Abstract
A means of determining the F2-region critical frequency using noise measurements made on-board satellites orbiting above the F2-peak is investigated. Values of foF2 observed on iomograms recorded by the ISIS 2 satellite were compared against corresponding records of the ISIS AGC voltage trace superimposed on the ionograms. The difference between foF2 and the frequency at which the AGC trace displayed a continuous enhancement above the cosmic background level, was taken as a measure of how well noise measurements can be used to infer the sub-satellite value of foF2. It is seen that above regions of the globe where ground-based HF noise is high and the ionospheric structure does not display severe horizontal gradients, measurements of noise breaking through the ionosphere can be used to determine foF2 generally to within one MHz. Comparison of daytime and nighttime measurements over the ocean shows a considerably smaller difference between foF2 and noise breakthrough at night. It is postulated that this feature is due to multihop propagation modes that are attentuated less at night than during the day.
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