Xudong Sun¹, Aimee Norton², Shin Toriumi³, Peter Schuck⁴, Jie Zhang ⁵
(1) Institute for Astronomy, University of Hawai‘i at Mānoa, Makawao, HI, USA
(2) Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA, USA
(3) Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan
(4) Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
(5) Department of Physics and Astronomy, George Mason University, Fairfax, VA, USA

Summary: NOAA active regions 13664/8, the solar source region responsible for the May 2024 extreme geomagnetic disturbances that caused the largest geomagnetic storm since 2003, broke the record of magnetic flux emergence rate in the SDO era.

Our Sun is entering the maximum phase of activity cycle 25. In May 2024, the NOAA Active Region 13664 and 13668 complex (hereafter AR 13664/8) generated a series of major eruptions, including ten GOES X-class flares (Fig. 1(a)) and 14 major CMEs from May 8 to 14. The three consecutive halo CMEs occurred on May 8, through their interaction in the interplanetary space, produced a complex interplanetary ICME sequence with the presence of extraordinary southward magnetic field (>70 nT), ultimately resulting in the third most intense geomagnetic disturbance in half a century (Fig. 1(b)). The maximum Dst index was -412 nT, only behind the March 1989 Quebec blackout (-589 nT) and the November 2003 Halloween event (-422 nT). Aurorae were reported across the globe, with rare sightings in Florida and Hawai‘i.

Figure 1 | Top: One-minute averaged GOES-18 soft X-ray (SXR) flux (0.1-0.8 nm band). The peak times of 10 GOES X-class flares associated with AR 13664/8 are noted by red ticks. Other X-class flares originated from AR 13663 in the northern hemisphere. The approximate Stonyhurst longitude of the AR centroid is noted at top. Middle: Hourly, real-time Dst index from World Data Center for Geomagnetism, Kyoto. Bottom: Mean unsigned magnetic flux of AR 13664/8 within 3-hour windows. The error bars show three times the standard deviation within the window.

We conducted a preliminary study of AR 13664/8 using photospheric observations from SDO/HMI. In particular, we calculated the unsigned magnetic flux for May 4-11 from the HMI 12-minute-cadence vector data series hmi.sharp_cea_720s_dconS[11149] (Fig. 1(c)). The new data series has stray light removed from the Stokes profiles prior to inversion, which results in sharper images and slightly higher fluxes. The flux started to increase rapidly on May 7, roughly one day before the onset of large eruptions: such a behavior is typical for flare-productive ARs [1]. By the end of May 11, the unsigned flux reached about 1.7×10²³ Mx. We further evaluated the unsigned magnetic flux emergence rate following ref. [2]. The average emergence rate over a 5-day period starting from May 7 00 UT is 6.9×10²⁰ Mx hr^-1. The maximum emergence rate over 6 hr chunks was reached on May 11, at about 2.2×10²¹ Mx hr^-1. Such an extraordinary “instantaneous” flux emergence rate is greater than that of AR 12673, which produced the most intense flare of cycle 24, and AR 12192, the largest sunspot group since 1990 (Fig. 2). It is likely the fastest flux emergence observed in the SDO era.

Figure 2 | Diagram of mean signed magnetic flux vs maximum signed flux emergence rate (half of unsigned values), for various observations and simulations, updated from ref. [2]. Red and blue symbols indicate simulations and observations, respectively. Four ARs during the HMI era are included as stars symbols, which shows the values at the time of the highest instantaneous flux emergence rate. Their multi-day evolutionary tracks are also plotted at 6 hr cadence. Large oscillations in flux emergence rate may be related to the known HMI systematics caused by varying spacecraft velocity. AR 13664/8 has the highest instantaneous emergence rate among them. Here only AR 13664/8 is based on deconvolved data.

The morphology of AR 13664/8 is complex. Below, we describe the photospheric evolutions from May 4 to 8 based on HMI continuum and vector magnetic map sequence (Animation of Fig. 3). These descriptions are by no means comprehensive: rather, they are intended to guide the eyes of the readers. A movie of NOAA 13664/8 flux emergence is available.

• May 4 – 5 (Fig. 3(a)). The bipolar AR 13664 on the western side exhibited moderate flux emergence. The two main magnetic polarities (N1/P1 in Fig. 3(a)) separated quickly. Group of smaller spots of the same polarity started to coalesce. A pair of smaller bipole on the southern side of P1 (N1a/P1a) underwent significant shearing.
• May 5 – 7 (Fig. 3(b)). Two pairs of bipole (N2/P2, N3/P3) emerged synchronously on the eastern side, forming the backbone of NOAA AR 13668. Soon N3 started to collide with P1.
• May 7 (Fig. 3(c)). Another two pairs of bipole (N4/P4, N5/P5) appeared, along with new emergence in N2/P2. These marked the onset of fast magnetic flux emergence. Both N4/P4 and N5/P5 emerged largely north-south oriented with initial tilt angles that were anti-Hale, in stark contrast to the existing components. The N4/P4 pair emerged earlier, was rather elongated, and quickly moved apart. The N5/P5 pair quickly moved westward after birth and showed significant shearing. Meanwhile, apparent collisions occurred in the P5/P1, N3/P1, N4/P5, and N2/P3 pairs. The older spot N1 had moved far west and became isolated; P1a appeared to have merged with P5.
• May 8 – 9 (Fig. 3(d)). Significant flux emergence sustained in N2/P2, N4/P4, and N5/P5. Additional flux emergence appeared near N1a, and for a small bipole N6/P6 to the north. The interaction inside the N3/P1 and N4/P5 pairs became more intense. The interaction between multiple flux components made it difficult to visually track individual bipoles.

Figure 3 | Photospheric evolution of AR 13664/8. Four snapshots are shown for HMI continuum (left) and radial magnetic field (right) in Cylindrical Equal Area projection. The western portion in the first row was denoted as AR 13664 first, and the eastern portion as AR 13668 as it emerged. Pairs of bipoles are marked with N/P plus number for negative and positive polarities, respectively. A movie of NOAA 13664/8 flux emergence is available.

We emphasize that collisional and shearing motions between multiple bipoles were ubiquitous in AR 13664/8. This aspect, coined as “collisional shearing” between non-conjugate polarity pairs, was hypothesized to be as crucial for producing intense eruptive activities as flux emergence [3]. Another interesting feature is the formation of δ-penumbrae: their area significantly increased as the bipole interactions intensified.

In addition to collisions, complex regions can be formed though 1) multi-loop buoyancy [4] and 2) deformation of flux tubes due to convective turbulence or a kink instability [4]. While we can infer (imperfectly) the subsurface geometry of the magnetic fields as the AR emerges, 13664/8 presents a challenge. The tilt angles of the emerging N4/P4 and N5/P5 bipoles are slightly anti-Hale and equal. They emerge within 24 hours of the N2/P2 and N3/P3 bipoles that emerge simultaneously with tilt angles obeying Hale and Joy’s law. How two, multi-loop buoyancy events can happen co-spatially within 24 hours with bipolar tilt angles 180 degrees different from each other is puzzling. The emergence of flux from an O-ring or bubble could explain the observed geometry, with N2/P2 and N3/P3 emerging from the top of the O-ring first then N4/P4 and N5/P5 emerging from the bottom of the O-ring later.

References

[1] Toriumi, S., & Wang, H. 2019, Living Rev. Sol. Phys., 16, 3
[2] Sun, X., & Norton, A. A. 2017, Res. Not. AAS, 1, 24
[3] Chintzoglou, G., Zhang, J., Cheung, M. C. M., & Kazachenko, M. 2019, Astrophys. J., 871, 67
[4] Toriumi, S., Schrijver, C. J., Harra, L. K., Hudson, H., & Nagashima, K. 2017, Astrophys. J., 834, 56
[5] Norton, A. A., Levens, P. J., Knizhnik, K. J., Linton, M. G., & Liu, Y., 2022, Astrophys. J., 938, 117