【Key points of this research】
・The geospace (※5) exploration satellite "Arase" (※6) (hereinafter referred to as "Arase" satellite) successfully observed the unique temporal and spatial variations in electron density in the plasmasphere (※3) and ionosphere (※4) associated with a massive geomagnetic storm (※2) that occurred on May 10, 2024 due to a solar flare (※1).
・It was discovered that the plasmasphere rapidly shrank to an altitude of 1.5 times the radius of the Earth, and that it took more than four days for it to recover to its original state.
・It was confirmed that the electron density in the ionosphere, which is the source of the charged particles in the plasmasphere, had decreased by up to 90% compared to quiet times over a wide area from high to low latitudes.
・This study is the first to demonstrate that anomalous prolonged ionospheric depletion inhibits the recovery of the plasmasphere, significantly extending the recovery time of the plasmasphere compared to other geomagnetic storm events.
[Research overview]
Professor Yoshiya Kasahara (Emerging Media Initiative/Advanced Research Center for Space Science and Technology, Kanazawa University), and Associate Professor Shoya Matsuda (Faculty of Electrical, Information and Communication Engineering, Institute of Science and Engineering/Advanced Research Center for Space Science and Technology), in collaboration with a research group including Assistant Professor Atsuki Shinbori of the Institute for Space-Earth Environmental Research, Nagoya University, analyzed observation data from the Global Navigation Satellite System (GNSS) (*7) and the Arase exploration satellite, and succeeded in observing the temporal changes and spatial structure of electron density in the plasmasphere and ionosphere during the massive geomagnetic storm that occurred on May 10, 2024.
Observation data showed that the plasmasphere, which typically extends to an altitude of 4 to 6 times the Earth's radius (6,378 km: equatorial radius), rapidly shrank to an altitude of 1.5 times the Earth's radius and took more than 4 days to recover to its original state. Statistical analysis also revealed that this recovery time was about twice as long as usual case of a normal magnetic storm. Ionospheric electron density measured by the GNSS showed that the ionospheric electron density decreased by up to 90% over a wide area from high latitudes to low latitudes compared to quiet conditions, and that this condition continued for at least two days.
This study is the first to show from observational data analysis that the anomalous prolonged ionospheric depletion caused by such a huge magnetic storm inhibits the recovery of the plasmasphere existing above the ionosphere. Ionospheric depletion causes failures in shortwave communication that rely on ionospheric reflection. In addition, structural changes of the plasmasphere during magnetic storms control the electromagnetic environment responsible for the generation of energetic particles in geospace. Accurate forecasting of these dynamics is essential for forecasting the space weather for the safe utilization of outer space. The findings of this research provide an important case study that contributes to the prediction of anomalous space environment variations around the Earth during infrequent but high-impact giant magnetic storms.
The research results were published in "Earth, Planets and Space", a comprehensive international journal of earth sciences published by Springer Nature, on November 20, 2025 at 17:00 JST.
【Glossary】
*1 Solar flare
A multi-wavelength brightening phenomenon that occurs on a timescale of several minutes to several hours, caused by a magnetic activity near the solar surface.
*2 Magnetic storm
A geomagnetic disturbance phenomenon in which the geomagnetic field decreases from its normal state over a period of several hours to a day, and then gradually recovers to its original strength over several days. The process in which the geomagnetic field decreases and a magnetic storm develops is called the main phase, and the process of recovery is called the recovery phase. The magnetic field on the ground that changes during a magnetic storm is about 1/1000 of the normal magnetic field on the ground, but in the case of a huge magnetic storm, the change can be as much as 1/100 of the normal magnetic field on the ground.
*3 Plasmasphere
A region of relatively high-density plasma consisting of low-energy (low-temperature) charged particle groups (plasma) of ionospheric origin. This region exists from the outer edge of the ionosphere to the inner edge of the Earth's magnetosphere, where the plasma density rapidly decreases by about one order of magnitude at the outer boundary. The boundary is called the plasmapause.
*4 Ionosphere
A region at an altitude of about 60 to 1,000 km where molecules and atoms in the upper atmosphere surrounding the earth are ionized by ultraviolet rays, X-rays, etc. This region absorbs, refracts, and reflects radio waves, which affects the propagation of radio waves in the shortwave band and other areas.
*5 Geospace
The space around the Earth that is becoming a zone of human activity.
*6 Geospace Exploration Satellite "Arase"
A scientific satellite launched by JAXA in December 2016, dedicated to exploring the electromagnetic environment in geospace.
The high-resolution electron density measurements essential to this discovery were derived from the Plasma Wave Experiment (PWE) aboard Arase, for which Professor Yoshiya Kasahara serves as the Principal Investigator (PI) and Associate Professor Shoya Matsuda as the Data Manager.
*7 Global Navigation Satellite System (GNSS)
A generic term for satellite positioning systems that perform position measurement, navigation, and time distribution using signals emitted from satellites such as the U.S. Global Positioning System (GPS), Japan's Quasi-Zenith Satellite (QZSS), Russia's GLONASS, and the European Union's Galileo. Among them, the well-known GPS consists of satellites orbiting approximately 20,000 km above the Earth (30 satellites in six orbital planes), a control station that tracks and controls the GPS satellites, and a receiver for the user to perform positioning.
Click here to see the press release【Japanese only】
Journal: Earth, Planets and Space
Researchers' Information: Yoshiya Kasahara
Shoya Matsuda
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