Origin of short‐period (30–300 s) Doppler frequency fluctuations of lower F region reflections in the equatorial electrojet region

Abstract

Measurements of phase path P of lower F region reflections at normal incidence at Kodaikanal (10°14′N, 77° 28′E, dip 3.0°N) revealed the ubiquitous presence of 30–300 s quasi‐sinusoidal variations in the time rate of change of phase path, P (Doppler frequency shift) during day time. A study is made of the influence of the irregularities in the equatorial electroject on the P fluctuations using simultaneous observations of F region phase path at Kodaikanal and of equatorial electrojet with the VHF backscatter radar at Thumba (08° 29′N, 76° 56′E, dip 0.9°S). It is shown that the spectral content of the Doppler fluctuations (quantified in terms of variance, σ2 computed from P time series synthesized through FFT−1 (fast Fourier transfrom) in the chosen period bands, 30–300 s/30–120 s of the FFT of original P time series) bears a significant positive linear relationship to the horizontal phase velocity (Vp) of electrojet irregularities (3‐m scale size)on a hourly basis. This result is in consonance with our earlier findings (Sastri et al., 1990) of a significant linear relationship of σ2 to the electrojet strength (estimated from H field data) and a practical cessation of the P fluctuations at times of disappearance of Esq on ionograms (partial/complete counterelectrojet). The present work substantiates the interpretation that the short‐period Doppler frequency fluctuations are due to phase path changes imposed on lower F region reflections by the refractive index variations associated with the convective motions of plasma density irregularities (type I and II) in the daytime equatorial electrojet.