Estimation of beam broadening spectrum of radar returns from field-aligned irregularities in ionospheric sporadic E region

Although the backscatter from field-aligned irregularities (FAI) associated with sporadic E (Es) layer are generally believed to be extremely aspect-sensitive, the beam broadening effect induced by the along- and cross-radar-beam drifts of the irregularities on Doppler spectral width of the echoes may be significant and cannot be totally ignored. Prior to the estimate of the beam broadening spectrum of the Es FAI, the locations of the irregularities in the echoing region should be precisely determined. With the knowledge of the interferometer-measured parameters, such as the angular position of the FAI from the radar beam axis and the zonal extent of the irregularities in the echoing region, combined with the observed Doppler velocity, the components of the along- and cross-radar-beam drift velocities can be estimated. Experimental results show that for the present case the magnitudes of the components of the along- and cross-radar-beam drift velocities are, respectively, 22.4 and 11.9 m/s. Taking advantage of these estimated drift velocities, we calculate corresponding beam broadening spectrum in accordance with the expression developed by [Chu, Y.H., Wang, C.Y. Plasma structures of 3-meter type 1 and type 2 irregularities in nighttime mid-latitude sporadic E region. J. Geophys. Res. 108 (A7), 1282, doi:10.1029/2002JA009661, 2003]. A comparison of observed and beam broadening spectral widths shows that the portion of the observed Doppler spectral width which is caused by the beam broadening effect can be as large as more than 26%. Therefore, the beam broadening effect caused by the drift of the field-aligned irregularities in the echoing region can play a decisive role in broadening the Doppler spectral width of the Es FAI and cannot be neglected from the observed Doppler spectrum.