Radar echoes from 3-m plasma irregularities of ionospheric sporadic E (E-s) layer are analyzed and investigated in this article by using the Chung-Li VHF radar. The interferometry result indicates that the E-s echoes extending from about 109-122 km altitude are extremely aspect sensitive with enormously narrow aspect angle of +/-0.25 degrees. After reconstructing the three-dimensional structure of the echoes, we find that the object responsible for the radar returns is an isolated and solid blob-like plasma structure with elliptical cross section. The average lengths of the major and minor axes of the cross section are, respectively, 8.7 and 6.4 km, in which the major axis is primarily parallel to north-south direction and minor axis lies in east-west direction. A comparison between the so-called trace velocity V-t deduced from the bulk displacement of the plasma structure projected on three mutually orthogonal planes and the true velocity V-a estimated from the least-squares fit is also made. We find that for the present case in average V-a is systematically larger than V-t by a factor of 2. In addition, interferometry analysis shows that the positive slope of the echoes structure (range rate) in the range-time-intensity plot and the negative Doppler frequency shift of the echoes are attributed to the westward motion of the plasma structure. This result strongly suggests that it is inappropriate to infer the dynamic behavior of the E-s irregularities only from the information in the range-time-intensity plot and the Doppler spectra, without acquiring the actual angular position and true velocity of the targets. (C) 2001 Elsevier Science Ltd. All rights reserved.
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JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS Volume: 63 Issue: 2-3 Pages: 123-133
