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 (45.4øN) to low latitudes (14.7øN) in east Asia. Recognizable wave structures were observed mainly on low sounding frequency, virtual height hf 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 (31.2øN). 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 (22.3øN) 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 (14.7ø-45.4øN). munications, Tokyo. '•Telecommunication Training Institute, Taipei. •Manila Observatory. larly prevalent during magnetic storms (see review by Hunsucker [1982]). The so-called "large-scale" AGWs of period 30 m•n to 3 hours have been observed to propagate equatorward over distances of several thousand kilometers, giving rise to periodic ionospheric variations differing in phase at meridional, spaced stations [e.g., Morgan, 1983]. The "medium-scale" AGWs of period 15 min to 1 hour, though, seem to propagate over much shorter distances (less than 1000 km) and may have as their sources meteorological disturbances I-Waldock and Jones, 1986]. Previous investigations of AGWs have been mostly confined to mid and high latitudes. Recently, the detected wave propagation characteristics have been obtained from simultaneous measurements at closely separated stations, for example, using ionosonde sounding at 150 km spacing [Morgan, 1983; Tedd and Morgan, 1985] and using HF Doppler at 80 km spacing [Waldock and Jones, 1987]. Traveling ionospheric disturbances (TIDs) have been detected, moving over long distances (3000 km), from recognizable "kinks" in ionograms [e.g., Heisler, 1963]. Although such disturbances may have been associated 867 868 WALKER ET AL.' IONOSPHERIC WAVES IN EAST ASIA WALKER ET AL.: IONOSPHERIC WAVES IN EAST ASIA 869
