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Hourly values of the critical frequency of the ionospheric F region, foF2, obtained at Chongqing ionospheric observatory&#13; (geographic 29.50N, 106.40E), China, during the interval of 1977 to 1997 (solar cycle 21 and 22) have&#13; been used to investigate the dependence of the monthly median foF2 on solar activity and geomagnetic activity,&#13; and to construct single-station model (SSM) using Fourier expansion. The results of the present analysis show&#13; that there is a significant nonlinear relationship between monthly median foF2 and sunspot number R especially&#13; at daytime in each month and nighttime in summer. Furthermore, introducing geomagnetic index Ap further&#13; improves the description of variation of monthly median foF2, and the standard deviations decrease also dependently&#13; on time and month. Thus the complex influence of solar activity and geomagnetic activity may be approximately&#13; expressed by a general multiple nonlinear function at Chongqing station, China. The basic aim of this&#13; paper is to show that considering the nonlinear influence both of solar activity and geomagnetic activity improves&#13; the monthly median model. The SSM is in good agreement with observations, with standard deviation&#13; 0.65MHz, while IRI model has a lower accuracy with standard deviation 0.96MHz

A single-station spectral model of the monthly median foF2 over Chongqing, China