Dependence of Аnnual Asymmetry in NmF2 on Local Time

Based on the global empirical model of the median of the critical frequency of the F2 layer (SDMF2), the properties of diurnal variations in the annual asymmetry in the maximal concentration NmF2 of the F2 layer for different values of the solar activity index F are analyzed. The AI index characterizing the relative difference in NmF2 averaged over all latitudes and longitudes between January and July at a given local time is used as a parameter of this asymmetry. It is found that a semidiurnal mode prevails in the diurnal variations in the AI index with maxima at the daytime and nighttime hours. The daytime maximum of the AI index hardly depends at all on the solar activity level. The nighttime AI maximum decreases with an increase in solar activity. The daytime and nighttime maxima in AI almost coincide by the amplitude when AI = 16–17% for low solar activity. The difference in the solar radio emission flux between January and July due to the ellipticity of Earth’s orbit relative to the Sun contributes substantially into the AI index at all hours of the day. On average, it is 3–4% and could reach 5% for low solar activity during the nighttime hours. The difference in the AI index for low and high solar activity according to the International Reference Ionosphere model IRI (with the URSI and even more with the CCIR coefficients) is overestimated with respect to the SDMF2 model at almost all hours of the day, apparently due to limited amount of the experimental data used to obtain the CCIR and URSI coefficients, especially over oceans.