Dynamics of the tongue of ionizations at different altitudes during the geomagnetic storm on September 7, 2015

Using the electron density (Ne) observations from the Defense Meteorological Satellite Program (DMSP), and Constellation Observing Systems for Meteorology, Ionosphere, and Climate mission (COSMIC) and simulations from the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM), we investigate the dynamic evolution of the polar tongue of ionization (TOI) from double to single structures at different altitudes during a geomagnetic storm. The modeled Ne depicted that double and single TOIs occurred at altitudes above 300 km, respectively. During the northward turning of IMF Bz, the afternoon TOI disappeared and the morning TOI was reduced. The plasma transport due to neutral winds and ambipolar diffusion facilitated (prevented) the depletion of plasma density of the morning TOI at 300 (500) km, with a relative contribution of 42.8% and 28.6% (-15.4% and -76.9%), respectively. Downward E × B drifts led to an enhancement/reduction of plasma density in the SED region in the lower/upper ionosphere. During the duskward turning of IMF By, the morning TOI could be mostly attributed to the anti-sunward plasma drifts (75.8% at 300 km, 100% at 500 km), with a relatively stronger role of the zonal component than that of meridional E × B drifts. The upward E × B drifts were important/ignorable in the upper/lower ionosphere. Both the neutral winds and ambipolar diffusion resulted in an accumulation of plasma density of the morning TOI at 300 km indirectly (24.2%), however, their roles were minor at 500 km.