Trends in critical frequencies and layer heights over Tromso and their consequential impact for radio system modelling Original Research Article

A unique resource providing continuous observations of the critical frequencies since 1935 and layer heights since 1952 has beenused to determine the long-term trends above Tromsø in Northern Norway (69°N, 19°E). These, and the recent results of Ulich et al. [Ulich, T., Cliverd, M.A., Rishbeth, H. Determining the long-term change in the ionosphere. EOS Trans. 84(52), 581–585, 2003] have then been used to illustrate the effects of layer height changes on radio system modelling and operation by reference to high frequency ground range and skip zone variations. The analysis ignores the effects of the variations in the underlying E-region and changes in the F2 layer critical frequencies. Consequently, the system impact is expected to be higher still. It is shown that a 16 km drop in hmF2 will introduce ground errors of ∼100 km with higher values under certain circumstances. The Tromso analysis indicate that these ground range errors will not occur until the year 2110, however, the results of Ulich et al. [EOS Trans. 84(52), 581–585, 2003] indicate that this condition occurred in the year 2000. This study demonstrates an urgent need to reach a consensus understanding of long-term ionospheric trends. A reassessment of the CCIR and URSI worldwide maps of ionospheric characteristics is likely necessary.