Ionospheric evidence for a nonlinear relationship between the solar e.u.v. and 10.7 cm fluxes during an intense solar cycle

Abstract

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 ( F 10.7) varied from 66 to 303, are presented. Diurnal maximum IEC value (IEC max) increases linearly with ( F 10.7), as expected, for values of ( F 10.7) less than about 200; at higher values of ( F 10.7), 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-α (1216 Å) and He I (10,830 Å) 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 ( F 10.7).