Overshielding electric field identified by ground-based magnetometer associates plasmaspheric shoulders seen in the global EUV images

We examine the overshielding electric field using the ground-based magnetometer data associated with the formation of the plasmaspheric shoulders. Decreasing deflections of horizontal north-south component of the magnetic field at the magnetic equatorial regions indicates the overshielding electric field. We survey the plasmaspheric images taken by IMAGE&KAGUYA and find shoulders. Clear connections are shown between the overshielding electric field and the emergence of the shoulders.