IMF‐driven overshielding electric field and the origin of the plasmaspheric shoulder of May 24, 2000

Between the hours of 6–10 UT on May 24, 2000, the IMAGE extreme ultraviolet (EUV) instrument observed a shoulder‐shaped bulge in the morning sector plasmapause [ Burch et al. , 2001a , 2001b ]. Simulation results of the data‐driven Magnetospheric Specification Model (MSM) have reproduced the formation (during 4:00–5:15 UT) and subsequent evolution of the shoulder. In the model, the shoulder is created by a dusk‐to‐dawn overshielding electric field, triggered by two sudden, strong northward (N wd ) turnings of the IMF. Overshielding causes antisunward flow of pre‐dawn plasma, producing an asymmetric bulge that rotates eastward.