The electron density distributions in the D-region during the night and pre-sunrise period

Abstract

Experimental measurements of reflection and conversion coefficients over a range of frequencies from 16 to 71 kHz are used to deduce electron density distributions for night-time and the pre-sunrise period. A full-wave integration method which takes account of the energy dependence of the electron collision frequency is used to compute propagation parameters for waves reflected from a postulated model of electron density. These computations are made for azimuth and incident angles relevant to the paths for which experimental data are available, the appropriate angles of incidence being found using a stationary phase technique. The electron density distribution is then suitably modified until the computed values agree with the experimental measurements. In this way the electron density distributions representative of night-time and subsequent times up until ground sunrise are deduced for Summer, high solar activity conditions at middle latitudes. The electron density distribution at night-time shows a ledge of ionization of density about 100 cm −3 situated between 82 and 90 km, and a sharp gradient near 90 km. The presunrise changes in electron density first occur near 80 km and the electron densities at lower heights increase progressively as the solar zenith angle is reduced. The deduced electron densities are compared with the results obtained by direct measurements.