Some recent measurements of the direction of arrival of HF signals propagated over a path tangential to the mid-latitude trough

Horizontal electron density gradients (tilts) in the ionosphere are responsible for the deviation of HF (330 MHz) radiowaves from the normal great circle propagation path (GCP). The resulting deviations are often far in excess of the instrumental accuracy (~0.1°) of direction finding (DF) systems and can amount to errors of over 50° at high latitude sites where relatively large ionospheric tilts may occur. One region of the ionosphere in which strong electron density gradients occur is the mid-latitude trough. This trough is an F-region electron density depletion region a few degrees wide in latitude on the equatorward edge of the auroral oval in which the critical frequencies drop by a factor of 2-4 or more and the altitude of the electron density maximum rises by over 100 km. The mean positions of the poleward and equatorward walls of the trough and its extent in local time as a function of Kp have been calculated by means of the regression equations of Halcrow and Nisbet (1977). An example of the modelled trough positions for an equinox day is presented for various levels of geomagnetic activity