Summery. Variation of E-layer critical frequency with solar activity

Ionospheric f0E data over one or, where available, two solar cycles from 47 stations all over the world are correlated with the corresponding data of each of the three solar indices, the Zÿrich relative sunspot number R, the solar radio noise flux ¿ at 10.7 cm and the ionospheric index of solar activity IF2, and numerical expressions are derived. There is no difference in the degree of correlation between the variates of each of the above expressions at the 5% level. At each station, the monthly values of the sensitivity to solar activity changes are independent of season and their 12-month average is independent of the latitude of the station. The uncertainty in the relations is mainly due to inconsistency between solar-cycle variations of f0E at different stations, butR appears less reliable than the other two parameters.