Spectral Behavior of VHF Backscater From Ionospheric Sporadic E Irregularities in the Equatorial Anomaly Crest Zone

Abstract

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 type2 radar spectra and ofEs and fbEs is also made in this article, where ofEs and fbEs scaled form the trace of a HF ionogram record are the critical frequency and blanket frequency of the sporadie 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.

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