DOPPLER FREQUENCY FLUCTUATIONS OF LOWER THERMOSPHERIC REFLECTIONS IN THE EQUATORIAL ELECTROJET REGION

A common and characteristic feature of lower F-region (approximately 200 km) reflections at vertical incidence over Kodaikanal (dip 3.0-degrees-N) during the daytime is the presence of quasi-sinusoidal variations in the time rate of change of phase path, P (Doppler frequency shift, DELTA-f) with periodicities of 30-600 s. We have studied here the dependence of the lower-period segment (30-300 s) of the Doppler frequency variations on the ambient electrojet strength and associated ionospheric conditions. The level of wave activity in P in the period range 30-300 s is quantified by computing the variance of the P fluctuations synthesized through reverse FFT, in the chosen frequency band, of the FFT of original data. The variance of P (computed at approximately 1 h intervals) is found to bear a significant linear relationship to the ambient electrojet strength. P fluctuations in the range 30-120 s are found to be most sensitive to changes in electrojet strength compared to the other two sub-bands 120-210 s and 210-300 s, and tend to dominate, in general, the spectral content of P fluctuations in the range 30-300 s. The Doppler fluctuations in band 30-300 s are consistently seen to practically cease at times of disappearances of Esq on bottomside ionograms (i.e. during partial/complete counter-electrojet conditions). The short-period fluctuations in P are interpreted in terms of phase path changes imposed on lower F-region reflections by the refractive index variations associated with the convective motions of plasma density irregularities (types I and II) in the daytime equatorial electrojet.