We describe a new class of nonthermal plasma density irregularities observed in the postmidnight topside equatorial ionosphere under low solar flux conditions. They are distinct from irregularities associated with equatorial spread F (ESF) in terms of their morphology and because they exhibit strong spectral sidebands at the lower-hybrid frequency. The coherent echoes were observed in a series high-altitude radar experiments performed at Jicamarca utilizing long- and coded double-pulse modes and a dual-beam mode. The coded double-pulse mode was used to measure the low-frequency characteristics of the echoes with fine range resolution. Doppler shifts of the main backscatter line were observed to fall between ±150 m/s. The long-pulse mode was employed for high-frequency spectral analysis which revealed the presence of strong spectral sidelobes at the lower-hybrid frequency. A dual-beam mode was used to investigate the horizontal structure of the echoes. Zonal drift speeds of 50–70 m/s were inferred with this mode, and longitudinal dimensions of approximately 270 km were estimated. The study summarizes with a discussion of different mechanisms that may be responsible for the phenomenon and the lower-hybrid sidebands in particular.
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