Coherent backscatter cross-section ratio measurements in the midlatitude Eregion ionosphere

Dual-frequency coherent backscatter measurements from a fixed volume in the midlatitude E region ionosphere, made simultaneously with two radars opera ting at significantly different VHF radio frequencies, 144 MHz and 50 MHz, were used to study the scattering cross-section ratios and the spatial spec trum dependence in the 1-m to 3-m wavelength range. To calibrate the measur ements, simultaneous aircraft echo intensities observed by the radars durin g the experiment were used. It was found that the 1-m to 3-m plasma wave sc attering cross-section ratios, Sigma = sigma(1m)/sigma(3m) were always belo w unity, but their values depended strongly on the type of irregularities o bserved, either type 1 (large phase velocity Farley-Buneman) or type 2 (low phase velocity secondary) waves. Mean values for Sigma were near similar t o 0.4 for type 1 echoes and similar to 0.06 for type 2 echoes, suggesting t hat secondary waves are more difficult to generate at 1-m than at 3-m wavel ength even though type 1 waves are nearly always seen simultaneously at bot h 50 and 144 MHz. Assuming a negative power law k dependence for the irregu larity spatial spectrum, I-k proportional to k(-beta), the spectrum slope b eta 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 t heory of Sudan [1983] for both types of irregularities, but particularly fo r type 2 waves; for type 1 waves the agreement is not as good, but the theo retical predictions are not as well defined either.