Oblique ionogram features associated with off-great circle HF propagation at high and sub-auroral latitudes

Oblique ionograms received over northerly paths are often difficult to interpret, with many features not being of the form observed on the more familiar mid-latitude soundings. In particular, the ionograms contain unusual traces at frequencies close to and above the conventional or basic maximum usable frequency, i.e. above the junction frequency of the high and low angle modes. A series of experiments have been undertaken over a period of several years to investigate off-great-circle propagation mechanisms over high latitude and sub-auroral oval paths. In addition to measurements of the signal direction of arrival, oblique ionograms were also recorded over several of the paths. By these means, the characteristic ionogram signatures associated with off-great-circle propagation effects arising due to the presence of convecting patches of enhanced ionisation or sun-aligned arcs within the polar cap ionosphere, or to the presence of steep gradients to the side of the path within the mid-latitude trough region, have been identified.