Abstract
A review of the studies of long-term trends in the ionospheric E and F regions is given. It is shown that there are two principal methods of trend determination: absolute and relative. The main details of both methods are described and illustrated by particular results. It is noted that there are some disadvantages in the absolute trend method which might have led to significant differences of the results obtained by this method from those obtained by the relative trend method. The most significant result of the studies by the relative method is determination of a strong relation of the foF2 and hmF2 trends derived upon geomagnetic activity. Nevertheless one cannot explain all the trends in foF2 and hmF2 obtained by this method only in the scope of a geomagnetic control concept. An assumption that there is a “nongeomagnetic” component of the F2-region parameter trends seems inevitable. In the E region the results obtained by various authors using the absolute method give positive trends of foE which agree with the trends of the ion composition.
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