Relative Timing of Nightside and Dayside Plasmapause Motion: Two Events in June 2001

We examine the relative timing of dayside and nightside plasmapause motion following southward interplanetary magnetic field turnings on 2 and 9 June 2001. For both events the delay between the dayside and nightside plasmapause response is less than the temporal resolution of Imager for Magnetopause‐to‐Aurora Global Exploration extreme ultraviolet observations (10 min). Our result thus establishes a possible upper limit ( ≤ 10 min) on the day‐to‐night onset delay. From analysis of the extreme ultraviolet‐observed plasmapause motion we find on 2 June the time‐averaged plasmapause E field was (in mV/m) 0.61 near noon magnetic local time, 0.35 near midnight magnetic local time, and 0.44 overall. The normalized plasmapause speed data (both dayside and nightside) are fitted to the curve ( 0 . 09 ± 0 . 05 ) L 3 , consistent with a spatially uniform and time‐constant dawn‐dusk penetration E field that is 9% of the solar wind field. On 9 June the time‐averaged E field values (mV/m) were 0.24 (dayside), 0.28 (nightside), and 0.26 (overall); the plasmapause motion was consistent with a penetration E field 10% of the solar wind field. Plasmasphere erosion is a fundamental element of the dynamic magnetospheric response to solar wind driving. This study yields an important observational constraint on the day‐to‐night timing of the plasmapause response. The result also has implications for the two main models for the cause of erosion (convection and interchange).