A nighttime sporadic E (Es) layer responsible for range spread Es trace in ionogram and quasi-periodic echoes scattered from 5-m field-aligned electron density irregularities (FAIs) is investigated by using a ground-based 30 MHz coherent scatter radar, ionosonde and FORMOSAT-3/COSMIC satellite. A comparison shows a positive correlation between the 30 MHz radar backscatter and critical frequency foEs of the Es trace, which are consistent with earlier observational results. Interferometry measurement indicates that the spatial patterns of the radar echoes from the FAIs were in patchy form with vertical extent of about a few kilometers and horizontal dimensions of about 10-20 km. The echo structures experienced quasi-periodic oscillations not only in vertical but also in horizontal directions with periods of approximately 5-6 min. Observed Doppler velocities of the FAIs also exhibited quasi-periodic oscillations with the same periods. From their downward phase movement, these oscillations were very likely attributed to the modulations of upward propagating gravity waves with vertical wavelengths of about 10-20 km. In addition, vertical shears of the FAI Doppler velocity were observed, which are believed to result from the change in the phase of gravity wave with height. A physical process is proposed in this article to account for the relation between gravity wave-perturbed neutral wind and height variation of the FAI Doppler velocity. It is believed that the oscillations in the Doppler velocity profiles are associated with the wave-induced polarization electric fields in a finite plasma structure in the azimuth direction. (C) 2010 Elsevier Ltd. All rights reserved.
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JOURNAL OF ATMOSPHERIC AND SOLAR-TERRESTRIAL PHYSICS
