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Ann. Geophys., 24, 1401-1409, 2006
www.ann-geophys.net/24/1401/2006/
© European Geosciences Union 2006
Equatorial ionospheric disturbance observed through a transequatorial HF propagation experiment
T. Maruyama and M. Kawamura
National Institute of Information and Communications Technology, Koganei, Tokyo, 184-8795, Japan
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.
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