Abstract
Hourly values of the critical frequency of the ionospheric F region, foF2, obtained at Chongqing ionospheric observatory (geographic 29.50N, 106.40E), China, during the interval of 1977 to 1997 (solar cycle 21 and 22) have been used to investigate the dependence of the monthly median foF2 on solar activity and geomagnetic activity, and to construct single-station model (SSM) using Fourier expansion. The results of the present analysis show that there is a significant nonlinear relationship between monthly median foF2 and sunspot number R especially at daytime in each month and nighttime in summer. Furthermore, introducing geomagnetic index Ap further improves the description of variation of monthly median foF2, and the standard deviations decrease also dependently on time and month. Thus the complex influence of solar activity and geomagnetic activity may be approximately expressed by a general multiple nonlinear function at Chongqing station, China. The basic aim of this paper is to show that considering the nonlinear influence both of solar activity and geomagnetic activity improves the monthly median model. The SSM is in good agreement with observations, with standard deviation 0.65MHz, while IRI model has a lower accuracy with standard deviation 0.96MHz.
Highlights
Among ionospheric characteristic parameters, the critical frequency of the F region, foF2 is one of the most important ones
Because global models may smear out features unique to a particular region, single-station models are very useful and widely viewed as the milestone of any ionopheric services (Holt et al, 2002, Pancheva and Mukhtarov, 1998; Liu et al, 2004), and there is no stationspecific model over Chongqing, we developed a SSM of monthly median foF2 based on the observations from 1977 to 1997
Rather than monthly average or monthly median indices. The latter may present the influence of solar or geomagnetic activity on ionopheric average action more effectively in specific month, and some empirical models chosen these indices (e.g. Zolesi et al, 1993; Liu et al, 2004), the indices are difficult to predict with a long lead time because of their severe variations
Summary
The critical frequency of the F region, foF2 is one of the most important ones. To take into account this behavior, a second-degree relationship between foF2 and solar activity indices is used frequently (Sizun, 1992; Xenos et al, 1996; Pancheva and Mukhtarov, 1998). This «hyseresis» effect depends on latitude (Sethi et al, 2002) and historical solar activity (Rao and Rao, 1969; Liu et al, 2004). Pancheva and Mukhtarov (1996) developed a single-station spectral model over Sofia with R and KR and demonstrated that in many cases the increase in the monthly standard deviation coincided with the increase in the geomagnetic activity index aa.
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