Calculations of long-term trends in the ionosphere are critically dependent on solar activity (solar cycle) correction of ionospheric input data. The standard technique is to compute the dependence on solar activity from the whole analyzed data set and analyze trend of deviations from solar activity dependence. However, if the solar activity dependence changes with time, the solar correction calculated from the whole data set may result in miscalculating the ionospheric trends.

As an ionospheric parameter we consider foE, which is equivalent to electron density maximum in the ionospheric E region. Here we use data of two European stations with the best long data series of parameters of the ionospheric E layer, Slough/Chilton and Juliusruh over 1975-2014 (40 years). Noon-time medians (10-14 LT) are analyzed. The trend pattern after removing solar influence with one correction for the whole period is complex. For yearly average values for both stations first foE is slightly decreasing in 1975-1990, then the trend levels off or a very little increase occurs in 1990-2005, and finally in 2006-2014 a remarkable decrease is observed. However, when the solar correction is calculated separately for the three above periods, we obtain a smooth slightly negative trend. While solar corrections for the first two periods are similar (even though not equal), the solar activity dependence of foE in th third period is clearly weaker.

Thus the stability of solar correction should be carefully tested when calculating ionospheric trends. This could also explain some changes of trends in time.