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 Dopp ler experiment in Crete, which operates at 50 MHz and observes coheren t backscatter from field-aligned irregularities in the midlatitude E r egion ionosphere (L = 1.35). In this paper a large database of high-re solution Doppler spectrum observations from over one and a half years of radar operation is utilized to investigate the properties of cohere nt radii, wave scattering. The statistics of the spectral moments show that echo Intensities are in the 5- to 15-dB range, Doppler velocitie s point northward and upward having magnitudes usually less than 100 m /s, whereas spectrum widths are narrow taking values mostly in the ran ge 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 gen erated during conditions of weak plasma, turbulence. A striking featur e is the seasonal and diurnal echo occurrence. The echoes are overwhel mingly present in the summer months and appear only during nighttime, mostly in the premidnight local time sector. Knowing the close connect ion with the midlatitude sporadic layers, E(s), the echo occurrence ca n 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 th e gradient drift instability is inoperable during the day, probably be cause of electron density gradient smoothing due to strong solar phpto ionization production. Finally, the mean Doppler velocities are larger in amplitude but vary approximately in phase with the ambient northwa rd and upward E x B drifts, inferred from past incoherent scatter rada r measurements, and model predictions of meridional neutral winds.