The Densities and Compositions of Background Cold Ions Based on the Van Allen Probe Observations.

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 proton ions show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen increases, and the percentage of protons 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. Plain Language Summary: The cold ions play an important role in magnetospheric dynamics since they are the source of thermal plasma and they could affect the magnetic reconnection and wave generation. However, the main population of cold ions is difficult to measure due to their low energy and spacecraft charging. Magnetospheric convection and/or induced electric field could increase the energy of cold ions sufficiently above the spacecraft potential so that these ions can be detected by particle instrument. In this study, we investigate the properties of background cold ions when the total ion density is comparable to the background electron density. We found the cold ion could often be measured near the dusk sector and a clear dawn‐dusk asymmetry is observed for all ion species. Similar to the cold electrons, cold protons also show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen ions increases, and the percentage of protons decreases as geomagnetic activity increases whereas the percentage of helium ions is generally small. Our results provide important information on cold ion density for the study of wave‐particle interactions and magnetic reconnection in the Earth's magnetosphere. Key Points: We statistically analyzed the cold ion densities and compositions based on Van Allen Probe observationsThe density above L = 3 decreases as geomagnetic activity increases for all three ion species, suggesting the shrinking of plasmasphereThe percentage of cold oxygen ions increases as geomagnetic activity increases [ABSTRACT FROM AUTHOR]