ₀ of a large-scale field and azimuth φ_e of a small-scale dipole field. According to the numerical survey of K12, there are two specific cases: opposite polarity (OP) and reversed shear (RS) fields (we call these two “flare-trigger fields”), which can trigger a flare. We previously developed a way to measure the angles of θ₀ and φ_e using the photospheric magnetic field and chromospheric brightening observed by Hinode and SDO^[3,4]. In this study, we aim to evaluate the consistency between the flare-trigger condition of K12 and observations, focusing on geometrical structures. Specifically, we determine what fraction of the flares are consistent with the OP- or RS-types and whether a flare occurs with some conditions other than OP or RS. For that purpose, we investigated 32 flare events that are stronger than M5.0.


Figure 2| List of the analyzed events and classification results.

We classified the selected 32 events into six types by evaluating the HMI-observed photospheric magnetic field configuration, presence of precursor brightenings, and shape of the initial flare ribbons using the method of our previous studies^[3,4]. The summary of classification procedure, classification results, and examples of each event are shown in Figures 1, 2, and 3, respectively. The simplified description of the definitions for each type are as follows.

OP Type (proposed by K12):

The event satisfies 0° ≦ θ₀ ≦90° and 124° ≦ φ_e ≦ 225° at the location and timing of the last precursor brightening. The magnetic field in the small-bipole region has an OP pattern relative to that averaged over the whole AR.


RS Type (proposed by K12):
The event satisfies 40° ≦ θ₀ and 225° ≦ φ_e ≦ 335° at the location and timing of the last precursor brightening. The local magnetic shear in the small-scale bipole region is toward the opposite direction of the global magnetic shear of the AR.


Contradicting-K12 Type:

The event occurs with the “no-flare” condition of K12: 0° ≦ φ_e ≦ 120° and φ_e ≦ 250° with a small θ₀ value.


Multiple Trigger Candidates Type:

There are multiple small-scale bipole fields that show the important features of K12’s flare-trigger fields, either the OP or the RS.

No-precursor Brightening Type:
The event clearly shows two sheared ribbons in the initial flare phase, however, it does not show any precursor brightening over the local PIL of the small-scale bipole field located at the center of the two ribbons.


Complicated Ribbon Type:

The event shows complicated initial flare ribbons that are nothing like the flare ribbons expected in the numerical simulation of K12.

Figure 3| Examples of each event type.

From the classification, we found three important results. First, there was no Contradicting-K12 type event that satisfied the “no-flare” condition of K12. Second, 30% of the events, including six RS-type events and four multiple trigger candidates types, were consistent with K12. Third, approximately 70% of the events (22 of the analyzed 32 events) were classified as either the no-precursor brightening or complicated ribbon types that did not clearly show the key features suggested by K12. Then we consider whether the 70% of the events contradict the K12’s flare-trigger conditions. We found that 8 of the 11 complicated ribbon type events showed precursor brightenings on the PIL. Accordingly, we propose that it is possible that the flare-triggering site for the eight events may be found through a detailed analysis, and we need a precise analysis for each individual event. Meanwhile, we found that physical process(es) different from those proposed by K12 cannot be ruled out for the events that did not show any precursor brightenings, such as the no-precursor brightening type events and the three complicated ribbon type events. We consider other physical processes such as the magnetic flux cancellation model^[5] to trigger these events. Extended studies are required to reveal the physical process(es) that cause the different types of flares. Even so, our result that 30% of the events investigated in this study were consistent with the flare-trigger model of K12 leads to the conclusion that the observable features and (θ₀, φ_e)parameters of K12 are important to understand the flare triggering.


References

[1] Y. Bamba, & K. Kusano, 2018, ApJ, 856, 43

[2] K. Kusano, Y. Bamba, T. T. Yamamoto, Y. Iida, S. Toriumi, & A. Asai, 2012, ApJ, 760, 31

[3] Y. Bamba, K. Kusano, T. T. Yamamoto, & T. J. Okamoto, 2013, ApJ, 778, 48

[4] Y. Bamba, K. Kusano, S. Imada, & Y. Iida, 2014, PASJ, 66 (SP1), S16

[5] van Ballegooijen, A. A., & Martens, P. C. H. 1989, ApJ, 343, 971