IONOSPHERIC EVIDENCE FOR A NONLINEAR RELATIONSHIP BETWEEN THE SOLAR EUV AND 10.7 CM FLUXES DURING AN INTENSE SOLAR-CYCLE

Results obtained from an analysis of the ionospheric electron content (IEC) data collected at several stations in the Northern Hemisphere during December 1980-December 1985, when the 10.7 cm solar flux index (F10.7) varied from 66 to 303, are presented. Diurnal maximum IEC value (IEC(max)) increases linearly with F10.7, as expected, for values of F10.7 less than about 200; at higher values of F10.7, contrary to expectation, IEC(max) saturates at all stations. The observed variation of IEC(max) is interpreted as convincing ionospheric evidence for a nonlinear relationship between solar e.u.v. and 10.7 cm fluxes during intense solar cycles. Variation of the e.u.v. flux obtained from the latest version of the SERF2 solar e.u.v. flux model for the intense solar cycle 21 agrees with this interpretation. Lyman-alpha (1216 angstrom) and He I (10,830 angstrom) equivalent width data, measured during the same period as the IEC observations and used as independent data sets in the solar e.u.v. flux model, also show nonlinear variations with F10.7.