MODELING STUDIES OF IONOSPHERIC VARIATIONS DURING AN INTENSE SOLAR-CYCLE

Modeling studies are carried out using the Sheffield University plasma sphere-ionosphere model to investigate the relative importance of neut ral winds, neutral densities, and solar EUV fluxes in leading to the s aturation of ionospheric ionization observed during the intense solar cycle 21. Values of mean daytime (1100-1700 LT) ionospheric electron c ontent, peak electron density (Nmax), and peak height (hmax) computed for four midlatitude stations for the month of October 1980-1985, when the 10.7-cm solar activity index (F10.7) varied from 66 to 303, incre ase nonlinearly with F10.7 with saturation for high values of F10.7. N eutral winds, neutral densities, and solar EUV fluxes (obtained from e mpirical models based on observed data) used in the model computations also undergo similar nonlinear increase with F10.7. The study reveals that (1) the nonlinear increase of neutral winds and neutral densitie s has no net effect on the saturation of ionospheric ionization, and ( 2) the saturation of ionization is caused by the saturated production of ionization due to the nonlinear increase of the solar EUV fluxes. T he model values of ionospheric height saturate mainly due to the nonli near increase of the neutral densities determined by solar EUV and UV radiations. The study concludes that the ionosphere (and atmosphere) r esponds linearly to the solar EUV (and UV) inputs and nonlinearly to F 10.7 because the expected linear relationship between the EUV (and UV) fluxes and F10.7 breaks down during solar maximum. Thus it is recomme nded that the F10.7 proxy, which has conventionally been used as an in dex of solar activity, must be replaced or be used carefully, particul arly during solar maximum.