Yijun Hou1,2, Ting Li3,2,1, Shuhong Yang1,2, Leping Li1, Yingjie Cai1,2, Xiaofeng Liu4,5,1, Shuo Yang6, Yilin Guo7, Shihao Rao8,9, Chuan Li8,9, Guiping Zhou1,2
1. State Key Laboratory of Solar Activity and Space Weather, National Astronomical Observatories, Chinese Academy of Science, Beijing 100101, China
2. School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
3. State Key Laboratory of Solar Activity and Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China
4. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210034, China
5. School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
6. North China University of Technology, Beijing 100144, China
7. Beijing Planetarium, Beijing Academy of Science and Technology, Beijing 100044, China
8. School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China
9. Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, China
Solar flares are observed as sudden wide spectrum electromagnetic brightening, driven by the release of free magnetic energy via magnetic reconnection, but how free magnetic energy accumulates in the pre flare coronal magnetic field remains a key open question. Previous studies found that major flares tend to take place in the vicinity of the polarity inversion lines (PILs), and active regions (ARs) with compact shearing PILs are often flare-productive[1,2]. In the source region of the largest X9.3 flare of Solar Cycle 24 (AR 12673), up to five pairs of magnetic bipoles successively emerged and continuously interacted to form a strongly sheared PIL and a complex multiple-magnetic-flux-rope system above it[1]. The “collisional shearing” between the non conjugated polarities of two emerging bipoles within the same AR is believed to drive continuous magnetic flux cancellation, thereby triggering a series of solar eruptions[3].
In September 2024, AR 13842 appeared at the east solar limb and rapidly evolved into a flare-productive AR with a highly sheared PIL, producing two X class flares (X7.1 and X9.0) and over ten M class flares in three days; the X9.0 flare is so far the largest in Solar Cycle 25. In this work, we investigate the magnetic evolution of the flaring region in AR 13842 and why free energy accumulated so quickly that major flares could recur there in such a short time.
Figure 1. Rapid magnetic flux emergence with perpendicular separation directions in AR 13842. (a1)–(a3): Sequence of HMI continuum intensity maps overlaid with the LOS magnetic field contours (red and blue curves) showing the magnetic evolution of AR 13842 from September 30 to October 3. Black solid arrows indicate separation directions of two emerging bipoles perpendicular to each other. The black rectangles outline the evolving region of the two polarities around the flaring PIL region. (b1): Temporal evolution of the unsigned magnetic flux (solid curves) and its emergence rate (dotted curves) in the whole AR 13842. (b2): Temporal evolution of the negative (blue curves) and positive (red curves) magnetic flux around the flaring PIL region.
As shown in Figure 1, AR 13842 initially consisted of a conjugated magnetic polarity pair (N1 and P1). Since 2024 September 30, more magnetic bipoles successively emerged between them, and separated along the northeast-southwest direction. Meanwhile, continuous flux emergence (n1 and p1) also took place to the southwest of N1, and then separated along the northwest-southeast direction. As a result, collisional shearing occurred between the opposite nonconjugated polarities (N1 and p1), accompanied by frequent flux cancellations. Thus, a highly-sheared flaring PIL began to form. It is revealed that the unsigned flux of the whole AR gradually increased to reach 2.3 × 1021 Mx late on October 3 with a maximum emergence rate of 3.5 × 1019 Mx hr-1.
Figure 2. Evolution of photospheric nonpotentiality and three-dimensional (3D) magnetic topology in flaring PIL region of AR 13842. (a1)–(a3): Sequence of free energy density (ρfree) maps showing the PIL region with high ρfree. (b1)–(b3): Top view of sheared arcades and twisted magnetic flux ropes (MFRs) above the sheared PIL region. (c1)–(c3): Distribution of magnetic twist Tw in the vertical planes based on the green cuts labeled in (b1)–(b3). The yellow curves outline the general section shape of MFRs appearing before the onset of the X7.1 and X9.0 flares, respectively.
During the rapid formation of the collisional shearing PIL, a great deal of free magnetic energy was also synchronously accumulated, accompanied by the rapid formation of twisted magnetic flux ropes (MFRs) in the PIL (see Figure 2). On September 30, the PIL between the main sunspot (N1) and satellite pore (p1) already had high free energy density (ρfree), with sheared arcades but no twist. By October 1, before the X7.1 flare, an MFR with a maximum Tw of ~2.0 had already formed as the high ρfree region became curved and longer due to shearing. It’s worth noting that despite having erupted during the X7.1 flare, a more twisted MFR (maximum Tw of ~2.7) still appeared in the same PIL region before the X9.0 flare 38 hours later. It indicates that the formation of the MFR driven by the shearing motion between the PIL was continuous during that period, which was repeatedly but briefly interrupted by the series of major flares.
Figure 3. Temporal profiles of magnetic parameters (normalized area, total free energy, average ρfree, and average horizontal magnetic field strength) of the flaring PIL region with high ρfree (≥ 9 × 104 erg cm-3).
We also noticed another interesting fact in Figure 3 that the area, average ρfree and horizontal magnetic field strength of the flaring PIL region with high ρfree exhibited a gradual pre-flare decrease followed by a rapid post-flare increase. The post-flare rise is attributed to magnetic field collapse, while the pre-flare decline is explained by the gradual formation and subsequent slow ascent of an MFR lying above the PIL[4]. Such a signature is a promising precursor for major eruptive flares. To further confirm its universality and physical nature, we are analyzing magnetic fields in more major flare source regions, with corresponding numerical simulations also underway.
For more details of this work, please refer to our publication Ref. [5].
References:
[1] Hou, Y., Zhang, J., Li, T., et al. 2018, A&A, 619, A100
[2] Toriumi, S., & Wang, H. 2019, LRSP, 16, 3
[3] Chintzoglou, G., Zhang, J., Cheung, M. C. M., & Kazachenko, M. 2019, ApJ, 871, 67
[4] Liu, Y., Welsch, B. T., Valori, G., et al. 2023, ApJ, 942, 27
[5] Hou, Y., Li, T., Yang, S., et al. 2026, ApJ, 1004, 36