The May 2024 Superstorm Lit Up Auroras Over Half the Planet. Japan's Arase Satellite Watched It Gut Earth's Outer Radiation Belt, and Physicists Have Now Reconstructed How.

In May 2024, the Sun threw the most intense geomagnetic storm at Earth since the Halloween storms of October 2003. Auroras were photographed from Florida to Mexico and across the Southern Hemisphere, GPS-guided tractors stalled in the American Midwest, and, unseen from the ground, the outer Van Allen radiation belt abruptly lost a large fraction of its most energetic electrons. A study published this month in AGU Advances by Xingzhi Lyu and colleagues delivers the first comprehensive account of how that dropout happened, and it finds that the standard models used for space weather forecasting cannot reproduce it.

The Van Allen belts are two doughnut-shaped zones of charged particles, mostly electrons and protons captured from the solar wind, held in place by Earth's magnetic field. The outer belt, which stretches from roughly 13,000 to 60,000 kilometers above the surface, is where geostationary communications satellites and GPS constellations live. Its population of "killer electrons," moving at close to the speed of light, can bury themselves in spacecraft electronics and cause charging failures. During big storms the belt can swell, shrink or empty out within hours, and predicting which will happen is one of the central problems of space weather.

Lyu's team used data from the Japan Aerospace Exploration Agency's Arase satellite, which flies an elliptical orbit through the belts, to quantify how much of the outer belt was lost during the superstorm. They then ran the event through the Versatile Electron Radiation Belt model, known as VERB, a simulation designed to reproduce belt dynamics from first principles. The simulations pointed to two well-studied processes. The first is magnetopause shadowing: when the solar wind's pressure compresses the sunward side of the magnetosphere, the outer edge of the belt is suddenly outside the magnetic boundary, and electrons on those orbits simply drift off into interplanetary space. The second is local wave scattering, in which plasma waves inside the magnetosphere knock electrons onto trajectories that dive into the upper atmosphere, where they are absorbed.

The crucial finding was about timing. The simulations only matched Arase's measurements when the outward transport from magnetopause shadowing began almost simultaneously with the strong compression of the magnetosphere that occurred during the storm's main phase. Wave scattering then finished the job, sweeping remaining electrons inward. Which process dominated varied from one region of the magnetic field to another. Treat the two as separate, or offset them by even a modest interval, and the model fails to produce the extreme loss that actually occurred.

That is a problem for forecasters, because the electron transport models commonly used in operational space weather prediction do not capture this tight coupling between storm dynamics and particle loss. The authors argue their results can guide the development of better models, which matters more with every launch: the number of active satellites has roughly quadrupled since 2019, and the next solar maximum will not wait for the software to catch up.

The May 2024 event was a rare natural experiment, and its lesson is that the belt's response to an extreme storm is not simply a bigger version of its response to an ordinary one. The sequence and synchronization of events matter as much as their strength.