Statistical investigation of the saturation effect in the ionospheric foF2 versus sunspot, solar radio noise, and solar EUV radiation

This study explores the saturation effects in the ionospheric foF2 due to sunspot number R, solar radio noise (10.7 cm) flux F10.7, and solar EUV fluxes. To locate the R, F10.7, or EUV value at which the foF2 values are saturated, a two-segmented regression model is built based on the data of the strictly rise period of the 21st solar cycle recorded by eight ionosonde stations scattering roughly between 40°N and 407deg;S geomagnetic latitude. Results show that clear saturation features appear around the equatorial anomaly crest regions. The regression model is then fitted into the foF2 data observed at Chung-Li station (13.8°N, geomagnetic) to investigate the hourly variation of the saturation effect. The saturated foF2 slope and the value of the R or EUV at which the foF2 values are saturated in the fitted models are closely related to the overall mean and standard deviation of foF2. The hourly results demonstrate that the daily ionospheric equatorial fountain and prereversal enhancement are important for the saturation features.