A Pileup of Coronal Mass Ejections Produced the Largest Geomagnetic Storm in Two Decades

Abstract: The largest geomagnetic storm in two decades occurred in 2024 May with a minimum $D_{\rm st}$ of $-412$ nT. We examine its solar and interplanetary origins by combining multipoint imaging and in situ observations. The source active region, NOAA AR 13664, exhibited extraordinary activity and produced successive halo eruptions, which were responsible for two complex ejecta observed at the Earth. In situ measurements from STEREO A, which was $12.6^{\circ}$ apart, allow us to compare the geo-effectiveness" at the Earth and STEREO A. We obtain key findings concerning the formation of solar superstorms and how mesoscale variations of coronal mass ejections affect geo-effectiveness: (1) the 2024 May storm supports the hypothesis that solar superstorms areperfect storms" in nature, i.e., a combination of circumstances resulting in an event of an unusual magnitude; (2) the first complex ejecta, which caused the geomagnetic superstorm, shows considerable differences in the magnetic field and associated ``geo-effectiveness" between the Earth and STEREO A, despite a mesoscale separation; and (3) two contrasting cases of complex ejecta are found in terms of the geo-effectiveness at the Earth, which is largely due to different magnetic field configurations within the same active region.

• The paper demonstrates that a series of CMEs from NOAA AR 13664 piled up to generate an extreme ejecta with a Dst minimum of -412 nT, marking the largest geomagnetic storm in two decades.
• It employs multi-point observations and the GCS forward model to reconstruct CME kinematics and quantify mesoscale magnetic variations that critically affect geoeffectiveness.
• The study reinforces the 'perfect storm' hypothesis by showing how CME-CME interactions and active region dynamics synergize to amplify storm intensity, presenting challenges for current forecasting models.

CME Pileup and the May 2024 Geomagnetic Superstorm: Analysis and Implications

Introduction

The work "A Pileup of Coronal Mass Ejections Produced the Largest Geomagnetic Storm in Two Decades" (2409.11492) provides a comprehensive assessment of the largest geomagnetic storm since 2003, which reached a minimum $D_{\rm st}$ of $-412$ nT in May 2024. By leveraging extensive multi-point observations and advanced modeling, the paper characterizes the solar and interplanetary precursors, revealing how a complex interaction of several sequential coronal mass ejections (CMEs) from the highly active region NOAA AR 13664 produced an exceptionally geoeffective complex ejecta at Earth. The study further exploits the mesoscale spacecraft separation to quantify variations in magnetic structure and geoeffectiveness, offering critical insights into the nature of solar superstorms.

Solar and Interplanetary Precursors

NOAA AR 13664, itself a merger of two large active regions, exhibited persistent eruptive activity, generating multiple X- and M-class flares between May 8 and May 13. The study identifies eight full halo CMEs originating from this region, and through combined SOHO/LASCO and STEREO A imaging alongside the GCS forward model, reconstructs their kinematics and geometry. Importantly, the CMEs were not especially extreme in initial velocity (<2000 km s$^{-1}$), but appeared in two tightly clustered groups (CMEs 1–4 and 5–7) with temporal and spatial proximity conducive to CME-CME interaction during propagation.

Time-elongation plots reveal clear evidence of merging CME fronts—a signature of pileup and in-transit interaction—characterized by the formation of two large, complex ejecta as detected in situ at 1 AU.

In Situ Properties and Mesoscale Magnetic Structure Variation

Wind in situ observations unambiguously demonstrate two complex ejecta, each bounded by forward shocks. The first ejecta, formed from the merging of CMEs 1–4, exhibited extraordinary magnetic amplification: peak field strength $\sim$72 nT and maximum southward $B_z$ of 59 nT. These properties are far above typical ICME values at 1 AU and provided the necessary precondition for an extreme geomagnetic response. The second ejecta, attributed to the interaction of CMEs 5–7, was less geoeffective, displaying weaker southward components and minimal impact on $D_{\rm st}$.

Key results are obtained via comparison with STEREO A, which, at only $12.6^\circ$ longitudinal offset from Earth, observed both earlier (by 2.6 hours) and stronger ejecta fields ($D_{\rm st, min} \approx -494$ nT by simulation) compared to Earth ($D_{\rm st, min} \approx -378$ nT by the same model). Notably, significant differences appeared in the internal magnetic vector components on this mesoscale separation, indicating that local geoeffectiveness can be highly sensitive to small changes in impact parameter even for the largest solar wind structures.

Mechanisms of Superstorm Formation: The "Perfect Storm" Hypothesis

Analysis of the event confirms the synthesis of multiple reinforcing effects necessary for superstorm formation, in line with the "perfect storm" framework previously articulated in [Liu et al. 2014, Liu et al. 2019]:

• Prolonged, high-cadence CME production from a single active region: Maximizes probability of in-transit CME pileup.
• In-transit CME-CME interaction: Preserves and amplifies ejecta magnetic field strength via compression and inhibition of expansion, yielding exceptionally strong fields at 1 AU.
• Preconditioning: Preceding ejecta clear solar wind obstacles, reducing deceleration and enhancing subsequent CME impact.

The exceptional geoeffectiveness—despite moderate initial CME velocities—underscores the necessity of considering complex CME interaction sequences and active region field topology, rather than single eruption parameters alone, in extreme space weather risk assessments.

Theoretical and Practical Implications

Geomagnetic Storm Forecasting

The pronounced variation in ejecta field structure and simulated $D_{\rm st}$ at Earth and STEREO A highlights a profound forecasting challenge: even with multipoint upstream sampling, accurate prediction of southward field strength and subsequent ring current enhancement is highly nontrivial for complex ejecta. The results support prior findings that mesoscale differences (on the order of $10^\circ$ in longitude) result in substantial uncertainty in geoeffectiveness estimation, particularly if only off-axis or off-point measurements are available.

Space Weather Hazard Assessment

The event demonstrates that solar superstorms—regardless of initial CME speed—can arise through convergent factors. This has major implications for risk estimation of Carrington-class and lesser but still damaging events. Reliance on CME catalogs that focus only on isolated fast events is insufficient; comprehensive models must integrate history, topology, and interaction dynamics of active regions and their ejecta chains.

Implications for Theoretical CME Modeling

The distinct contrast between two successive complex ejecta (with only the first causing extreme geoeffectiveness) further demonstrates the crucial role played by the source region magnetic field distribution, eruption clustering, and interplanetary interaction geometry. These findings necessitate improvements in MHD modeling of CME-CME interactions, flux-rope internal structure evolution, and shock-ejecta coupling, especially given that the strongest response was not associated with the highest initial velocity nor an isolated CME.

Future Prospects

The increasing availability of multi-vantage-point measurements (e.g., STEREO, Solar Orbiter) enables more robust characterizations of CME mesoscale and substructure. Further, these results demand integration of time-dependent, 3D CME interaction models with empirical field reconstructions for operational space weather prediction. Improved active region magnetograms and connectivity diagnostics, perhaps augmented by machine learning classification systems, will be needed to capture precursor and interaction cues essential for probabilistic forecasting of superstorms.

Conclusion

The analysis of the May 2024 superstorm establishes critical new empirical benchmarks for understanding CME pileup and space weather extremes. The findings robustly support the “perfect storm” scenario, with interacting CMEs from a prolific active region yielding the most intense geomagnetic storm since 2003. Strong mesoscale variations in ejecta magnetic structure and geoeffectiveness highlight ongoing challenges for storm prediction. Theoretical models and operational systems must increasingly account for complex CME interaction dynamics and active region temporal evolution to advance forecasting and mitigate hazards from extreme solar activity.