Japan's Arase Satellite Found High-Energy Electrons Inside a Barrier Thought to Block Them

Japan's Arase satellite recorded electrons carrying millions of electronvolts deep inside the inner part of Earth's radiation belt during the superstorm of May 2024, in a region that space physicists have long called an "impenetrable barrier" to particles that energetic. Y. Miyoshi of Nagoya University's Institute for Space-Earth Environmental Research and 19 co-authors report the measurement in Geophysical Research Letters.

The inner belt has been treated as closed to electrons at those energies. Miyoshi and colleagues find that it can be breached when space weather turns extreme.

The evidence for where the electrons came from is the shape of the distribution. If they had been driven in from the outer belt, their numbers would thin steadily inward from the edge. Instead, the phase space density profiles, which count how tightly particles are packed at a given energy and orbit, peak inside the barrier. The authors read that peak as acceleration happening on the spot, building a source that then fed particles both inward and outward.

Arase also recorded intense chorus waves, electromagnetic waves that can accelerate electrons, inside a region where the cold, dense plasma had largely drained away. They call that combination consistent with acceleration driven by the chorus waves.

The measurements come from one storm, the May 2024 event, and from Arase's continuous observations near the magnetic equator. The paper was published online Oct. 26, 2026, and is open access under a Creative Commons license.