SPREAD-ES STRUCTURE PRODUCING APPARENT SMALL-SCALE STRUCTURE IN THE F-REGION

When transmitting on 5.8 MHz the Bribie Island HF radar array synthesi zes a beam that is 2.5-degrees wide. The beam can be steered rapidly a cross the sky or left to dwell in any direction to observe the fading rates of echoes within a small cone of angles. With the beam held stat ionary, the time scale associated with deep fading of F-region echoes is usually more than 5 min. This is consistent with the focusing and d efocusing effects caused by the passage of ever-present medium-scale t ravelling ionospheric disturbances (TIDs). On occasion the time scale for deep fading is much shorter, of the order of tens of seconds or le ss, and this is thought to be due to the interference of many echoes f rom within the beam of the radar. It is shown that the echoes are not due to scatter from fine structure in the F-region, but rather due to the creation of multiple F-region paths with differing phase lengths b y small, refracting irregularities in underlying, transparent spread s poradic-E, (spread-E(s)). The natural drift of the spread-E(s) causes the phase paths of the different echoes to change in different ways ca using the interference. Two methods are used to investigate the rapidl y fading F-region signals. Doppler sorting of the refracted F-region s ignal does not resolve echoes in angle of arrival suggesting that many echoes exist within a Fresnel zone [WHITEHEAD and MONRO (1975), J. at mos. terr. Phys. 37, 1427]. Statistical analysis of F-region amplitude data indicates that when the range spread in E(s) is severe on ionogr ams, then a modified Rayleigh distribution caused by the combination o f 10 or so echoes is most appropriate. Using knowledge of the refracti ng process the scale of E(s) structure is deduced from these results. Both methods find a spread-E(s) irregularity size of the order of 1 km or less. It is proposed that the Rayleigh type F-region signals seen by JACOBSON et al. [(1991b), J. atmos. terr. Phys. 53, 631 are F-regio n signals refracted by spread-E(s).