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

[1] 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.