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Akebono Observations of Long-Term Variations of the Radiation Belt

The Akebono satellite was launched in February 1989 primarily to reveal generation mechanisms of auroras. Nearly half of the onboard instruments are still working with their performance sustained, although it has been more than 19 years since the launch. This allows us to study long-term variations of the plasma environment in near-Earth space with the identical instruments.

It is known that the plasma environment in near-Earth space is strongly influenced by 11-year-period solar activity. The figure shows the relationship between the development of the radiation belt and the sun spot number (i.e., solar activity) from the launch to 2006. The radiation belt significantly develops as solar activity increases, and vice versa.

There have been almost no satellites which continuously observed the radiation belt for a very long time. Recently Akebono's extended operation until March 2011 was approved, so that Akebono will be the first satellite in the world to observe the 22-year variation, which corresponds to the period of the reversal of the Sun's magnetic polarity.

Ayako Matsuoka (ISAS/JAXA)

June 2008