2024 solar storm triggered ultrarelativistic electrons inside Earth's 'impenetrable barrier'

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 from Earth's center, electrons with extremely high energies seem to hit an invisible, impenetrable wall. This barrier was first discovered in 2014, and its source 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. The aurora borealis, or northern lights, appeared in night skies in Hokkaido and in places as far south as Hyogo prefecture. 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.