On the Nature of F-Region Bottom-Type Echoing Layer Over Gadanki

Bottom-type echoing layers observed from Gadanki are presented and analyzed. The analysis is based on observations made using a substantial database of the 30 MHz Gadanki Ionospheric Radar Interferometer (GIRI) during 2017-2019, collocated digisonde observations, and a few days of overlapping volume observations made using the GIRI and the 53 MHz Advanced Indian MST Radar (AIR) . The echoes commence well before the E- and F-region sunsets and last as late as 23:00 Local Time. The echoing layers occur in the height range of 220-300 km with a thickness of 10-15 km and the Doppler spectra are narrow with spectral width confined to 10 m s(-1). The echoing layers are very common during the entire observational period and do not show any noticeable seasonal dependence. The zonal velocities of the irregularities responsible for the radar echoes are close to zero irrespective of the height of the echoing region. The echoes are stronger at 30 MHz than at 53 MHz indicating strong Bragg scale dependence. The height of the echoing layer agrees very well with the height of the F-region peak electron density (h(m)F(2)). Also, early commencement of the echoing layer is found to be associated with low h(m)F(2) and corresponding high electron density (f(o)F(2)). While the results indicate toward the wind-driven interchange instability as their origin, the early onset of the echoing layer and its relationship with the background ionospheric parameters call for explanation. The results are discussed in the light of current understanding on the bottom-type echoing layer.