WAVE-NUMBER-RESOLVED OBSERVATIONS OF IONOSPHERIC WAVES USING THE VERY LARGE ARRAY RADIOTELESCOPE

During a campaign of observations in 1990, we used the Very Large Array (VLA) radiotelescope to study ionospheric waves via their transient signatures in the line of sight total electron content (TEC). The data analysis exploited the fact that VLA baselines (less-than-or-equal-to 35 km) are comparable with the waves' horizontal scale-lengths lambda/(2-pi), thus enabling us to measure the waves' horizontal phase velocity V. The observed waves consist of two main groups: as seen by numerous earlier studies, traveling ionospheric disturbances (TIDs) dominate at long periods (> 10(3) s); their horizontal phase speeds are typically V < 0.2 km s-1, and their azimuth distributions are quasi-isotropic. However, at short periods (< 3 x 10(2) s) the dominant waves are magnetic-eastward directed (MED), and these have phase speeds in the range 0.1 less-than-or-equal-to V < km s-1. The MED disturbances are too fast to be F-region drifting irregularities but are usually too slow to be acoustic waves. The MED waves observed to date cause very small r.m.s. TEC perturbations (less-than-or-equal-to 2 x 10(13) m-2). They are more active at night and are much more visible along South- rather than North-directed lines of sight. We are not aware of any prior reports of such waves.