On the mechanism of the post-midnight winter NmF2 enhancements: dependence on solar activity

The mechanism of the N_mF₂ peak formation at different levels of solar activity is analyzed using Millstone Hill IS radar observations. The h_mF₂ nighttime increase due to thermospheric winds and the downward plasmaspheric fluxes are the key processes responsible for the N_mF₂ peak formation. The electron temperature follows with the opposite sign the electron density variations in this process. This mechanism provides a consistency with the Millstone Hill observations on the set of main parameters. The observed decrease of the nighttime N_mF₂ peak amplitude with solar activity is due to faster increasing of the recombination efficiency compared to the plasmaspheric flux increase. The E × B plasma drifts are shown to be inefficient for the N_mF₂ nighttime peak formation at high solar activity.