Dependence of the F2-layer critical frequency median at midlatitudes on geomagnetic activity

Abstract

Abstract. We put forward a method of separating the geomagnetic activity contribution to the F2-layer critical frequency median, f o F2 med , at middle latitudes. It is based on the analysis of δf o F2, which is the ratio f o F2 med /f o F2 q in percent, where f o F2 q is the F2-layer critical frequency for quiet conditions. The quantities f o F2 q and δf o F2 depend on solar and geomagnetic activity respectively. These dependences are taken into account using indices F 12 (annual average solar radio emission flux at 10.7 cm) and Ap m (monthly average Ap index), thus facilitating the use of this method for forecasting f o F2 med . 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 δf o F2 dependence on Ap m 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 Ap m increase causes a δf o F2 decrease. For midnight, this feature is more pronounced than for midday. This regularity is also valid for annual average Ap m and δf o F2.

Estimating the maximum of the smoothed highest 3-hourly aa index in 3 d by the preceding minimum for the solar cycle

“…Among various indices to quantitatively describe geomagnetic activity, the aa index (Mayaud, 1972), derived from the 3-hourly K index at two near-antipodal midlatitude stations in England and Australia, is the longest time series (since 1868) and has been widely used to analyze the long-term trend of global geomagnetic activity (Russell and Mulligan, 1995;Marat et al, 2017;Du, 2011b;El-Borie et al, 2019) and its correlation with both climate change (Cliver et al, 1998;Dobrica et al, 2009;Gavrilyeva et al, 2017) and solar activity (Echer et al, 2004;Prestes et al, 2006;Lukianova et al, 2009;Du, 2011a, c;Du and Wang, 2012a;Singh and et al, 2019). The minimum aa index (aa min ), at or near the minimum of the solar cycle, has been widely used to predict the maximum amplitude of the sunspot cycle (R m ), the socalled Ohl's precursor method (Brown and Williams, 1969;Ohl and Ohl, 1979;Du et al, 2009).…”