Large HF bearing errors for propagation paths contained within the polar cap

Large electron-density gradients in the polar-cap ionosphere, associated with the convection of patches of overdense plasma and the presence of high-latitude auroral arcs, produce tilted reflecting surfaces which result in HF signals propagating over paths well displaced from the great circle. Measurements have been made at a very high-latitude receiving site of signals propagated over paths contained within the polar cap. Systematic swings in the bearing of the received signal of up to ±100° from the great circle path were recorded and their characteristics related to the directional components of the interplanetary magnetic field and the level of geomagnetic activity, as these geomagnetic parameters play an important role in determining the state of the high-latitude ionosphere.