Although it seems that nothing exists in the outer space around the Earth, tenuous but high-energy particles (ions and electrons) are trapped in the Earth’s magnetic field. This outer space, where these plasma environments are affected by the Earth and conversely strongly affect the Earth, is called “geospace.” The area in geospace between the altitude where the Space Shuttle flies and the geostationary orbit of the meteorological satellite Himawari is called the “inner magnetosphere.” The “radiation belts” exist there. The radiation belts are composed of ions and electrons with an energy level ranging from hundreds of keV (kilo electron volts) to tens of MeV (mega electron volts). The highest energy particles in geospace exist in the radiation belts.
Fig. 1 shows an illustration of the spatial structure of radiation-belt electrons and observation results by the Japanese satellite AKEBONO of electrons above 2500 keV. The electrons’ radiation belts have two belt-like distributions encircling the Earth known as the “inner belt” and “outer belt,” and a region between them called the “slot”. The ions’ radiation belt, although not shown in the figure, is a single not a double-belt structure.
Discovery of the radiation belts
The radiation belts were discovered by the U.S. satellite Explorer in 1958. It is named the “Van Allen Belt” after Prof. Van Allen who discovered it. From the 1960s to the 1970s, it has been the subject of observational and theoretical research. As a result, a quantitative explanation for the spatial structure of the radiation belts in its equilibrium state became possible. In the 1980s, research on the radiation belts temporarily stagnated, partly because satellite exploration areas shifted to the polar aurora region, the magnetotail connected to the polar aurora region in the magnetosphere, and other planets in the solar system.
Dynamically changing radiation belt
The upper section of Fig. 2 shows temporal variation of the relativistic electrons above 2500 keV as observed by AKEBONO. The horizontal axis indicates a period of half a year from January to June 1993 and the vertical axis indicates distance from the Earth. The lower section indicates the index of magnetic storms called the Dst index. Large changes to the negative shows the occurrence of a magnetic storm. We can see from the figure that, when magnetic storms occur, electrons in the outer belt disappear temporarily and then increase slowly to form the outer belt again. When the outer belt forms again, the electrons’ flux sometimes increases by more than two orders of magnitude. We also found that magnetic storms are not always accompanied with the reforming of the outer belt and that sometimes the outer belt does not form for a while after vanishing. In this way, the outer belt is a dramatically changing area responding to occurrences of magnetic storms.
The fact that the outer belt changes largely during magnetic storms was already reported by research in the 1960s to the 1970s. In the 1990s, however, the phenomenon was focused on again. One reason was that drastically changing events in the radiation belts were “rediscovered” by the U.S.’s CRRES and Japan’s Akebono satellites exploring the inner magnetosphere in the 1990s. The other reason was that satellite malfunctions caused by high-energy particles in the radiation belts became a significant social problem.