Secular increase of F2 peak altitude at northern temperate latitudes

Noon and midmight data over South-England of M(3000) F2 were compared for solar cycles 18 and 22. After suitable averaging the remaining long term variations were plotted as function of the increasing\decreasing smoothed sunspot number R. Systematic differences were found between the two cycles, in particular during increasing solar activity. With the well known relationship between the propagation factor M(3000)F2 and the height of the F2 peak, hmF2, the long term systematic increase in M(3000) F2 corresponds to a decrease of hmF2. The effect is, however, distinctly different for day and night and depends strongly on the phase in the solar cycle. The greatest decreases of hmF2 are found during night (median 31 km), This decrease, however, is different for different phases in the solar cycle. It varies from 8 km in the third quarter to around 40 km before R maximum. (C) 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.