Abstract
We put forward a method of separating the geomagnetic activity contribution to the F2-layer critical frequency median, foF2med, at middle latitudes. It is based on the analysis of dfoF2, which is the ratio foF2med/foF2q in percent, where foF2q is the F2-layer critical frequency for quiet conditions. The quantities foF2q and dfoF2 depend on solar and geomagnetic activity respectively. These dependences are taken into account using indices F12 (the average over 12 months flux of solar radiation at 10.7 cm) and Apm (the average over a month value of Ap-index), thus facilitating the use of this method for forecasting foF2med. With this method, from Slough station (51.5° N, 0.6° W) data for midday and midnight for 1954 to 1995 we have found that at midnight the dfoF2 dependence on Apm is significant at the 95 % confidence level for equinoxes and summer. For midday, this dependence is less pronounced and is significant only from April to July. At equinoxes and summer, an Apm increase causes a dfoF2 decrease. For midnight, this feature is more pronounced than for midday. This regularity is valid also for annual average Apm and dfoF2.
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