H.F. Propagation in the Arctic

This paper examines the results of several studies of oblique incidence frequency sounding and 30 Mc/s riometer absorption with the objective of using this information to increase the reliability of h. f. communications on auroral zone circuits. Studies have been carried out during low and high intensity polar-cap absorption (PCA) events, and moderate and severe geomagnetic storms, to determine the effect of these disturbances on h. f transmissions on a number of auroral zone point-to-point circuits and on a circuit from a ground station to an aircraft that was flown into, and through, the auroral zone. It was found that a transauroral and a subauroral zone circuit were less affected by polar-cap absorption than an inner auroral zone (polar-cap) circuit, while the transauroral circuit suffered the greatest deterioration during the main phase of moderately severe geomagnetic storms. Absorption measurements from riometer stations were used to establish that the majority of all occurrences of auroral absorption during certain geomagnetic storms were correlated over north-south distances of less than 900 km. It was confirmed that frequency sounding systems can be used to provide direct assistance to communication systems in the selection of optimum communications routes and operating frequencies during PCA events and geomagnetic storms. In order to match communications operating procedures to the highly variable characteristics of auroral sporadic-E and F-layer propagation during geomagnetic storms, it was found necessary to provide a well-spaced selection of assignments. Finally, because the reception quality on operating frequencies often changed markedly within 15 min, it was necessary to sample propagation conditions on available assignments at intervals not longer than this.