The effects of polar-cap absorption on HF oblique-incidence circuits

Data from HF oblique-incidence ionosonde circuits were studied to determine to what extent they were affected by the radio absorption events now known as polar-cap absorption (PCA). The data were recorded on three circuits terminating in Ottawa (from Churchill, Resolute Bay, and The Hague) and on a fourth circuit between Churchill and Resolute Bay. A relatively weak event caused an appreciable decrease in the range of usable frequencies on the Canadian circuits. More intense absorption resulted in blackout conditions on all circuits, including the transatlantic. The effects on these circuits could be interpreted from a consideration of the geometry of the propagation modes involved and of the pattern of PCA events that has been established with other experimental equipment, particularly riometers. The regular oblique ionosonde data on the Ottawa-Resolute Bay circuit were supplemented for a short time by amplitude measurements. These data showed that, although PCA reduced the signal strength on all frequencies received, the noise was absorbed at the same time; hence, the signal-to-noise ratio was not appreciably affected. This result is probably dependent on the location of the circuit.