摘要：Relativistic electrons at geosynchronous orbit (GEO) were persistently quiet in 2009 for almost a whole year. The solar wind speed, which has been known as a primary parameter controlling the outer belt electrons, was very slow in 2009 as expected and at a comparably low level as of 1997 when we did not observe such a persistently quiet condition. Since the interplanetary magnetic field (IMF) was quite different between 1997 and 2009, the difference in IMF is a possible cause of the difference in the electron flux levels, providing an important clue to understand the complex source and loss process of relativistic electrons at GEO. We suggest that the extremely weak IMF of the very slow solar wind plays an essential role in diminishing the source processes themselves associated with magnetic storms and substorms, and in turn to suppress the relativistic electron flux at GEO over the time scale of a year, as an inevitable consequence of extremely weak open magnetic field of the Sun associated with the extremely weak current solar minimum.
