资源环境科技发展态势分析平台

We present the first dual-frequency overlapping volume radar observations of daytime 150-km echoes made at 30 and 53 MHz from Gadanki, India. We show remarkably different responses of the 150-km echoing layers including the spectral characteristics of the echoes at 30 and 53 MHz. Results show that while the echoes in general are remarkably stronger with narrower spectral width at 30 MHz than that of 53 MHz the relationship between radar cross sections at the two frequencies varies depending on the echoing layer. We also show distinctly different spectral broadening in different echoing regions observed by the 53-MHz radar, which is missing in the 30-MHz radar observations. These new observations indicate involvement of naturally enhanced incoherent scattering as well as scattering from enhanced electron density fluctuations of plasma instability origin and hence are of utmost importance in understanding the origin of the puzzling 150-km echoing phenomenon.