Mechanism of the post-midnight winter night-time enhancements in NmF2 over Millstone Hill during 14–17 January 1986

Abstract

A time-dependent mid-latitude one-dimensional model of the ionosphere and plasmasphere has been used to study the causes of the post-midnight winter night-time enhancements in NmF2 over Millstone Hill during 14–17 January 1986. The mechanism proposed to explain night-time NmF2 enhancements in the current study is a combination of the plasmaspheric heating and refilling and chemical and plasma transport effects. A magnetic plasma flux tube refilling leads to an increase in the plasmaspheric H + density, and this causes an increase in a flow of H + ions from the plasmasphere to the ionosphere, i.e. the magnitude of [H +] is increased in the topside ionosphere. It leads to an increase in the night-time production rate of O +( 4S) ions due to charge exchanges between O +( 4S) and H and between H + and O. The magnitude of [O +( 4S)] is increased in the topside ionosphere under the influence of an increase in the night-time production rate of O +( 4S) ions, i.e. the absolute value of the night-time ionospheric O +( 4S) diffusion flux from the topside ionosphere to the F2- region is increased, resulting in an NmF2 increase. This implies that part of the night-time NmF2 enhancement is created by interactions between diffusion fluxes of O +( 4S), H +, and He + ions. It is shown that increases in electron and ion temperature created by an additional electron heating of the plasmasphere, Q ad, before midnight produce decreases in the absolute value of the night-time flux of O +( 4S) ions flowing from the topside ionosphere to the F2-region, resulting in an acceleration of the rate of the NmF2 drop before midnight. The rapid electron and ion temperature decay in the ionosphere due to the disappearance of Q ad leads to the rapid decrease in the absolute value of the night-time flux of O +( 4S) ions flowing from the topside ionosphere to the F2-region resulting in the corresponding increase in NmF2. As a result, an appearance of Q ad before midnight helps to create a post-midnight NmF2 enhancement. The model without Q ad produces a mismatch between the measured and modelled night-time electron densities and temperatures, giving an increase in calculated densities and a decrease in calculated electron temperatures by night. An appearance of Q ad inhibits the development of night-time enhancements in NmF2 caused by variations in the wind-induced plasma drift. As a result, night-time equatorward winds were not important drivers in the NmF2 enhancements for most of the nights. However, the neutral wind forces the F2 layer to descend to low altitudes of heavy chemical O +( 4S) ion losses, reducing NmF2 to low values before sunrise on 16 and 17 January forming the peaks in the NmF2 time dependence.