Arase observations from the May 2024 superstorm showed a growing local peak in the distribution of multi-MeV electrons near L*≈3. In a study published October 26, 2026, researchers interpreted that peak as evidence that electrons were accelerated in place before later spreading through the radiation belts. The study focused on a region usually described as the inner Van Allen-belt barrier.
The clue in the electron distribution
Phase-space density describes how particles are distributed across position and momentum. The study’s authors report that this measure developed a growing local maximum near L*≈3 in Arase’s near-equatorial observations. They interpret the maximum as evidence for a local source of multi-MeV electrons, rather than particles arriving there through inward transport alone.
The sequence matters: the authors say the local source formed before subsequent radial diffusion carried particles both inward and outward. Diffusion describes their later redistribution through the magnetosphere; it does not replace the study’s interpretation of how the source first formed. The paper reports the observations and that interpretation.
The result is best understood as a sequence: a local source near L*≈3, followed by inward and outward diffusion.
Chorus waves and depleted plasma
Intense whistler-mode chorus waves were observed in the same region as the local maximum, amid strongly depleted plasma. The authors say those conditions are consistent with chorus-driven acceleration. They also identify plasmasphere erosion, enhanced convection and a negative ionospheric storm as part of the setting for the event.
Nagoya University’s account says some observed electrons exceeded 3 MeV. It also describes the plasmapause as compressed to about L≈1.5. That L coordinate refers to the plasmapause measurement; the study’s local electron-distribution maximum is reported near L*≈3. Nagoya University’s summary identifies the Arase instruments involved as XEP, MGF and PWE.
What a separate low-orbit observation found
A separate study used Macau Science Satellite-1 and Arase observations to examine electrons above 1 MeV detected at low Earth orbit during the same May 2024 storm. It reported a continuous enhancement in the South Atlantic Anomaly over seven days during the recovery phase. Its authors considered several possible processes, including radial transport, radial diffusion, local acceleration and pitch-angle scattering. That study concerns a different location and measurement from the local source reported near L*≈3.
The plasma recovery measurement
Nagoya University reported that plasma density in the observed region took approximately 4.3–4.7 days to recover to more than half its pre-storm level. That interval measures the recovery of plasma density in that region.
