Climatic trends in E-region critical frequency and virtual height above Tromso (70 degrees N, 10 degrees E)

We have examined the long time series of observations of E-region virtual height (1948-2006) and critical frequency (1935-2006) hitherto made by the Tromso ionosonde at 70 degrees N, 19 degrees E. Combining a simplistic trend analysis with a rigorous treatment of errors we identify a negative trend in critical frequency. While a similar analysis of the virtual height h'E also suggests a negative trend, a closer examination reveals a possible weak positive trend prior to similar to 1975 and a strong negative trend from similar to 1975 to present. These two metrics of essentially the same feature of the ionosphere do not exhibit the same signature since critical frequency is controlled by photochemistry within the E-layer while height is controlled by pressure level. We further find that the trend in critical frequency is a daylight/summer phenomenon, no significant trend being evident in the winter subset of the data. On the other hand, the trends in virtual height are independent of season/daylight.