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

The mechanism of the NmF2 peak formation at different levels of solar activ ity is analyzed using Millstone Hill IS radar observations. The h(m)F(2) ni ghttime increase due to thermospheric winds and the downward plasmaspheric fluxes are the key processes responsible for the NmF2 peak formation. The e lectron temperature fallows with the opposite sign the electron density var iations in this process. This mechanism provides a consistency with the Mil lstone Hill observations on the set of main parameters. The observed decrea se of the nighttime NmF2 peak amplitude with solar activity is due to faste r increasing of the recombination efficiency compared to the plasmaspheric flux increase. The E x B plasma drifts are shown to be inefficient for the NmF2 nighttime peak formation at high solar activity.