Solar cycle variations of outer radiation belt and its relationship to solar wind structure dependences

The outer radiation belt shows solar cycle variation: the L-shell of the electron flux peak in the outer belt shifts inward during the period between the rising phase and the solar maximum, while it shifts outward between the beginning of the declining phase and the solar minimum. We show a possible mechanism which considers two typical types of magnetic storms categorized in accordance with solar wind drivers, namely coronal mass ejections (CMEs) and corotating interaction regions (CIRs). Large flux enhancements at the inner portion of the outer belt tend to occur during the recovery phase of great storms driven by CMEs, while large flux enhancements at the outer portion and at geosynchronous orbit tend to occur during the recovery phase of relatively moderate storms driven by CIRs. High-speed coronal hole streams which do not always cause large magnetic storms also effectively enhance the electron flux enhancement at the outer portion and in geosynchronous orbit. In this framework, the plasmapause always plays an important role in both flux enhancement and flux loss in the outer belt. The average plasmapause position depends on the storm amplitude, and the plasmapause reaches closest to the Earth during great storms driven by CMEs. CMEs themselves and CME-driven storms occur during maximum periods of solar activity, while CIRs themselves and CIR-driven storms occur during the solar declining phase. The observed long-term variations of the outer belt can therefore be understood in terms of their dependence on the large-scale interplanetary structures, varying depending on the phase of the solar cycle.

Research Highlights
► This paper clarifies the origin of the solar cycle variations of the outer belt.
► The flux enhancement of the outer belt significantly depends on the solar wind structures.
► The results provide a reliable guideline for the space weather forecast of outer belt electrons.