Editorial NexoScience
Earth’s radiation belts have looked the same in textbooks since 1958, when a Geiger counter aboard Explorer 1, the first American satellite, ran into a wall of trapped particles nobody had predicted. The picture that came out of that flight was two doughnuts of radiation wrapped around the planet, an inner one and an outer one, with a mostly empty lane in between. That lane didn’t stay empty after the solar superstorm of May 2024.
A satellite no bigger than a shoebox found two belts that shouldn’t have been there. REPTile-2 (Relativistic Electron and Proton Telescope integrated little experiment-2), the only science instrument aboard NASA’s CIRBE (Colorado Inner Radiation Belt Experiment) CubeSat, picked up a belt of very fast electrons sitting inside the supposedly empty slot region, roughly 7,900 miles (12,700 kilometers) above the surface, higher than the crowded low orbits where most satellites work and lower than the ring where GPS flies. Closer in, it found a second belt made of protons. Both showed up after the geomagnetic superstorm of May 10, 2024, the most violent in two decades, and the team reported them Feb. 6, 2025, in the Journal of Geophysical Research: Space Physics.
The belt only revealed itself because the instrument could tell electron energies apart. Other spacecraft had registered a smear of energetic particles in the slot region after the storm, but their detectors lumped everything above a threshold into one running count. REPTile-2 splits the same range into dozens of separate energy channels, and that resolution changed the reading entirely: electrons below 1.3 million electron volts, a measure of how much punch each particle carries, had been swept out of the region, while everything from there up to about 5 million electron volts was still trapped and circling. “The new electron belt could not have been correctly identified from such integral flux measurements alone,” wrote the team, led by Xinlin Li, a space physicist at the University of Colorado Boulder’s Laboratory for Atmospheric and Space Physics.
A faint hiss in Earth’s plasma does the sweeping. The slot region normally stays clear thanks to plasmaspheric hiss, a constant low-level static of radio waves inside the cloud of cold plasma that surrounds the planet. Those waves nudge trapped electrons a little at a time until the path they bounce along dips into the upper atmosphere, where they’re absorbed. The nudging is tuned to energy, and that’s the whole story of the new belt: by the study’s calculations, an electron of about one million electron volts survives only a few days in that region, while the faster ones in the new belt can hang on for months. The hiss works less like a broom than like a sieve, and the new belt is what the sieve left behind.
The proton belt looks even more durable than the electron one. Protons carrying between roughly 7 and 15 million electron volts jumped by more than a factor of ten over their pre-storm levels, a spike sharp enough to count as a new structure rather than a busier version of the old inner belt. Protons down there don’t lose energy to plasma waves. They lose it by colliding with electrons and atoms in the thin gas at the top of the atmosphere, a slow grind that usually plays out over about a year, and the authors expect this belt to last that long or longer. A second, far weaker storm on June 28, 2024, knocked a good part of the new electron belt down and left the protons untouched.
The spacecraft nearly missed the whole event. CIRBE had been working well for a year when it suffered an anomaly and fell silent a few weeks before the superstorm, and it didn’t return to normal science mode until roughly a month after. By the time the instrument opened its eyes again, the belts were already built. That gap is why the study can describe in fine detail what the storm left behind but not the moment of assembly.
The new belts matter most for spacecraft on their way up. Satellites bound for geostationary orbit are often dropped off in a lower transfer orbit and then creep outward on electric thrusters, a climb that can take many months and passes straight through the region where the new belts formed. More trapped protons mean more damage to solar cells, and more trapped electrons mean a higher total radiation dose for everything else on board.
The authors flagged several limits on what they can claim. CIRBE flies in a low orbit, so it only sampled particles whose bounce path dips down to its altitude, and the researchers wrote that they measured only the “tip of the iceberg” of belts that mostly sit higher up. The connection between the May storm and the belts rests on a before-and-after comparison with the instrument dark in between, so it’s an association in time rather than a formation caught in the act, and the paper says plainly that the physical processes that built the belts remain to be investigated. The electron lifetimes that explain the belt’s sharp lower edge come from a wave model, not from direct measurements of the waves in that region at that moment. And the risk to hardware isn’t quantified yet: further calculations are needed to pin down the real effect on spacecraft, the authors noted.
The next superstorm is the experiment nobody can schedule. The June storm that erased much of the electron belt hands the team a natural test of which waves scatter the most energetic particles, work the paper leaves for later. The proton belt is the slower clock, and the more interesting one: if it behaves the way the collision physics says it should, it’s still up there, thinning out month by month, waiting for the next big storm to either top it up or take it apart.
Source: Xinlin Li, Zheng Xiang, Yang Mei et al. “A New Electron and Proton Radiation Belt Identified by CIRBE/REPTile-2 Measurements After the Magnetic Super Storm of 10 May 2024.” Journal of Geophysical Research: Space Physics, volume 130, issue 2, published Feb. 6, 2025 (open access, CC BY). DOI: https://doi.org/10.1029/2024JA033504
