Observations and modelling of fast vertical sub-auroral plasma uplift during geomagnetic storms

During geomagnetic storms, ionospheric plasma is transported across high latitudes by the enhanced solar wind – magnetosphere coupling. The anti-sunward plasma convection results in polar cap plasma anomalies, such as the tongue of ionisation (TOI). Fast plasma uplifts at sub-auroral latitudes, due to the vertical coupling via electric fields and/or thermospheric neutral winds, are generally responsible for these long-lasting TOI anomalies. Particularly in the North American sector, TOI anomalies, as well as associated electric drifts, have been detected in observations using global positioning system signals (total electron content), in-situ satellite measurements, and altitude-resolved profiles from ground-based incoherent scatter radars. Recent modelling developments have enabled simulations of the TOI anomalies, even under extreme geomagnetic storm conditions. In this study, rare direct observations of plasma uplifts by the Millstone Hill incoherent scatter radar (288.5°E, 42.6°N) during the geomagnetic storm of November 2004 are analysed and compared to first-principles numerical simulations. These indicate that enhanced convection electric fields are the primary source of plasma uplifts, at least during the storm's main phase, and that the choice of plasma convection model is crucial for accurate modelling results.