This synoptic study of equatorial clutter used over 6200 long-range (8000 nautical miles, or 14,816 km) backscatter ionograms from the east coast over-the-horizon backscatter (OTH-B) radar system at Bangor, Maine, obtained from 1100 to 0500 LT during the 22-month period from October 1991 to July 1993. Data were collected in five 7.5°-wide azimuthal sectors (radar beams 1-6,1-8, 2-4, 2-8, and 3-5) from 58°T to 170°T. The backscatter clutter signatures can be identified with an equatorial clutter region covering a dip latitude range of ±40°. Equatorial clutter typically begins after sunset and continues through midnight, with a minimum of activity between 1700 and 2200 LT. The clutter range shows some dependence on solar activity, with a reduction in range from the radar from high to moderate solar activity periods. These data did not show any dependence of clutter on operating frequency or on magnetic activity. Synthetic backscatter ionograms were generated using the parameterized ionospheric model (PIM) for the ray trace technique. The analysis shows that the equatorial dome of the ionosphere favors the chordal mode of reflection from the south side (the farther side from the radar) of the dome, which produces horizontal traces as clutter signatures on the backscatter ionograms. The reduction in range at moderate solar activity is associated with the lowering of the mean height of the ionospheric dome. In beams 2-8 and 3-5 the ionospheric dome structure is associated with the equatorial (Appleton) anomaly. This dome structure geometry favors the chordal mode of reflection.
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