The Non-Linear Dependence of Daily Maximum Ionospheric Total Electron Content on F10.7
By Martin Cafolla (University of Warwick)
Solar Extreme Ultraviolet (EUV) radiation drives ionisation in the upper atmosphere to create the ionosphere. The variability of the intensity of this radiation results in regions of high electron number density across the ionosphere, characterised by the Total Electron Content (TEC). The daily solar flux at 10.7cm, the F10.7 index, is commonly used as a proxy to EUV in ionospheric models. Typically studies have shown how either the global averages or geographically local values of TEC vary with daily F10.7, F10.7A (the 81-day average) and F10.7p (a combination of daily F10.7 and F10.7A). We study how the daily maximum TEC correlates with daily F10.7 using 15-minute Global Ionospheric Maps (GIMs) from the Jet Propulsion Laboratory (JPL) between 2003-2024.
We find that for F10.7 ≳ 78 − 85 SFU, the daily maximum TEC saturates to a seasonally dependent value between 83−128 TECU. We asses the distribution of the residuals from linear and non-linear least squares fitting as a function of F10.7, as demonstrated in the figure below, and find that a tanh function out-performs a linear function for F10.7 ≥ 150 SFU. Our 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. Quantifying the value at which the daily maximum TEC saturates with F10.7, and its seasonal dependence, specifies the requirements of systems that are sensitive to extremes in TEC, important in planning of Low Earth Orbit satellite operations.
See publication for more details:
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
