A Japanese Satellite Caught Killer Electrons Inside Earth’s Radiation Barrier

A zone of near-Earth space long thought to be closed to the most energetic killer electrons was breached during the powerful solar storm of May 2024, according to a new study. A Japanese satellite recorded electrons being accelerated to extreme energies right inside the region, a result that could matter for how spacecraft and astronauts are protected.

The study, led by Yoshizumi Miyoshi of Nagoya University, was published in Geophysical Research Letters, according to both Universe Today and ScienceAlert.

What scientists thought the barrier did

Earth is surrounded by the Van Allen belts, two donut-shaped regions of energetic particles held in place by the planet’s magnetic field. The outer belt is filled mostly with electrons energized by processes driven by the Sun, while the inner belt is made up largely of protons, ScienceAlert reported.

In 2014, NASA’s Van Allen Probes found a surprisingly sharp boundary. Universe Today put it at around 11,500 kilometers above Earth’s surface, and ScienceAlert described it as about three Earth radii from the planet’s center. Beyond that line, the most energetic electrons rarely ventured. Researchers called it an “impenetrable barrier.”

The explanation was that electromagnetic waves in a region of cold plasma around Earth scatter the electrons out of their trapped orbits and send them into the atmosphere. Cracks in the idea had appeared before, ScienceAlert noted, with researchers suggesting as early as 2018 that the barrier might be partly an illusion created by the short duration of geomagnetic storms.

What happened in May 2024

The storm reached level G5, the top category on the geomagnetic storm scale. Universe Today said solar wind gusts topped out at 1,200 kilometers per second and that it was the most intense geomagnetic disturbance since the Halloween storm of 2003.

The storm crushed Earth’s plasmasphere, the cloud of charged gas that normally fills the area, back toward the planet. That left an unusually empty environment in which a type of wave called chorus could thrive. Miyoshi said that when converted into audio, it sounds like a bird song.

Japan’s Arase satellite, which has monitored the radiation belts since 2017, saw the result. During the storm’s recovery phase it detected a sharp rise in ultra-relativistic electrons inside the barrier. At energies around 3 megaelectronvolts, the flux increased by more than 5,000 times compared with levels before the storm, ScienceAlert reported.

The pattern of the increase pointed to local acceleration. Lower-energy electrons appeared first, higher-energy ones followed about half a day later, and electrons above 3 megaelectronvolts surged roughly a day after the initial rise. The electrons also formed a distinct peak near the barrier’s boundary, which is hard to explain if they had simply drifted inward from farther out. The researchers concluded the chorus waves boosted the particles right where Arase detected them.

Why it matters for spacecraft and astronauts

Killer electrons are a known problem in low-Earth space. According to ScienceAlert, they carry enough energy to penetrate satellite shielding, which can damage sensitive electronics and shorten the lifespan of spacecraft. They also pose an elevated radiation hazard to astronauts.

Universe Today described the finding as a warning about a new kind of space weather hazard: localized radiation caused not by a huge solar explosion alone but by a retreat of Earth’s own protective plasma. It suggested that, as humans expand into the region most affected, more shielding could eventually be needed for electronics and for people.

The effect also lasted. Universe Today and ScienceAlert both reported that Arase kept seeing the elevated electrons, which gradually declined, until another solar storm struck in October 2024 and changed the magnetic conditions again.

Miyoshi said, according to ScienceAlert, that the work highlights the need to understand and predict how energetic electrons behave under extreme space weather conditions. The result is based on observations from a single satellite during a single extreme event.

Around Prague, based on reporting by Universe Today, ScienceAlert