In May 2024, auroras were observed at unusually low latitudes across the globe, lighting up skies that rarely see such displays. Inside Earth's magnetosphere, the region of space surrounding our planet and dominated by its intrinsic magnetic field, something significant was finally being observed.
It started with a large sunspot firing a rapid series of powerful solar eruptions. Clouds of magnetized plasma merged as they traveled through space and impacted Earth's magnetosphere. No geomagnetic storm this powerful had ever been measured in the Earth's ring current region, a belt of charged particles in space near our planet.
Two sources of ring current ions are known: solar wind and Earth's ionosphere, the electrically charged upper layer of the atmosphere. For decades, scientists have debated how much each source contributes to the ring current. During most storms, both contribute. However, during a storm driven by a dense solar wind, some scientists expected solar wind ions to continue to play a notable role. Yet the first direct measurements of ring current composition from a super geomagnetic storm revealed that solar wind ion contributions were minimal, and the level of Earth-origin ion dominance had never been observed before.
The findings, published in Science Advances , suggest that understanding how much Earth's ionosphere contributes to the ring current may be essential to accurately predict the severity of super geomagnetic storms. The dominance of ionospheric ions, which are far heavier than solar wind particles, may have intensified the magnetic disturbance and concentrated the ring current peak unusually close to Earth. The researchers also make a case for a proposed Japanese multi-satellite mission to understand exactly how ion supply processes work.
Earth's ring current
On May 10 and 11, 2024, giant clouds of charged particles from the Sun struck Earth's magnetosphere. The resulting May 2024 super geomagnetic storm, also referred to as the "Gannon storm" or "Mother's Day storm," reached a minimum SYM-H index of −518 nanotesla, the second-largest value recorded since 1981. The last comparable geomagnetic storm was the November 2004 superstorm.
"Some super or extreme geomagnetic storms are not just impressive light shows—they pose radiation risks to spacecraft, disturb GPS signals and communications, and cause power outages. Understanding how a geomagnetic storm develops is not only a scientific question, but also one with real-world consequences," said Naritoshi Kitamura, lead author and designated assistant professor from the Institute for Space-Earth Environmental Research (ISEE) at Nagoya University.
The magnetic disturbance of a geomagnetic storm is caused by the ring current. This is a huge belt of energized ions, mostly oxygen and hydrogen, that drift slowly around Earth thousands of kilometers above the equator. The energized ions carry current, and that current generates a magnetic field that partially cancels Earth's own on the ground. This causes the disturbance that is observed by ground-based instruments.
Arase was ready: rare event, first of its kind observation
Japan's Arase satellite was launched in 2016 and has been operated by the Japan Aerospace Exploration Agency (JAXA). The ERG (Arase) science center is jointly operated by Institute of Space and Astronautical Science (ISAS)/JAXA and Institute for Space-Earth Environmental Research/Nagoya University.
Arase orbits the region where the ring current develops. The satellite carries specialized instruments to identify mass and energy of detected ions. It crossed through the ring current just after the storm began, and again near its peak.
"This is the first simultaneous observation of ring current ions and solar wind during a storm this large, and the data was clear—approximately 85% of ions were oxygen from Earth's own ionosphere," Kitamura explained.
"Near the peak of the storm, Arase detected a 40% decrease in magnetic field intensity at roughly 16,000 kilometers above Earth, and much closer to Earth than similar large decreases previously documented."
The same region also showed a simultaneous drop in high-energy electrons that normally orbit Earth in that zone. When a magnetic field weakens this severely, electrons drift out from their normal paths. Whether the magnetic field deformation caused the electron loss warrants further investigation.
The findings deepen our understanding of how super geomagnetic storms develop. Space weather forecasting models rely on solar wind conditions to predict storm severity, but this study suggests Earth's atmospheric state, and not just conditions at the Sun, may partly determine how severe a storm becomes.
The study also supports FACTORS, a two-satellite mission concept being prepared for JAXA's upcoming proposal opportunity, which would directly address this gap. FACTORS aims to improve our understanding of how Earth's atmospheric ions escape into the magnetosphere and contribute to geomagnetic storm development. It may ultimately help scientists more accurately predict how severe these storms will get.
