Positive plasma anomalies appear during the main phase of
geomagnetic storms at (sub)auroral latitudes, extending across the polar cap as tongue of ionisation (TOI). Physical mechanisms of TOI, including electrodynamic plasma transport and neutral wind forcing, are simulated with TIE-GCM during the superstorm of Nov. 2003. The simulations are compared with TEC observations and GNSS tomography. The electrodynamic transport (vertical ExB component in particular) is identified as the main mechanism controlling TOI anomaly during great storms (Dst < -300 nT). This makes the choice of high-latitude convection model critical for simulations
