First identification of the day-night asymmetric inner edge of the Earth’s plasmasphere

Abstract The Earth’s plasmasphere controls the lifetime of high-energy particles in the radiation belts and ring current regions (e.g., “killer electrons” with MeV energy). Although the configuration of plasmasphere has been studied for decades, its inner edge has not been identified so far, so that different inner edges of plasmasphere are assumed in different models. It is also assumed that the plasmaspheric inner edge or nearby plasma density is the same at all magnetic local times. Here, we show the day-night asymmetric image of the plasmaspheric inner edge and nearby plasma density from multi-satellite observations. By analyzing the lowest cutoff altitude of plasmaspheric hiss observed by Van Allen Probes and the lowest boundary of hydrogen and helium ions measured by DEMETER satellite, we find that the inner edge of the nighttime plasmasphere (altitude ~ 1274.3km) is higher than the daytime (~ 637.1km), but the ion’s density near the daytime inner edge is larger than the nighttime. These findings reveal the natural configuration of the Earth’s plasmasphere and supply key evidences for the modelling of the plasmasphere and its coupling regions such as ionosphere.