The Earth is surrounded by an invisible barrier that was thought to prevent highly energetic electrons from reaching closer to our planet. But in 2024, Japan's Arase space probe detected these electrons well inside the barrier. Now, research led by Nagoya University has found evidence that the electrons were accelerated locally rather than infiltrating from outer radiation belts.
The Earth's magnetic field traps energetic charged particles, namely protons and electrons, in two donut-shaped belts called the Van Allen radiation belts. These particles can potentially damage hardware and electronics in spacecraft flying through the belts.
At the inner edge of the second Van Allen belt, at roughly 3 Earth-radii away from Earth's center, electrons with extremely high energies seem to hit an invisible, impenetrable wall.
First discovered in 2014, the source of this barrier is not completely understood. One possibility is a phenomenon known as "plasmaspheric hiss": low frequency radio waves in the Earth's magnetosphere that produce static noise when played through a speaker. Human-made very-low-frequency radio transmissions, used to communicate with undersea submarines, may also help shape this boundary by scattering radiation-belt electrons into Earth's atmosphere.
Under typical conditions, this region of space is devoid of extremely fast-moving electrons. But space weather can be as precarious as what we experience on the planet.
In May 2024, high solar activity unleashed a geomagnetic "superstorm" on Earth. Night skies in Hokkaido and in places as far south as Hyogo prefecture witnessed aurora borealis or northern lights.
During and after this superstorm, the Japanese radiation space probe Arase , operated by JAXA, collected data from within the impenetrable barrier.
And sure enough, it recorded a large flux of high-energy electrons moving close to the speed of light. Some electrons had energies exceeding three megaelectronvolts — high enough to go through some satellite shields. The so-called impenetrable barrier seemed to have been broken.
The researchers looked for possible explanations.
"One of the key questions was how these highly energetic electrons appeared inside the barrier," said Yoshizumi Miyoshi, a professor at the Institute for Space-Earth Environmental Research (ISEE) at Nagoya University and the lead author of this study. "Were they transported inward from the outer radiation belt, or were they accelerated locally within this region?"
The first clue came from analyzing electron phase space density (PSD), a quantity that describes how electrons are distributed according to their energies and motion. Using measurements collected during multiple Arase orbits, the researchers examined how PSD varied with distance from Earth while accounting for changes in the electrons' motion.
They found that a distinct local peak in PSD developed near the usual barrier boundary and became stronger over time. Such a growing peak provides evidence that electrons were gaining energy locally, rather than merely being transported inward from the outer radiation belt.
But what gave them this boost? Arase also detected intense electromagnetic waves that repeatedly swept upward in frequency.
"When converted into audio, it sounds like a bird song," Miyoshi said. These chirping waves, called "chorus waves," are natural, low-frequency electromagnetic waves that ripple through the Earth's magnetic field. These waves can transfer energy to electrons when their oscillations match the electrons' spiraling motion around magnetic field lines.
Their effect was particularly pronounced in May 2024 because the storm had dramatically eroded Earth's plasmasphere, a region filled with relatively cool charged particles. Its outer boundary retreated unusually close to Earth, leaving the region around the previously impenetrable barrier much less densely populated with plasma. This allowed hotter electrons to reach farther inward and generate chorus waves.
Additionally, the depleted plasma density did not recover quickly. "Typically, the plasmasphere recovers on a timescale of about two days," Miyoshi said. "During the May 2024 superstorm, however, it took approximately 4.3 to 4.7 days for the plasma density to recover to more than half of its pre-storm level in the observed region."
This unusually slow recovery allowed the low-density plasma conditions to persist for several days, potentially providing favorable conditions for chorus-wave activity and the continued acceleration of energetic electrons inside the previously impenetrable barrier.
Miyoshi said these findings, published in the journal Geophysical Research Letters , demonstrate that even regions of near-Earth space previously considered relatively protected from extremely energetic electrons can experience substantial radiation enhancements during severe space weather events.
"This discovery highlights the need to understand and predict how energetic electrons behave under extreme space weather conditions," Miyoshi said. "Such knowledge is important for assessing radiation risks to spacecraft and improving future space weather predictions."
The findings also provide new insights into how the Earth's magnetosphere responds to extreme solar activity, emphasizing the importance of continuous observations of the near-Earth space environment.
