“Space is not empty it’s just waiting to mess with your GPS.” That could well be the takeaway from the May 2024 geomagnetic superstorm, when Earth’s magnetic shield and its surrounding plasma environment were squeezed to levels never before recorded. In a first, scientists got continuous, direct measurements that showed the plasmasphere’s outer boundary retreating from its usual altitude of about 44,000 kilometers to just 9,600 kilometers-a contraction to one-fifth its normal size.
Now called the Gannon storm, the event was driven by a series of enormous CMEs ejected in rapid succession from active solar region AR3664. Those CMEs, with speeds to over 2,000 km/s, merged en route to Earth and thus increased both their density and magnetic field strength. The shock compressed the magnetopause to about five Earth radii and yielded a disturbance storm-time index minimum of –406 nT and a Kp index pegged at 9 the strongest geomagnetic activity in more than two decades.
Fortunately, the Arase satellite of the Japan Aerospace Exploration Agency, launched in 2016 to study plasma waves and magnetic fields in the inner magnetosphere, was aptly positioned to observe the plasmasphere’s collapse. The satellite’s in situ measurement of electron density manifested a dramatic contraction of the high-density plasma region within nine hours after the storm’s arrival. This erosion was accompanied by extreme heating in the polar ionosphere, followed by a sharp depletion of oxygen ions-a signature of an ionospheric “negative storm.”
Negative storms occur when storm-driven neutral winds transport molecular-rich air (N₂, O₂) from high latitudes to lower latitudes, lowering the O/N₂ ratio and enhancing the ion recombination. Here, the effect was hemispherically asymmetric: an electron density reduction in the northern hemisphere of as much as 98%, which persisted for over two days, while there were localized enhancements in parts of the southern hemisphere. GOLD satellite observations showed that the composition change spread equatorward, with depleted O/N₂ regions crossing the equator in some sectors.
Whereas the recovery of the plasmasphere, in general, takes up to one or two days, it extended to over four days this time-the longest refilling time since Arase began operations. A statistical study of 77 CME-driven storms since 2017 confirms this recovery time as an extreme outlier and points to negative storms delaying the plasmaspheric replenishment.
The intensity of the storm also displaced auroral activity far toward the equator: enhanced solar wind-magnetosphere coupling and strong auroral electrojets expanded the auroral oval to latitudes as low as Japan, Mexico, and southern Europe. The rapid westward propagation of the intense currents was documented using ground magnetometer data and auroral imaging, while local peaks of GIC above 60 A were measured in parts of the UK transmission network.
From a technological standpoint, the Gannon storm stressed multiple systems. Nearly 5,000 satellites in low-Earth orbit executed autonomous orbit-raising maneuvers against increased atmospheric drag. This made collision avoidance more challenging due to degraded orbit prediction accuracy. In the U.S. Midwest, GPS-based precision agriculture faced guidance errors at the centimeter level during planting season. Estimated losses were above $500 million. Aviation operations also suffered due to impairments to the Wide Area Augmentation System. This temporarily reduced vertical guidance capability at about 2,000 airports. Transoceanic flights were also affected, resulting in high-frequency communications anomalies.
Advanced modeling with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) reproduced many of the observed features, including the super-fountain effect-an intensified equatorial plasma uplift driven by prompt penetration electric fields-and the subsequent suppression of the equatorial ionization anomaly by westward disturbed fields. The simulations emphasized how the interaction of the disturbed electric fields and the changed neutral composition resulted in a negative storm response that was prolonged and asymmetric.
The May 2024 superstorm has become a benchmark in space weather science, allowing an unprecedented level of detail in observation of how extreme solar activity reshapes Earth’s plasma environment. It showed that during such events, the behavior of the plasmasphere is not related just to magnetic compression but also to complex ionospheric chemistry and global thermospheric circulation-processes which directly impact the resilience of satellites, navigation systems, and communication networks.
