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Radar observations at VHF on equatorial spread F (ESF) made at Gadanki (13.5°N, 79.2°E; magnetic latitude 6.3°N) and Trivandrum (8.5°N, 77°E; magnetic laitude 0.3°N) are presented in the form of height-time maps of signal intensity and Doppler velocity. The peak signal intensities are found to be 30–40 dB above the noise level at Gadanki and, normalized to the same system sensitivity, about 6 dB higher at Trivandrum. The discrete plasma structures and the phase velocities of the 3m irregularities observed at Gadanki are well correlated to that observed at Trivandrum at the height linked by the same flux tube. The height-time-intensity maps show both updrafting and downdrafting of the plasma structures; the downdrafting observed on one occasion at Gadanki is somewhat unusual in that it extends down to the E region. The Doppler velocities observed at Gadanki show that the highest values are encountered in the rising plumes with the upward velocities ranging from 100 to 300 ms−1. The velocities are predominantly downward in the bottomside F region, particularly during the later phase of the ESF development. On occasion, the downdrafting is observed well into the topside, reaching as high as 550 km, which emphasizes the influence of electric field being extended to a greater height as pointed out by Anderson and Haerendel [1979] in their model based on flux tube integrated quantities. The downdrafting velocities range from 20 to 100 m s−1. These values being mostly well above the background plasma drift velocity, the downdrafting structures are to be regarded as plasma depletions. |
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