Long-term relationship between foF2 from China ionospheric stations and solar activity during the 24th solar activity cycle

This paper employs the complete data of six ionospheric observation stations in China in the 24th solar activity cycle and researches the variation of frequency of the F2 layer (foF2) and the relationship with the solar activity indices. We discover that the dependence of foF2 on the sunspot number (R) and the solar radio flux of 10.7 cm wavelength (F10.7) is strongest in mid-solar years, and weakest in high solar years. At the same time, the long-term variation trends of foF2, R and F10.7 in the complete 24th solar activity cycle are analyzed using the mutual information method in information theory. We utilize the linear and nonlinear methods to compare the effects of long term change relationship between foF2 and R during the rising and falling phases of solar activity. The findings reveal that the sunspot number is more suitable to depict the long-term trend of foF2 in China, and the relationship between them is better described by nonlinear quasi-polynomial. This result has important implications for improving foF2 forecasts and the prediction accuracy of some ionospheric models.