Characteristics of midlatitude coherent backscatter from the ionospheric E region obtained with Sporadic E Scatter experiment

Sporadic E Scatter (SESCAT) experiment is a continuous wave radio Doppler experiment in Crete, which operates at 50 MHz and observes coherent backscatter from field-aligned irregularities in the midlatitude E region ionosphere (L = 1.35). In this paper a large database of high-resolution Doppler spectrum observations from over one and a half years of radar operation is utilized to investigate the properties of coherent radii, wave scattering. The statistics of the spectral moments show that echo Intensities are in the 5- to 15-dB range, Doppler velocities point northward and upward having magnitudes usually less than 100 m/s, whereas spectrum widths are narrow taking values mostly in the range from 50 to 150 m/s. On the basis of the data, we postulate that 50-MHz midlatitude echoes are largely due to secondary irregularities generated during conditions of weak plasma, turbulence. A striking feature is the seasonal and diurnal echo occurrence. The echoes are overwhelmingly present in the summer months and appear only during nighttime, mostly in the premidnight local time sector. Knowing the close connection with the midlatitude sporadic layers, E(s), the echo occurrence can be understood in terms of the well-known morphology of intense E(s) layers. On the other hand, the absence of daytime echoes means that the gradient drift instability is inoperable during the day, probably because of electron density gradient smoothing due to strong solar phptoionization production. Finally, the mean Doppler velocities are larger in amplitude but vary approximately in phase with the ambient northward and upward E x B drifts, inferred from past incoherent scatter radar measurements, and model predictions of meridional neutral winds.