Some initial results on mid-latitude spread- F irregularities using a directional ionosonde

Abstract

Specific phase delays imposed on returning ionospheric signals received at 8 antennas in one antenna arm (500 m long) are effective in producing an interference pattern (which is displaced for off-vertical angles) on what would otherwise be a standard ionogram. Recordings from 2 antenna arms at right angles allow the calculations of zenith angles and azimuths-of-arrival for returning ionospheric signals. The system is therefore a directional ionosonde. The records show that ionospheric spread- F signals arrive predominantly from directions in the northwest quadrant consistent with earlier experimental results from Brisbane. Also, the records suggest that at these times of spread-F occurrence it is not unusual to find ‘tongues of ionization’ which extend some tens of kilometres below the normal level of the F2-layer. These observations are consistent with earlier reports of the existence of almost-vertical ionospheric contours of equal-ionization density. For one particular event which was analysed in detail an enhanced maximum electron-density of the F2-layer was found at the position of the ‘tongue’. The occurrence of isolated patches of sporadic E with associated spreading of the E s trace (spread- E s ) at the times of occurrence of 18 ‘tongue’ events has been found to be statistically significant. It is suggested that a knowledge of the existence of these tongues at the time of occurrence of mid-latitude spread- F may give a new lead in the quest to explain how and why spread- F irregularities are formed.