A barrier long thought to protect Earth's inner radiation zone from the most dangerous electrons was breached from within, according to a new paper in Geophysical Research Letters using data from Japan's Arase satellite during the May 2024 solar storm.
The study, by Yoshizumi Miyoshi of Nagoya University and co-authors, suggests that during the storm, nearby electrons may have been accelerated to relativistic speeds inside the region where ultra-relativistic “killer electrons” had previously been held at bay.
The findings concern Earth's Van Allen radiation belts, two regions of charged particles held by the planet's magnetic field. The inner belt is made mainly of protons, while the outer belt contains high-energy electrons. In 2014, NASA's Van Allen Probe mission discovered a boundary at about 11,500 kilometres above Earth's surface that "killer electrons" seemed unable to cross. Arase, also known as the ERG mission, has flown through Earth's magnetosphere since 2017 and, until May 2024, observed the barrier holding.
During that storm, solar wind gusts topping 1,200 kilometres per second compressed Earth's magnetic field and triggered the most intense geomagnetic disturbance since the Halloween storm of 2003. At the storm's peak, Arase counted ultra-relativistic electrons at levels 5,000 times higher than the typical background. The researchers used a mathematical method called electron phase space density, which tracks particle distribution by energy and motion. If electrons had come in from outside the barrier, the metric would show a downward trend toward Earth. Instead, Arase saw a distinct peak right where the barrier used to be, a result described as a “smoking gun” for local acceleration.
The paper's explanation is a “chorus wave”. Normally, the inner radiation belt is covered by a shroud of charged gas known as the plasmasphere, which dampens high-frequency electromagnetic waves by absorbing their energy. The May 2024 storm crushed the plasmasphere toward Earth, and a “negative ionospheric storm” further depleted local plasma densities, creating an empty electromagnetic environment. In those conditions, low-frequency chorus waves generated by lower-energy particles were able to transfer their energy efficiently to surrounding electrons, boosting them from sub-million electron volt energies to multi-MeV ranges. The new radiation then spread both toward and away from Earth. Its effects were visible to Arase for months, until another solar storm hit five months later, in October 2024, and changed the magnetic field situation again.
The article describes the result as a warning about a new type of space weather hazard: localised radiation caused not by a massive solar explosion but by the retreat of Earth's own protective plasma sheath. It says expanding into the affected region might require more shielding to protect sensitive electronic and biological cargo.
Analysis
Why This Matters
- The event represents a new type of space weather hazard, where dangerous radiation is generated locally when Earth's plasma sheath retreats, rather than arriving directly from the Sun.
- Satellites and future crewed missions operating in the inner radiation zone may need stronger shielding against multi-MeV electrons that can persist for months after a storm.
- The May 2024 storm, already known for auroras seen as far south as Florida and Mexico, continues to reshape scientific understanding of the near-Earth radiation environment.
Background
Earth's radiation environment is dominated by the Van Allen belts, discovered early in the Space Age by Explorer 1. The inner belt is mainly protons, while the outer belt is filled with high-energy electrons. In 2014, NASA's Van Allen Probe mission found that ultra-relativistic "killer electrons" appeared to stop at a boundary roughly 11,500 kilometres above Earth, acting as a shield for the area closest to the planet. Japan's Arase satellite (the ERG mission) has been flying through the magnetosphere since 2017, providing the continuous observations needed to catch the May 2024 breach.
What to Watch
- Whether a future major solar storm produces the same effect inside the barrier, which would confirm how repeatable the mechanism is, with Arase continuing to monitor the magnetosphere.
- The state of the plasmasphere after the October 2024 storm, and how long the locally generated radiation takes to clear.
- Whether spacecraft shielding designs change in response to the finding that multi-MeV electrons can be created inside the previously protected zone.
Checked
12 claims in this story checked against 1 source: 10 supported.
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- SupportedEarth’s radiation shield was breached from within during the May 2024 solar storm.Universe Today: “Earth’s “Impenetrable” Space Radiation Shield Was Just Breached From Within.”
- SupportedThe study suggests nearby electrons accelerated to relativistic speeds using chorus waves.Universe Today: “a new paper published in Geophysical Research Letters by Yoshizumi Miyoshi of Nagoya University and his co-authors shows it might have allowed some nearby electrons to accelerate to relativistic speeds using something called a chorus wave.”
- SupportedArase data point to electrons being boosted to multi-MeV energies inside what is normally an exclusion zone.Universe Today: “In a depleted plasma environment, these waves are able to transfer their energy very efficiently to the surrounding low energy electrons, boosting them from sub-million electron volt energies to multi-MeV ranges - essentially creating high energy electron radiation within what is normally an exclusion zone.”
- Partly supportedThe Van Allen belts are charged-particle regions held by Earth’s magnetic field.Universe Today: “It’s dominated by what scientists call the Van Allen belts. These were discovered early on in the Space Age by Explorer 1, and they are Earth’s primary defense against high energy electrons and other ions spewed out by the Sun.”
- SupportedIn 2014, NASA’s Van Allen Probe mission found a boundary about 11,500 kilometres above Earth’s surface that killer electrons seemed unable to cross.Universe Today: “in 2014, NASA’s Van Allen Probe mission discovered a noticeable boundary - the ultra-relativistic “killer electrons” that move near the speed of light seem to hit some sort of glass wall at around 11,500 kilometers above the Earth’s surface.”
- SupportedArase had observed high-energy electrons being held at bay from 2017 until May 2024.Universe Today: “Japan’s Arase satellite ... has been flying through Earth’s magnetosphere since 2017, was watching. For its entire lifetime up to that point, it noted that high energy electrons were being held at bay. Until May 2024.”
- Partly supportedSolar-wind gusts during the storm topped 1,200 kilometres per second.Universe Today: “During that storm, solar wind gusts topping out at 1,200 kilometers per second compressed Earth’s magnetic field”
- SupportedThe storm triggered the most intense geomagnetic disturbance since the Halloween storm of 2003.Universe Today: “and triggered the most intense geomagnetic disturbance since the arguably even more famous Halloween storm of 2003.”
- SupportedAt the storm’s peak, Arase counted ultra-relativistic electrons at 5,000 times the typical background level.Universe Today: “As the storm peaked, Arase counted ultra-relativistic electrons at levels 5,000 times higher than the typical background.”
- SupportedThe electron phase space density method tracks particle distribution by energy and motion, and Arase’s peak indicated local acceleration.Universe Today: “This tracks the particle distribution of those electrons by energy and motion. ... However, Arase saw a very distinct peak right where the barrier used to be. To space physics, that represents a smoking gun for “local” acceleration”
- SupportedA depleted plasma environment let chorus waves efficiently transfer energy to lower-energy electrons, boosting them to multi-MeV ranges.Universe Today: “In a depleted plasma environment, these waves are able to transfer their energy very efficiently to the surrounding low energy electrons, boosting them from sub-million electron volt energies to multi-MeV ranges”
- SupportedArase saw the effects for months, until another solar storm hit five months later in October 2024 and changed the magnetic-field situation.Universe Today: “Over time this new high energy radiation spread out both towards and away from Earth - and the effect lasted for months. Its effects were visible to Arase until another solar storm hit five months later in October 2024, and changed the magnetic field situation up again.”