HF Doppler observations associated with spread F

Abstract

This paper discusses HF Doppler spectrum observations at mid‐latitudes when spread F was observed on local ionograms. The data indicate that Doppler broadening is related to spread F (they both commence simultaneously). The results can be simulated by using different ionospheric models. Calculations show that a rippled contour can cause HF Doppler broadening and range spreading on ionograms if the structure is traveling horizontally. Although this mechanism can lead to HF Doppler broadening, scattering at 4.5 and 7.5 MHz in an anisotropic medium from near overhead can also explain the experimental results. The observation of aspect‐sensitive scatter does, however, depend critically on the statistical description of the irregularity spectrum. It is suggested that an experiment designed to measure the Doppler shift as a function of zenith angle may enable some of the current ambiguities associated with spread F to be eliminated.