Low altitude quasi-periodic radar echoes observed by the Gadanki VHF radar

We report here on the low altitude quasi-periodic (LQP) radar echoes from low latitude sporadic E layer (Es) observed by the Gadanki (13.5°N, 79.2°E; geomagnetic latitude 6.3°N) VHF radar. The LQP echoes occurred both during daytime and nighttime and are confined to a slowly descending layer with a thickness of about 2-4 km in the height range of 90-100 km. The periods are found to range from tens of seconds to less than about 3 minutes. The Doppler velocities vary over a range of -20 to +20 m/s during daytime and 0 to 10 m/s during nighttime and are known to be sensitive to the layer height, being dominated by the effect of zonal electric field above and meridional neutral wind below a height of ∼97 km for the type 2 irregularities [Krishna Murthy et al., 1998]. The spectral widths are found to be of the order of 50 to 75 m/s during daytime and 30 to 60 m/s during nighttime. The LQP echoes reported here are similar to that observed recently over midlatitudes [Rao et al., 2000; Urbina et al., 2000], but distinctly different from the widely reported quasi-periodic (QP) echoes occurring at higher altitudes (>100 km). The observations are discussed briefly in terms of the potential source mechanisms.