The nightside‐to‐dayside evolution of the inner magnetosphere: Imager for Magnetopause‐to‐Aurora Global Exploration Radio Plasma Imager observations

It took about 12 h for the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite to fly from the outbound pass to inbound pass of every inner magnetosphere crossing. This provides a unique opportunity to study the nightside‐to‐dayside evolution of the corotating inner magnetosphere when the outbound pass is in the nighttime while the inbound pass is in the daytime because a flux tube observed on the nightside may be observed again on the dayside in such situation. The differences between the two observations may be caused by the evolution of the flux tube. By analyzing both the passive and active sounding measurements from the IMAGE Radio Plasma Imager, we study two cases concerning this evolution. One, under a quiet geomagnetic condition, shows a typical evolution process during which the plasmapause was observed on both the nightside and the dayside. The other, during the recovery phase of a magnetic storm, shows a different inner magnetospheric structure in which the distinct plasmapause observed on the nightside becomes unidentifiable on the dayside as the density in the nearly empty nightside plasmatrough increases to a level similar to that of the plasmasphere. It is shown that the evolutions of the inner magnetosphere in both cases were primarily controlled by the fast plasma refilling of the flux tubes from the ionosphere as the flux tubes drift from the nightside to the dayside. In the former case the fast refilling was confined inside L = ∼6.3, while in the latter case the fast refilling extended to at least L = 10. The present observations provide an example for fast refilling as a possible cause of the smooth density transition from the plasmasphere to the subauroral region and demonstrate the importance of the plasmasphere‐ionosphere coupling in controlling the structures of the inner magnetosphere.