May 2024 G5 – Extreme geomagnetic storm caused record ionospheric depletion

The May 2024 G5 – Extreme geomagnetic storm is now classified as one of the most powerful in modern records. For three days, disturbances driven by multiple coronal mass ejections hammered Earth’s magnetic field, pushing activity to the highest NOAA G-scale level.

It was the strongest geomagnetic storm since the Halloween events of October 2003, which also reached G5 intensity and caused widespread power grid failures and satellite anomalies. In terms of geomagnetic indices, the May 2024 storm reached a Kp of 9, the maximum possible value, and recorded a disturbance storm time (Dst) index of about –412 nanoteslas. Such levels place it firmly among the most severe storms ever measured, rivaled only by a handful of modern events and surpassed only by the legendary Carrington Event of 1859.

But unlike the 1989 Québec storm that shut down electricity grids, or the 2003 storms that disabled satellites and disrupted GPS, the May 2024 superstorm left its most visible mark on the ionosphere.

This high-altitude plasma region, extending from roughly 60 km (37 miles) to over 1 000 km (620 miles) above Earth’s surface, acts as the planet’s natural mirror for radio waves. During the storm, however, electron densities across the Northern Hemisphere collapsed, leaving HF radio signals with nothing to reflect against.

The collapse was unprecedented in both scale and speed. Over East Asia, total electron content fell by more than 100 TEC units, a near-complete depletion. Stations in the Chinese Meridian Project (CMP) network, including CSSL, SAYA, GLDC, and PUJI, reported that ionosonde echoes disappeared for hours. This meant that shortwave frequencies between 3 and 30 MHz, normally used for transcontinental communication, aviation, and maritime operations, simply passed into space.

The sudden silence on HF bands offered a stark reminder of how dependent modern systems remain on this century-old technology. Even with satellites and fiber optics, HF radio remains a backbone for polar aviation, emergency backup, and naval operations. Its collapse during a G5 storm highlights a different vulnerability than the grid failures or satellite damage often feared in space weather scenarios.

While the Northern Hemisphere ionosphere was collapsing, the Southern Hemisphere’s mid- to low-latitude regions showed significant electron density enhancements. This dramatic asymmetry, depletion in the north but strengthening in the south, had rarely been observed in past storms.

Multi-instrument observations combined with TIEGCM simulations revealed the mechanisms. Summer-to-winter neutral winds and differences in oxygen-to-nitrogen ratios between hemispheres played key roles. Disturbances in neutral composition propagated from high to low latitudes, while a westward electric field created by overshielding penetration and disturbance dynamo processes deepened the collapse in equatorial regions.

The study confirms that critically low ionospheric electron density prevents effective reflection of HF radio waves. Instead of bouncing back, signals escaped into space. As a result, HF communications, still vital for aviation, maritime navigation, and military operations, suffered interruptions during the superstorm.

Unlike the March 1989 storm that crippled Québec’s power grid, the 2024 event did not cause widespread blackouts. Its most visible impact was the collapse of global HF radio. The contrast highlights how storms of similar intensity can damage entirely different systems, depending on the physical drivers.

The unprecedented depletion poses a challenge to existing storm models. Accurately predicting extreme-event blackouts now requires incorporating hemispheric asymmetry into models. The authors emphasize that future work must refine thresholds for depletion, quantify latitudinal dependencies, and improve how coupled magnetosphere–ionosphere models handle asymmetries.

The findings provide new benchmarks for space weather research. For forecasters, they show the need to integrate observations from networks like CMP with advanced modeling. The lessons are crucial, since aviation, shipping, military and emergency systems remain highly vulnerable to HF radio disruption.

The Mother’s Day storm of 2024 now joins the Carrington Event of 1859 and the March 1989 storm as one of the great case studies in space weather. While it did not collapse power grids, it left behind the most extreme ionospheric depletion ever recorded.

The storm is a reminder that Earth’s upper atmosphere can be radically restructured in hours, and that the risks posed by geomagnetic storms extend far beyond electricity grids.

References:

¹ Chinese Meridian Project reveals: Storm-time ionosphere collapse disrupts HF radio propagation – EurekAlert! – August 21, 2025

² The extreme depletion of ionospheric electron density and its hemispheric asymmetry during the May 2024 Storm – Yanhong Chen et al. – National Science Review – August 1, 2025 – 10.1093/nsr/nwaf307 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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