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Abstract

The cold ions, which are generally ‘invisible’ to most instruments, have strong impacts on plasma wave and magnetic reconnection. Under particular situations, these cold ions could be accelerated and thus become detectable. In this study, we statistically investigated the properties of background cold ions based on Van Allen Probe observations. The cold ions could often be detected near the dusk sector, and a clear dawn-dusk asymmetry is observed for all ion species with higher density at the dusk side, showing plasmaspheric plume-like structures. Similar to the cold electrons, cold ions except for oxygen ions show a clear boundary of plasmapause with its location moving towards the Earth as geomagnetic activity increases. Furthermore, the oxygen composition increases and the proton composition decreases as geomagnetic activity increases whereas the helium composition is generally small. Our results provide important information on ion compositions for the understanding of cold-plasma dynamics in the inner magnetosphere.