Day-time variations of the equivalent electron concentration in the non-sounded lower ionosphere

Summary Based on data on the lowest reflected frequencyf_min and on information on the lower and upper boundaries of the non-sounded lower ionosphere, an equivalent electron concentration for all concentrations below the correspondingf_min was determined. Day-time variations of the equivalent concentration are investigated, confirming that there is a cosine relation to the solar zenith angle. The power index of that relation has an outlined seasonal course with a maximum in April and October, while the absolute seasonal minimum is during the winter (the summer minimum is slightly outlined). The mean yearly values of the index are almost constant:n_N0.5 for solaractivity,I₁₅₀₀ to 115.10^–22 W Hz^–1 m². During higher activityn_N changes correspondingly toI₁₅₀₀ according to relation (12). The variations ofn_N during high solar activity show that the altitude gradient and temperature gradient in the low ionosphere are becoming proportional toI₁₅₀₀ when the solar x-ray radiation exceeds a certain level. The results obtained confirm the reliability of the method developed for employingf_min in aeronomic investigations.