Solar cycle variation and single-station model of the monthly median foF2

The hourly values of the critical frequency of ionospheric F region, f(o)F(2), observed at nine ionosonde stations in China about two solar cycles have been used to investigate the dependence of the monthly median f(o)F(2) on solar activity and geomagnetic activity, and to construct a single-station model (SSM) using Fourier expansion. The results of the present analysis show that there is a significant nonlinear relationship between monthly median f(o)F(2) and sunspot number R especially at daytime in each month and nighttime in summer and the nonlinear relationship is more evident as the latitude decreases. Furthermore. introducing geomagnetic index Ap further improves the description of variation of monthly median f(o)F(2), and the standard deviations decrease also dependently on time and month. Based on the nonlinear relationships, we develop a new SSM for the selected stations and the SSMs are compared with IRI, which shows a much better agreement with observations.