VARIATIONS OF IONOSPHERIC IONIZATION AND RELATED SOLAR FLUXES DURING AN INTENSE SOLAR-CYCLE

Variations of ionospheric ionization (represented by ionospheric elect ron content (IEC)) and related solar fluxes with the 10.7-cm solar flu x index (F10.7) are studied for the intense solar cycle 21 when F10.7 was as high as 367. The IEC data collected at several stations during 1980-1985, the solar EUV (50-1050 Angstrom) fluxes obtained from the E UV91 solar EUV flux model, and the measured values of Lyman a (1216 An gstrom) flux and He I(10,830 Angstrom) equivalent width (EW) are used for the study. It is shown that daily values of diurnal maximum IEC (I ECmax) saturate (remain constant) when F10.7 (or its 81-day running av erage) exceeds a threshold (approximately 160-200) which depends sligh tly on season and latitude. Variations of the model values of the sola r EUV fluxes reveal that when F10.7 exceeds the threshold: (1) the int egrated solar EUV (50-1050 Angstrom flux increases at a very low rate, and (2) the fluxes of the important (for thermospheric heating) chrom ospheric lines and intervals generally saturate (remain constant), whi le those of the coronal lines and intervals increase at a reduced rate . Lyman a flux and He I EW, which are used as input data in the solar EUV flux model, also increase at a very low rate when F10.7 exceeds th e threshold. The saturation of ionospheric ionization, observed for hi gh values of F10.7 during the last three solar cycles 19-21, is the re sult of the nonlinear variation of the solar EUV and Lyman alpha fluxe s with F10.7. IECmax increases linearly with the integrated solar EUV flux, Lyman alpha flux and He I EW.