On the features of great Forbush effect during May 2024 extreme geomagnetic storm

Abstract: The work investigates the features of galactic cosmic ray density and anisotropy behavior and their relation to solar sources, interplanetary and geomagnetic disturbances from May 8 to May 13, 2024. During this time, powerful solar flares and fast CMEs were recorded, leading to registration of an extreme geomagnetic storm along with one of the most significant Forbush effects for the entire observation period. All the calculations of cosmic ray characteristics are made using the data of global neutron monitor network and unique methods maintained at IZMIRAN: the Global Survey Method and the Ring of Stations Method. It is determined that the magnitude of Forbush effect under study was 15.7% (for particles with 10 GV rigidity) and as an extreme geomagnetic storm was recorded there was a significant magnetospheric effect observed in the data of neutron monitors (~4%).

• The paper presents an in-depth analysis of a record Forbush decrease with a 15.7% amplitude and a 2-3 hour delay in cosmic ray response post-SSC.
• It employs GSM and RSM methods with global neutron monitor data to quantify GCR density, anisotropy, and magnetospheric effects from multiple fast CMEs and X-class flares.
• Findings constrain solar-terrestrial coupling models and enhance space weather forecasting and radiation risk assessment.

Extreme Forbush Decrease and Geomagnetic Effects during the May 2024 Solar Event

Introduction

The paper "On the features of great Forbush effect during May 2024 extreme geomagnetic storm" (2501.08029) conducts an exhaustive analysis of the unprecedented galactic cosmic ray (GCR) modulation and associated heliospheric and geomagnetic phenomena observed from May 8 to May 13, 2024. This period was marked by an exceptional cluster of solar flares and coronal mass ejections (CMEs), leading to an extreme geomagnetic storm with near-record parameters and a simultaneous Forbush decrease (FD) of notable amplitude. Through comprehensive exploitation of global neutron monitor (NM) data and advanced analytical techniques—namely, the Global Survey Method (GSM) and Ring of Stations Method (RSM)—the work delivers detailed characterizations of GCR density, anisotropy, and their relationship to the heliospheric drivers and geomagnetic activity.

Solar, Interplanetary, and Geomagnetic Context

The discussed episode originated from active region AR13664, which produced 10 X-class and 48 M-class flares between May 2 and 15, 2024. Multiple fast halo and partial-halo CMEs, several exceeding 1000 km/s linear speed, interacted and compounded in interplanetary space. Wind spacecraft data revealed successive shocks, with solar wind (SW) parameters reaching extrema: $V_{SW} > 900$ km/s, IMF magnitude $|B|$ peaking at 69.8 nT—unprecedented in modern records—and plasma density peaking at 48.1 particles/cm³.

Geomagnetic indices responded acutely. Sudden storm commencement (SSC) occurred at 17:05 UT on May 10. The 3-hour Kp index attained 9, while Dst index plummeted to -412 nT (May 11), situating this storm among the most intense since systematic monitoring began. The Ap-index reached 271. The compounding effects of successive interplanetary drivers yielded a prolonged state of heightened disturbance.

Cosmic Ray Variations and Forbush Decrease Analysis

Applying the GSM to data from the worldwide NM network, the authors quantify the FD for GCRs with $\sim$10 GV rigidity. The primary FD (FE #1) commenced at the time of SSC, with the following characteristics:

• FD amplitude: 15.7%, the most prominent since the 2003 Halloween storms, and among the deepest FDs since 1957.
• Min. hourly decrement: $-4.4\%$, exceeded on record only by the 1991 and 2003 events.
• Min. two-hour decrement: $-8.8\%$.

A salient point is that the initial CR decline was delayed 2-3 hours post-SSC, identifying a rare lag vis-à-vis typical FD profiles. The RSM identified precursor signatures approximately 7 hours prior to the event. The absolute CR minimum may have been deeper had the Earth's position been central to the affected region.

The GCR density did not reach pre-event values for days, in part due to subsequent interplanetary perturbations. Furthermore, during the FD's main phase, the GLE #74 was registered, ascribed to the May 11 X5.8 flare. The combination of deep FD and GLE is rare, previously seen only in exceptional historical events.

GCR Anisotropy

The event exhibited anomalously low GCR anisotropy. Maximum equatorial anisotropy ($A_{xy}$) was 1.9% (minimum across all $>10\%$ FDs in the FEID archive), with mean $A_{xy}$ of only 0.9%. North-south anisotropy variation (Az) was also reduced. This suggests that the Earth's position relative to the ICME and its magnetic structure was offset, resulting in the minimum CR depression not coinciding with Earth's longitude.

Magnetospheric Effects

A significant ground-based NM “magnetospheric effect” (as described in prior literature) was observed, contributing a decrease of up to ~4% in CR counts attributable to geomagnetic cutoff variation during the intense storm. This effect accentuates the need to correct raw NM counts in extreme events for reliable heliospheric diagnostics.

Magnetic Cloud Structure

GCR anisotropy variations, including abrupt changes in the direction of equatorial and north-south components, are interpreted as evidence for the traversal of an interplanetary magnetic cloud (MC) between 11:00 and 17:00 UT on May 11. This is consistent with select SW data segments, even as the MC identification in catalogues remains ambiguous for this event.

Theoretical and Practical Implications

The quantification and interpretation of the May 2024 FD within the context of an extreme geomagnetic storm extends the empirical basis for understanding the solar modulation of high-energy CRs. The small anisotropy parameters, rarely seen in deep FDs, underline the complex spatiotemporal structure and orientation of interacting ICMEs with respect to Earth. This event highlights the intricate coupling among solar eruptive activity, heliospheric transport, and terrestrial response, and demonstrates the sensitivity of GCR parameters—especially as measured through harmonized global NM networks and advanced inversion methods—to interplanetary structures.

The occurrence of a GLE coincident with the deepest phase of the FD further enables cross-calibration of CR and SEP transport models, with implications for space weather forecasting and radiation risk assessment.

Methodologically, the work validates the GSM and RSM as robust, high-temporal-resolution approaches for diagnosing global CR variation and anisotropy, resolving both the response to and the propagation properties of interplanetary disturbances. The evidence for significant magnetospheric modulation during extreme Dst excursions mandates careful consideration in interpreting ground CR data under similar conditions.

Prospects for Future Research

The 2024 event constitutes a benchmark for multi-messenger studies of solar-terrestrial coupling. Future efforts should focus on:

• High-fidelity modeling of FD and GLE superposition during complex CME interactions.
• Disambiguation of MC substructure signatures in CR anisotropy, particularly under non-central crossing geometries.
• Systematic evaluation of NM magnetospheric corrections for extreme storms.
• Data assimilation frameworks integrating NM-derived density and anisotropy with heliospheric and magnetospheric models for predictive capabilities.

Further statistical analysis of the increasing FD + GLE coincidences in upcoming solar cycles will also advance the understanding of source acceleration conditions and CR propagation in highly disturbed heliospheres.

Conclusion

This study provides a thorough and technically rigorous dissection of the May 2024 solar-terrestrial event, documenting an FD with one of the largest amplitudes recorded in the modern era, alongside rare features such as low GCR anisotropy and coincident GLE. The results reinforce the critical interplay between evolving solar disturbances, interplanetary propagation, and Earth's magnetospheric response, offering significant constraints for models of GCR modulation and providing key insights for space weather science and operational forecasting.