Simultaneous observations of E region coherent radar echoes at 2‐m and 6‐m radio wavelengths at midlatitude

Abstract

Two continuous wave Doppler radars, using scaled antenna arrays at the same receiving and transmitting sites in order to observe the same volume in the E region ionosphere, made simultaneous observations of coherent backscatter from the island of Crete, Greece. The radars operated at different frequencies, that is, 50 MHz and 144 MHz, which made it possible for the first time in midlatitudes to observe simultaneously aspect sensitive backscatter from about 3‐m and 1‐m irregularities, respectively. In this paper we introduce the experiment and present an overview of the observations. Both primary and secondary E region irregularities were observed at both radar frequencies, but the 144 MHz echoes were considerably weaker than those at 50 MHz. The observations demonstrated clearly the different character of E region type 1 and type 2 irregularities. The 144‐MHz type 2 echoes were completely absent during times of weak to moderately strong 50‐MHz backscatter activity and appeared only when the signal at 50 MHz became rather strong with relative intensities nearing and exceeding 20 dB above noise. The large differences in the observed signal levels between the 50‐ and 144‐MHz radar type 2 echoes suggest a very steep wavenumber spectrum in the irregularity wavelength range from 3‐m to 1‐m. On the other hand, and in sharp contrast to type 2 echoes, there was one to one correspondence in the occurrence of 50‐MHz and 144‐MHz type 1 echoes, even when the signal at 50 MHz was only a few decibels above noise. Although at times the observed velocities of the simultaneous 3‐m and 1‐m type 1 irregularities were the same, on the average the evidence favored a slightly higher threshold for excitation at 144 MHz than at 50 MHz. The results are in general agreement with what is anticipated from basic theory of E region meter‐scale plasma waves.