Spectral behavior of VHF backscatter from ionospheric sporadic E irregularities in the equatorial anomaly crest zone

In this article, the spectral behavior of the radar returns from ionospheric irregularities of the sporadic E layer occurring in equatorial anomaly crest zone are investigated using 52 MHz Chung-Li VHF radar. It is shown that the mean Doppler velocity and spectral width for the type 1 radar spectra are within 28∼35 m/s and 3∼5 Hz, respectively. Compared with the observational results made with other VHF radars, the Doppler shifts of type 1 spectra obtained here are significantly smaller than those in the equatorial and auroral zones. On the other hand, the mean Doppler velocities of the type 2 radar spectra with the spectral width of 16∼30 Hz are height-dependent and have values between -50∼60 m/s for the present case. The correlation between the Doppler spectral parameters of the type 2 irregularities, namely, the echo intensity, mean Doppler shift, and the spectral width, are also analyzed in this paper. It is demonstrated that a high correlation exists between echo intensity and spectral width, while a considerably poor correlation prevails between mean Doppler frequency shift and spectral width. These results indicate that the primary mechanism causing the breadth of the Doppler spectrum of type 2 irregularities is the ionospheric electron density fluctuations, not the beam broadening effect. The analysis shows that there is a good correlation between mean Doppler shift of type 2 radar spectra and the slope of echo contour in the range-time-intensity plot, implying that the Es targets responsible for type 2 echoes are localized and discrete in the resolution volume. The comparison between the echo intensity of the type 2 radar spectra and foEs and fbEs is also made in this article, where foEs and fbEs scaled from the trace of a HF ionogram record are the critical frequency and blanket frequency of the sporadic E layer, respectively. This shows that the VHF peak power of the type 2 radar spectra is proportional to the difference between foEs and fbEs. A detailed discussion on this feature is given in the text.