SIMULTANEOUS OBSERVATIONS OF E-REGION COHERENT RADAR ECHOES AT 2-M AND 6-M RADIO WAVELENGTHS AT MIDLATITUDE

Two continuous wave Doppler radars, using scaled antenna arrays at the same receiving and transmitting sites in order to observe the same vo lume in the E region ionosphere, made simultaneous observations of coh erent backscatter from the island of Crete, Greece, The radars operate d 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 l-m irregularities, r espectively, In this paper we introduce the experiment and present an overview of the observations, Both primary and secondary E region irre gularities were observed at both radar frequencies, but the 144 MHz ec hoes were considerably weaker than those at 50 MHz, The observations d emonstrated clearly the different character of E region type 1 and typ e 2 irregularities, The 144-MHz type 2 echoes were completely absent d uring times of weak to moderately strong 50-MHz backscatter activity a nd appeared only when the signal at 50 MHz became rather strong with r elative 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 i rregularity wavelength range from 3-m to l-m, On the other hand, and i n 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 time s the observed velocities of the simultaneous 3-m and l-m type 1 irreg ularities were the same, on the average the evidence favored a slightl y higher threshold for excitation at 144 MHz than at 50 MHz. The resul ts are in general agreement with what is anticipated from basic theory of E region meter-scale plasma waves.