Rui Wang^1,2,3, Ying D. Liu^1,2,3, Huidong Hu^1,2, Xiaowei Zhao^4,5, & Chong Chen⁶

1 State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China
2 Solar Activity and Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China
3 University of Chinese Academy of Sciences, Beijing, China
4 Key Laboratory of Space Weather, National Satellite Meteorological Center (National Center for Space Weather), China Meteorological Administration, Beijing 100081, China
5 School of Earth and Space Sciences, Peking University, Beijing 100871, China
6 School of Microelectronics and Physics, Hunan University of Technology and Business, Changsha, China

Active Regions (ARs) 13664/13668 unleashed consecutive X-class flares accompanied by persistent coronal mass ejections (CMEs) in May 2024. The geomagnetic response index soared to G5, Kp index peaked at 9, and the Dst index decreased to -412 nT. The only storm in the 21st century stronger than this one is the 2003 November storm, which had a minimum Dst of -422 nT. The May 2024 storm can be classified as an extreme space weather event, i.e., a low-probability but high-consequence event otherwise called a solar superstorm. A well-known example of extreme space weather before the space era is the 1859 Carrington event, with a minimum Dst estimated to be about -850 nT. The most severe event of the space age is the 1989 March storm with a minimum Dst of -589 nT. Liu et al.^[1] performed a comparative study of the 2012 July and 2017 July events. They identified similar patterns in both cases: “a long-lived eruptive nature of the source AR, a complex event composed of successive eruptions from the same AR, and in-transit interaction between the successive eruptions resulting in exceptionally strong ejecta magnetic fields at 1 AU.” They indicated that the concept of “preconditioning” is a necessary condition for a Carrington-class storm to occur. They further propose a hypothesis that solar superstorms are essentially “perfect storms”^[1,2], i.e., a combination of circumstances that produces an event of an unusual magnitude. It is intriguing to see how the May 2024 case will fit or challenge this hypothesis.

Figure 1| Solar wind parameters observed at Wind and measured Dst index. From top to bottom, the panels show the proton density (a), bulk speed (b), proton temperature (c), magnetic field strength (d), proton β (e), magnetic field components (f)-(h), and Dst index (i). The shaded regions indicate the intervals
of the two complex ejecta, and the vertical dashed lines mark the associated shocks. The red curve in panel (c) denotes the expected proton temperature calculated from the observed speed. The red curve in panel (i) represents the estimated Dst index by combining two kinds of formulas.

Figure 1 shows that the Dst index underwent a rapid decrease on 2024 May 10 (as shown in Figure 1), reaching its minimum on May 11. It then experienced a prolonged recovery phase, returning to normal levels only by May 13. In-situ data indicates that the rapid decrease and slow recovery of Dst can be attributed to the effects of two complex ejecta, as highlighted in the shaded regions. Ejecta 1 exhibited a strong southward magnetic field component, while Ejecta 2 showed no significant southward component, which explains why the Dst index did not continue to decrease on May 11.

Figure 2| Magnetic flux emergence and CME source regions. (a) Mean unsigned magnetic flux (green, 3-hour windows) and 6-hour average flux emergence rate of AR 13664/8 (red). (b)-(h) CME (E1-E7) source regions in AIA 304 Å and 131 Å. The green and red bars point to the locations of filament/hot channel. The arrows in the right panel mark the propagation directions of the full halo CMEs from May 8 to 13 obtained through CME reconstruction, with the lengths of the arrows indicating CME velocities near the Sun. Red (blue) arrows represent CMEs that likely contributed to the first (second) complex ejecta at the Earth.

Studies reveal that the eruption source region (Figure 2b-2h) underwent rapid formation and merging of ARs, accompanied by fast magnetic flux emergence. Figure 2a shows that by the end of the May 11, the unsigned magnetic flux of the AR reached approximately 1.5×10²³ Mx. The average magnetic flux emergence rate from the May 7 to the 9 reached a remarkable ∼10²¹ Mx h^-1, with the peak emergence rate occurring on the May 8 at ∼1.6×10²¹ Mx h^-1. This magnetic flux emergence rate is considered historic when compared to past events ( http://hmi.stanford.edu/hminuggets/?p=4216 ), surpassing both AR 12673 (known for its most intense flare activity) and AR 12192 (which hosted the largest sunspot group). However, such historic-level magnetic flux emergence alone does not fully account for the unprecedented geomagnetic response.

Research shows that the AR underwent a “collisional-shearing” process, which is considered a typical mechanism for producing homologous CMEs^[3]. This collisional-shearing process refers to the convergence and shearing between two pairs of non-dipolar magnetic fields during flux emergence, leading to the formation of strong-gradient polarity inversion lines (PILs). Following this model, analysis reveals that the seven consecutive Earth-directed eruptions primarily originated from two collisional PILs (indicated by the two red arrows in Figure 3). CMEs from these two PILs formed the aforementioned two complex ejecta through CME-CME interactions in interplanetary space. It indicates that the magnetic field from the lower PIL exhibited strong southward components in both its toroidal and axial components, while the upper PIL showed predominantly northward components. This explains why the Dst index did not continue to decrease.

Figure 3| Overview of GOES 1-8 Å flux (top) and AR 13664/8 (bottom). The vertical dashed blue lines show the flare peak times. The arrows indicate the axial fields of potential MFR structures along different PILs (red and purple), with reverse azimuthal fields (blue and green).

It indicates that another crucial factor in the formation of the two complex ejecta was the relatively slow speed of the 5th CME (as shown in Figure 2), which prevented it from catching up with the previous eruption, thus failing to meet the necessary conditions for CME-CME interaction. The underlying reason for this was that the rapid decrease in magnetic flux emergence rate directly led to the reduction in eruption intensity.

Additionally, the necessity of the collisional shearing mechanism for eruption formation is verified by analyzing the correlation between the changes in horizontal gradients of the vertical magnetic field in each eruption source region and the onset time of each eruption. Therefore, magnetic field gradients at collisional PILs play a crucial role in predicting homologous CMEs. This can serve as an important precursor for future space weather forecasting. Furthermore, the study used STEREO-A data to estimate local Dst index values, which could reach nearly -500 nT. This indicates that mesoscale separation in solar storms can lead to considerable differences in the magnetic field and associated geoeffectiveness.

For details of this work, please refer to our full publications, Ref. [4] and [5].

REFERENCES

[1] Liu, Y. D., Zhao, X., Hu, H., Vourlidas, A., & Zhu, B. 2019, ApJS, 241, 15
[2] Liu, Y. D., Luhmann, J. G., Kajdic, P., et al. 2014, Nature Comm., 5, 3481
[3] Wang, R., Liu, Y. D., Yang, S., & Hu, H. 2022, ApJ, 925, 202
[4] Liu, Y. D., Hu, H., Zhao, X., Chen, C., & Wang, R. 2024, ApJL, 974, L8
[5] Wang, R., Liu, Y. D., Zhao, X., & Hu, H. 2024, A&A, 692, A112