Propagating ionospheric waves observed throughout east Asia during the WAGS October 1985 campaign

The propagation of acoustic gravity waves has been observed by an investigation of their effects upon the ionosphere over distances ranging up to 3400 km and extending from mid-latitudes (45.4°N) to low latitudes (14.7°N) in east Asia. Recognizable wave structures were observed mainly on low sounding frequency, virtual height h'f and to a lesser extent foF2 time variations, obtained from ionograms taken at 10 ionosonde stations, sounding at 5-min intervals throughout the period October 15-19, 1985. Investigations were mainly confined to the nighttimes, since during the day large latitudinal gradients of ionization (associated mainly with the equatorial anomaly) together with movements of the equatorial anomaly crest obscured the observation of waves. In general, inferred propagation of the waves was southward, but three cases of northward propagation were observed, the former having a range of periods 40-210 min, while the latter were consistently about 70 min. The waves structures, mostly, differed depending upon whether they were located to the north or south of Yamagawa (31.2°N). There was some evidence of the predomination of longer-period waves to the south, the shorter-period waves observed to the north being apparently damped out in their propagation southward. The southward propagating waves exhibited upward energy propagation, while at Hong Kong (22.3°N) there was some evidence of downward phase propagation with a shift to lower periods at lower heights. It has not been possible to identify the origins of the detected waves, but it is clear that sources existed within and outside the latitude range covered by the ionosonde sounding stations (14.7°-45.4°N).