Solar Cycle Influence on the Lower Ionosphere and on VHF Forward Scatter

A brief survey of published literature indicates that the electron concentration in the D region of the ionosphere changes with the solar cycle. Such changes should also be apparent in VHF forward scatter signal intensities. Analyses have therefore been made of seven years´ results, at 49.8 mc from the Cedar Rapids, Iowa, to Sterling, Va., forward scatter path (1243 km). On an annual basis, it is found that the monthly median received signal intensity, for the noon and afternoon period only, follows the same trend as the mean monthly sunspot number. The effect is still more pronounced when comparison is made with magnetic disturbance indexes. However, when the comparison with magnetic indexes is made using a direct comparison in each 3-hour period, and eliminating long-term trends, no observable increase of the signal intensity is found as the magnetic conditions become disturbed. This is in sharp contrast to the behavior of such circuits in auroral regions. Further analysis shows that a weak increase in signal intensity with a rising magnetic index does occur if the analysis is confined to the midday period. It is concluded, therefore, that any magnetic influence is smaller on middle than on high latitude scatter circuits, but an effect can be clearly discerned by studying the magnitude of the received signal through a solar cycle. These results show that VHF forward scatter signals are not caused by reflection from meteor trails alone.