Day-time variations of the equivalent electron concentration in the non-sounded lower ionosphere
Based on data on the lowest reflected frequencyfmin and on information on the lower and upper boundaries of the non-sounded lower ionosphere, an equivalent electron concentration for all concentrations below the correspondingfmin 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 N ≈0.5 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 employingfmin in aeronomic investigations.
KeywordsRadiation Temperature Gradient Solar Activity Electron Concentration Zenith Angle
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