The method of determination of the long-term trends in the F2 region independent of geomagnetic activity

A detailed analysis of the f oF2 data at the Sverdlovsk station is performed to answer the question: whether the long-term trends of the F2-layer parameter detected recently are mainly due to the long-term changes in geomagnetic activity during the recent decades. Two methods to derive trends independent of geomagnetic activity are developed. It is found that both methods agree well and give a relative nongeomagnetic trend of about −0.0015 per year (or an absolute nongeomagnetic trend of about −0.015 MHz per year). The close relation of f oF2 to Ap found by several authors is mainly due to a high correlation between the f oF2 deviations from the regression line and year-to-year changes in the Ap index rather than to systematic long-term changes of Ap as has been suggested earlier.