The dynamic evolution of the double tongue of ionization (TOI) into a single TOI at 400 km altitude 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 increased southward B-z and weak positive B-y, while the single TOI occurred in the presence of northward turning of B-z and duskward turning of B-y. 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 x B drift. The transition process is due to both the northward and duskward turning of interplanetary magnetic field (IMF). The northward turning of IMF B-z weakens the storm enhanced density (SED) and the TOI in both afternoon and morning sectors, while the increasing duskward IMF B-y strengthens the morning TOI. Plain Language Summary A tongue of ionization (TOI) is a persistent "tongue"-like structure of electron density enhancement at high latitudes and usually elongates in the noon-midnight direction, which is a hot topic in the field of magnetosphere-ionosphere coupling. However, previous studies mainly focused on the formation and evolution of single TOI events. The dynamic evolution from double TOIs to single TOI events has never been reported, which is an important topic in understanding the dynamics of the magnetosphere-ionosphere coupled system. Our work suggests that the temporal variations of interplanetary magnetic field B-y and B-z control the transition by regulating polar ionosphere convection.
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