A TIEGCM numerical study of the source and evolution of ionospheric F-region tongues of ionization: Universal time and interplanetary magnetic field dependence

The National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIEGCM) has been employed to systematically study the source and evolution of the ionospheric F-region Tongue of Ionization (TOI), which is electron density enhancement in the polar region. The model is run for different Universal Times (UT), season and Interplanetary Magnetic Field (IMF) conditions. It is found that: (1) The TOI formation is critically dependent on UT, preferentially near 2000 UT in the Northern Hemisphere (NH) and near 1600 UT in the Southern Hemisphere (SH). These are the intervals when the high-latitude ion convection throat is closer to the middle-latitude high plasma density source region, so that more plasma can be directly transported into the polar cap region; (2) this different UT dependence between the two Hemispheres occurs, not only because of the different separation of the magnetic poles from the geographic poles in the two hemispheres, but also because of the UT dependence of the mid-latitude source locations (local time, latitude) and the magnitude of plasma density enhancements; (3) the TOI is generally stronger in the SH than it is in the NH, and in winter than in summer; (4) IMF By operates in the opposite sense in the two hemispheres in terms of the TOI pattern such that positive/negative IMF By tends to deflect the TOI toward the morning/afternoon sector in the NH. The opposite condition occurs in the SH.