Abstract
Abstract. A transequatorial radio-wave propagation experiment at shortwave frequencies (HF-TEP) was done between Shepparton, Australia, and Oarai, Japan, using the radio broadcasting signals of Radio Australia. The receiving facility at Oarai was capable of direction finding based on the MUSIC (Multiple Signal Classification) algorithm. The results were plotted in azimuth-time diagrams (AT plots). During the daytime, the propagation path was close to the great circle connecting Shepparton and Oarai, thus forming a single line in the AT plots. After sunset, off-great-circle paths, or satellite traces in the AT plot, often appeared abruptly to the west and gradually returned to the great circle direction. However, there were very few signals across the great circle to the east. The off-great-circle propagation was very similar to that previously reported and was attributed to reflection by an ionospheric structure near the equator. From the rate of change in the direction, we estimated the drift velocity of the structure to range mostly from 100 to 300 m/s eastward. Multiple instances of off-great-circle propagation with a quasi-periodicity were often observed and their spatial distance in the east-west direction was within the range of large-scale traveling ionospheric disturbances (LS-TIDs). Off-great-circle propagation events were frequently observed in the equinox seasons. Because there were many morphological similarities, the events were attributed to the onset of equatorial plasma bubbles.
Highlights
After the pioneering work done by Rottger (1973), not much attention has been paid to high-frequency transequatorial propagation (HF-TEP) experiments using a direction finding technique in ionospheric studies, except for a short campaign at a Peruvian longitude (Flaherty et al, 1996)
If the local time pattern of changes in the layer height at a given location is equivalent to the longitudinal structure over the equator at a given instance, the bottomside isodensity surface will tilt over 3000 km, reaching higher altitudes in the east, causing the shift of the direction of arrival (DOA) to the west
An HF transequatorial propagation experiment was conducted by receiving broadcasting signals of Radio Australia transmitted from Shepparton near Melbourne, Australia
Summary
After the pioneering work done by Rottger (1973), not much attention has been paid to high-frequency transequatorial propagation (HF-TEP) experiments using a direction finding technique in ionospheric studies, except for a short campaign at a Peruvian longitude (Flaherty et al, 1996). Rottger (1976) suggested that a macro-scale structure of the equatorial ionosphere that causes off-great-circle propagation is related to the formation of equatorial spread-F (ESF) irregularities through a spatial resonance mechanism that amplifies traveling ionospheric disturbances. The nonlinear evolution of the Rayleigh-Taylor instability forms density depleted plasma bubbles, which intrude into the topside and extend along the magnetic field line over several thousand kilometers. Satellite in-situ observations revealed density biteouts in the topside ionosphere. Weber et al (1982) related the topside density biteouts measured by the AE-E satellite to bottomside signatures through airglow and ionosonde measurements. Local uplifts of bottomside isodensity surfaces cause a 630-nm airglow depletion and oblique ionosonde echoes from local electron density gradients (Weber et al, 1980).
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