Periodic Response of Plasmasphere to Solar Wind High‐Speed Streams

Abstract We report the first observation of the plasmasphere's periodic response to solar wind high‐speed streams (HSS) during the declining phase of Solar Cycle 23, based on plasmapause location data from the IMAGE and THEMIS satellites. In both 2005 and 2008, the daily variability of the plasmapause exhibits a strong anti‐correlation with solar wind speed, oscillating coherently at specific timescales. A similar anti‐correlated variation is identified in the latitude of the midlatitude ionospheric trough (MIT) minimum, derived from electron density measurements by the DMSP F16 satellite. Periodogram analysis reveals a distinct 9‐day periodicity in 2005, and both 9‐ and 13.5‐day periodicities in 2008 across all parameters. These findings provide direct evidence of magnetospheric modulation by recurring solar wind drivers and establish a clear connection between the plasmasphere and the midlatitude ionosphere under periodic solar forcing.