Seasonal-latitude variations of ionospheric E-layer critical frequencies dependence on solar activity in empirical models.

The ionospheric electron concentration near E-layer maximum varies with solar activity presumably due to variations of EUV fluxes in the lines 97.7 and 102.6 nm and soft X-ray (0.1-10 nm) emission. The scale of variations of these fluxes in solar activity cycle is different for EUV (∼ 2 times) and X-ray (∼ 5 times) emission. The relative contribution R=q_x/q_u of EUV and soft X-ray emission into an ionization rate q near the E-layer maximum varies with season and latitude, R∼ 0.1 near equator at all seasons, but in the middle (∼ 50°) latitudes it varies from R∼ 0.08 in winter to R∼ 0.32 in summer. It results in the fact, that dependence of E-layer critical frequencies FoE on the solar activity level is remarkably different for summer and winter in the middle latitudes, and also differs for low and mid and high latitudes. In linear approximation the slope in equation for FoE dependence on F_10.7 activity index may differ by ∼ 20% for summer and winter and for low and middle latitudes. Up to the present time this effect was not taken into account in existing semi-empirical models. A simple analytical formula is proposed, wich is "in style" of IRI model, to take into account this effect. The work was partially supported by RFBI (grant Nr. 03-02-17109).