Ting Li^1,2, Anqin Chen³, Yijun Hou^1,2, Astrid M. Veronig⁴, Shuhong Yang^1,2, & Jun Zhang⁵

1. CAS Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, P. R. China
2. School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3. Key Laboratory of Space Weather, National Center for Space Weather, China Meteorological Administration, Beijing 100081, P. R. China
4. Institute of Physics & Kanzelhöhe Observatory for Solar and Environmental Research, University of Graz, A-8010 Graz, Austria
5. School of Physics and Materials Science, Anhui University, Hefei 230601, P. R. China

Solar flares and coronal mass ejections (CMEs) are the most catastrophic phenomena in the solar system, driven by sudden releases of magnetic energy stored in the solar corona. Large solar flares are often, but not always, associated with CMEs. We dub flares with a CME as “eruptive” and flares without a CME as “confined”. Previous studies about the magnetic causes of confined flares have mainly focused on two aspects. One focus is the constraining effect of the background magnetic field overlying the flaring region, i.e., the strength of magnetic field or its decay with height^[1,2]. The other focus is magnetic complexity and non-potentiality of active regions (ARs)^[3], such as free magnetic energy, relative helicity, magnetic twists, etc. Until now, the key magnetic parameters of ARs governing the eruptive character of solar flares are unknown based on statistical results. In our recent work^[4], we carried out the first statistical study that investigated the flare–CME association rate R as a function of characteristics of the ARs that produce the flares, in terms of their total magnetic flux (Ф_AR).

We analyzed 719 GOES flares stronger than C5.0 during 2010–2019, including 251 eruptive flares and 468 confined flares (Figure 1a). For each event, we calculated the total unsigned magnetic flux of ARs before the flare onset by using the available vector magnetograms from the HMI Active Region Patches. We investigated the flare-CME association rate R as function of both the flare class (FSXR) and the total flux of the source AR. Ф_AR was then divided into five subintervals. The relations of the association rate R with FSXR within the five subintervals revealed that for each AR subinterval, R clearly increases with FSXR (Figure 1b)., i.e., stronger flares are more likely associated with a CME. What is new and particularly important in our study is the clear difference of R, with the slope of R as a function of flare intensity being a monotonically decreasing function of Ф_AR (Figure 1c). This means that flares of the same GOES class but originating from an AR of larger magnetic flux are much more likely to be confined.

Figure 1| Relations of flare–CME association rate (R) with flare peak soft X-ray flux (FSXR) and total unsigned magnetic flux of ARs (Ф_AR).Panel a: Scatter plot of magnetic flux vs. SXR flare magnitude. Blue (red) circles are the eruptive (confined) flares. Panel b: Association rate R as a function of FSXR separately for five different subintervals of Ф_AR. Slopes and Spearman rank order correlation coefficients r’s are shown at the bottom right. Panel c: Plot of slopes α vs. Ф_AR.

Based on these solar observations, we can speculate on the association rate R for solar-type stars by taking a representative stellar AR magnetic flux of 10²⁴ Mx. For X100-class “superflares” on solar-type stars, no more than 50% flares can generate stellar CMEs. This may help to provide an explanation why the detection of stellar CMEs is rare^[5].

In summary, our results imply that the magnetic flux of an AR is a key factor determining the eruptive character of its solar flare, as it provides a global parameter relating to the strength of the background field confinement. Large flux means strong confinement^[2] and thus the flare-CME association rate is relatively low compared to active regions with small fluxes. Our results have important implications to the prediction of CMEs occurring associated with large flares as well as to the solar-stellar connection, where the solar flare-CME association rates can be used to estimate stellar CME occurrence frequencies.

References:

[1] Wang, Y., & Zhang, J. 2007, ApJ, 665, 1428
[2] Li, T., Hou, Y., Yang, S., et al. 2020, ApJ, 900, 128
[3] Sun, X., Bobra, M. G., Hoeksema, J. T., et al. 2015, ApJL, 804, L28
[4] Li, T., Chen, A., Hou, Y., et al. 2021, ApJL, 917, L29
[5] Vida, K., Leitzinger, M., Kriskovics, L., et al. 2019, A&A, 623, A49