Ionospheric Irregularities and Propagation at Frequencies Above the ‘Classical’ M.U.F.

Abstract

Sweep frequency pulse measurements, carried out elsewhere, have shown that the maximum usable frequency via the F1 and F2 layers may be extended by an anomalous mode of propagation. The extension may be 10–15 per cent above the value calculated from classical theory and is observed only during daytime. It is absent at night. The phenomenon has been ascribed by some authors to scattering by irregularities present in the reflecting layers.

Attention is drawn in this connection to the weak short-wave reflections observed at vertical incidence from heights below the normal E layer. These reflections are obtained during day-time and the reflecting layer merges during night-time with the meteoric E layer responsible for V.H.F. forward scatter propagation. It is argued that the extension of the ‘classical’ M.U.F. may be due to scattering produced by the irregularities responsible for the above weak reflections. It is possible that a part of the energy enters the skip zone due to forward scatter during the upward and the downward path of the wave through the region containing the irregularities.