This study explores the relationship between the sunspot number (SSN) and t
he ionospheric foF2. It is of interest to locate the SSN value at which the
foF2 values are saturated. A regression model is built based on the data o
f the strictly rise period of the 21st solar cycle recorded by eight ionoso
nde stations scattering roughly between 40 degrees N and 40 degrees S geoma
gnetic latitude. Results show that clear saturation features appear around
the equatorial anomaly crest region. (C) 2000 Elsevier Science Ltd. All rig
hts reserved.