Daily Maximum Total Electron Content Saturation with Daily F10.7: Seasonal and Hemispheric Effects

The daily solar flux at 10.7cm, the F10.7 index, is commonly used as an input in ionospheric models. Typically studies have focused on either global averages or geographically local values of Total Electron Content (TEC), and how these vary with F10.7. Daily F10.7; F10.7A, which is the 81-day average; and F10.7p, which is a combination of these, are all considered. We study how the daily maximum TEC correlates with daily F10.7 [1]. We find that for F10.7 ≳ 78 − 85 SFU, the daily maximum TEC saturates to a seasonally dependent value between 83 − 128 TECU. This saturation of TEC with F10.7 is not generally seen in global averaged TEC or F10.7A/F10.7p. Using 15-minute Global Ionospheric Maps (GIMs) from the Jet Propulsion Laboratory (JPL) between 2003-2024, we apply linear/non-linear least squares fitting on how the daily global maximum TEC varies with daily F10.7 and assess the quality of each fit and how the parameters vary in season for solar cycle 24. We examine the distribution of the residuals as a function of F10.7 and find that a tanh function out- performs a linear function for F10.7≥ 150 SFU. These results are sensitive to different hemispheres, as a result of the construction of JPL-GIMs. Finally, we find that the daily F10.7 clearly resolves the saturation of daily maximum TEC, while F10.7 based on the average does not. Reproducibility of the observed climate of TEC maxima, that is, how daily maximum TEC correlates with daily F10.7, provides a benchmark for ionospheric models forecasting the space weather response of TEC. This is vital for the integrity of position, navigation and timing systems and for the planning of Low Earth Orbit satellite operations.

[1] Cafolla, M.A., Chapman, S.C., Watkins, N.W., & Verkhoglyadova, O.P. (2026). The non-linear dependence of daily maximum ionospheric total electron content on F10.7. Space Weather, 24, e2025SW004745. https://doi.org/10.1029/2025SW004745