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<p><strong>Abstract.</strong> We have examined the long time series of observations of E-region virtual height (1948&ndash;2006) and critical frequency (1935-2006) hitherto made by the Tromsø ionosonde at 70&deg; N, 19&deg; 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 <i>h'E</i> also suggests a negative trend, a closer examination reveals a possible weakdoi:10.5194/angeo-25-2351-2007 fatcat:iwef4mspgjcg3jzhr3eidwqlyi