Coherent backscatter cross-section ratio measurements in the midlatitude E region ionosphere

Dual‐frequency coherent backscatter measurements from a fixed volume in the midlatitude E region ionosphere, made simultaneously with two radars operating at significantly different VHF radio frequencies, 144 MHz and 50 MHz, were used to study the scattering cross‐section ratios and the spatial spectrum dependence in the 1‐m to 3‐m wavelength range. To calibrate the measurements, simultaneous aircraft echo intensities observed by the radars during the experiment were used. It was found that the 1‐m to 3‐m plasma wave scattering cross‐section ratios, Σ = σ1m/σ3m, were always below unity, but their values depended strongly on the type of irregularities observed, either type 1 (large phase velocity Farley‐Buneman ) or type 2 (low phase velocity secondary ) waves. Mean values for Σ were near ∼ 0.4 for type 1 echoes and ∼ 0.06 for type 2 echoes, suggesting that secondary waves are more difficult to generate at 1‐m than at 3‐m wavelength even though type 1 waves are nearly always seen simultaneously at both 50 and 144 MHz. Assuming a negative power law k dependence for the irregularity spatial spectrum, Ik ∝ k−β, the spectrum slope β was found on the average to be about 1.0 and 2.8 for type 1 and type 2 irregularities, respectively, which suggests that the k spectrum is nearly 3 times steeper for type 2 than type 1 waves in the meter wavelength range. These figures are roughly consistent with the predictions of the unified theory of Sudan [1983] for both types of irregularities, but particularly for type 2 waves; for type 1 waves the agreement is not as good, but the theoretical predictions are not as well defined either.