Midday magnetopause shifts earthward of geosynchronous orbit during geomagnetic superstorms with Dst ≤ −300 nT

Geomagnetic storm intensity, as measured by the Dst (SYMH) index, shows no limit as the solar wind dawn‐to‐dusk electric field increases. We show that the magnetopause around noon erodes earthward with increasing storm intensity. The panoramic geosynchronous B Z signatures for the magnetic storm groups with different intensity are differ significantly from each other. For superstorms with SYMH ≤ −300 nT, the magnetopause around noon erodes to inside the geosynchronous orbit, which causes the B Z reversal near local noon. The necessary conditions for superstorms with SYMH ≤ −300 nT to occur include the following: (1) interplanetary magnetic field (IMF) B Z < −27 nT lasts for at least ∼1 h; (2) solar wind dynamic pressure, P d > ∼ 12 nPa; (3) the projected interplanetary electric field, E K − L > ∼30 mV/m.