Modeling and observations of the low latitude ionosphere-plasmasphere system at long deep solar minimum

The behavior of the low latitude ionosphere-plasmasphere system in American-Brazilian longitudes (30°W-120°W) in three seasons at the long deep solar minimum (2006-2010) is investigated using the theoretical model SUPIM and FORMOSAT-3/COSMIC electron density and GIM-TEC data. The data and model reveal some new aspects primarily for the low solar EUV fluxes. The ionosphere develops as a thin layer in the morning though becomes nearly as strong as that at normal solar minimum at around diurnal maximum (14 LT). However, after sunset the ionosphere-plasmasphere system decays rapidly to become an extremely weak cold system prior to sunrise (05 LT) when the ionosphere contracts to about 1/3rd as strong as that at normal solar minimum, with peak density only about 1.8 × 10⁵ cm^-3, half width only about 150 km, and O⁺/H⁺ transition height as low as 475 km where the ion densities are only 10⁴ cm^-3 and ion temperatures are only 600 K. The mechanical effects of the neutral wind dominate over other processes, which causes the disappearance of the well known winter anomaly in TEC and Nmax, and lowest O⁺/H⁺ transition height (∼650 km at 14 LT and 475 km at 05 LT) occurring at around ±15° magnetic latitudes where the mechanical effects optimize. In addition, the ionosphere becomes weakest about 7 months after the solar activity dipped to the lowest level in 2008.