Secular change of the ionospheric peak height at northern midlatitudes

The routinely reduced ionospheric parameter M(3000)F2 [M] is inversely related with the peak altitude (Shimazaki, 1955; Bilitza et.al., 1979). It undergoes quite important diurnal, day-by-day, seasonal and solar cycle variations. A potential secular change, though apparent in the original data for midnight is by day drowned in the regular seasonal variation. We show that it can even by day be identified after elimination of all shorter term variations i.e. after suitable averaging. Doing so with the monthly median data of a british station at (51.5o N, 0.6o W), and comparing the data of solar cycles 22 and 18 (i.e. over an interval of about 40 years) we find a clear effect by day as well as by night. This change is, however, far from being uniform, since it depends of the phase in the solar cycle. Long-term changes of the peak altitude between 0 km and more than 40 km were found with smaller values by day than by night. Particularly small secular changes occur during the period just after the maximum of the solar cycle.