Diurnal, seasonal and solar-cycle variations of electron densities in the ionospheric D- and E-regions

Abstract

Mean electron density profiles for the height region between 65 and 110 km are derived for medium latitudes in dependence on solar zenith angle, solar activity and season from radio wave propagation data in the frequency range between about 10 and 2500 kHz. Starting with an initial profile, the electron density profile is iteratively adjusted until the theoretical propagation data calculated from this profile satisfactorily agree with the experimental data. The results show a general enhancement of electron density with increasing solar activity in the whole height region. Evaluation of the seasonal variation shows that the lowest values of electron density in the lower ionosphere are found in April, whereas in winter the electron density is strongly enhanced above 83 km. This winter-time increase of electron density together with the enhancement of collision frequency causes the winter anomaly well-known from radio wave propagation measurements.