Arase detects electron acceleration in the inner Van Allen belt — ScienceAlert

Japan’s Arase satellite recorded electrons accelerating to megaelectronvolt energies inside Earth’s inner radiation belt during a powerful geomagnetic storm in May 2024. This region had previously been considered a natural barrier that prevents extremely energetic electrons from penetrating closer to the planet, ScienceAlert reports.

Observations during the storm

The study by a team led by space physicist Yoshizumi Miyoshi of Nagoya University was published in the journal Geophysical Research Letters. The scientists analyzed measurements from Arase, which has been observing Earth’s radiation belts since 2017.

During the recovery phase after the storm, the spacecraft recorded a sharp increase in the number of ultrarelativistic electrons inside the previously defined boundary. For particles with energies of about 3 megaelectronvolts, the flux was more than 5,000 times higher than before the storm.

At first, the number of lower-energy electrons increased. About half a day later, the number of higher-energy particles rose, and after about a day, the flux of electrons with energies above 3 megaelectronvolts increased. According to the researchers, this sequence indicates that the particles gained energy directly in this zone rather than merely moving there from other regions.

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Conditions inside the barrier

In 2014, observations by NASA’s Van Allen Probes mission identified a sharp boundary at about three Earth radii from the planet’s center, beyond which the most energetic electrons almost never penetrated. It was explained by the action of electromagnetic waves in cold plasma, which scattered the electrons and directed them into the atmosphere.

The G5-level geomagnetic storm in May 2024 significantly reduced the amount of cold plasma in Earth’s plasmasphere. This created a rarefied environment in which chorus-type electromagnetic waves could propagate. According to the study’s authors, these waves may have accelerated electrons inside the previously defined boundary.

After the storm, Arase continued to record a gradual decline in ultrarelativistic electron levels for several months. Observations continued until October 2024, when another solar storm occurred near Earth.