The Non‐Linear Dependence of Daily Maximum Ionospheric Total Electron Content on F10.7

Abstract Daily 10.7 cm solar flux, the F10.7 index, is a key parameter for the ionospheric state. Global maps of ionospheric Total Electron Content (TEC) offer almost continuous multidecadal coverage. We find that the global daily maximum of TEC saturates with daily F10.7. We determine how F10.7 orders the properties of daily maximum TEC measured from 15‐min Global Ionospheric Maps (GIMs) available for calendar years 2003–2024. These maps are generated by the Jet Propulsion Laboratory (JPL), California Institute of Technology. For each day of data we identify the largest TEC GIM maximum. For the full solar cycle 24 we compare linear and non‐linear least squares fitting of the relationship between the daily F10.7 and the daily TEC maximum. We find that a tanh function provides a better fit to the full range of F10.7 than the linear fit, enabling us to quantify the saturation values of the daily maximum TEC, which are seasonally dependent between 83 and 128 TECU for daily F10.7 values 78–85 SFU. We quantify the quality of fit as a function of F10.7. For F10.7 150 SFU, the residuals for the tanh fit follow a normal distribution more closely than those of the linear fit. At lower values of F10.7, both linear and tanh fits perform well. The JPL‐GIMs span the declining phase of cycle 23, the full cycle 24 and the ascending phase of cycle 25. Results for the partially sampled, but more active cycles are qualitatively consistent with that found for cycle 24, and motivate ongoing study of cycle 25.