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

<p>The dynamic evolution of the double tongue of ionization (TOI) into a single TOI at 400 km during the geomagnetic storm on September 7, 2015 was studied using the Defense Meteorological Satellite Program observations and Thermosphere Ionosphere Electrodynamic General Circulation model simulations. The double TOIs occurred in the presence of&#160;increased southward Bz and weak positive By, while the single TOI occurred in the presence of northward turning of Bz and duskward turning of By. In both double and single TOI events, the plasma at middle latitudes in the afternoon (prenoon) sector was greatly enhanced due to the local upward (upward) and zonal (meridional) E &#215; B. The transition process is due to both the northward and duskward turning of IMF. The northward turning of IMF Bz weakens the SED and the TOI in both afternoon and morning sectors, while the increasing duskward IMF By strengthens the morning TOI.</p>