Spatial distribution of random velocity fluctuations of 3-m field-aligned irregularities in ionospheric sporadic e layer

In this article we attempt to estimate the spatial distribution of the random velocity fluctuation of 3-m field-aligned irregularities (FAI) in sporadic E (E_s) region, which were responsible for the quasi-periodic (QP) echoes in range-time-intensity (RTI) plot. As previously shown by Chu and Wang (2003), beam broadening effect may be significant in spectral width measurements. However, using interferometry technique this effect could be removed by estimating the zonal drift velocity and the interferometry-resolved zonal extent of the plasma irregularities. By applying this correction to 1 hour of data we found that the ratio of the beam broadening spectral width to the observed spectral width vary between 5 and 40% with a mean value of 14.2% and a standard deviation of 6.3%. After removing the beam-broadening contribution from observed Doppler spectral width, the mean velocity fluctuations of the 3-m plasma irregularities at a height of 105 km are in the range of 5 to 12 m/s. In addition, the spatial distribution of the FAI velocity fluctuation is nearly uniform in the echoing region over a span of 8° (corresponding to a horizontal extent of about 20 km) in geomagnetic zonal direction. The plausible mechanism responsible for the uniform distribution of the random velocity fluctuation of the 3-m FAI in the nighttime E_s region is discussed.