OSCILLATIONS IN F-REGION DOPPLER VELOCITY IN THE DIP EQUATORIAL REGION ASSOCIATED WITH ULF GEOMAGNETIC-PULSATIONS

Measurements of phase path, P, of F-region reflections at vertical inc idence over Kodaikanal (dip 3-degrees-N, 77-degrees-28'E) on 24 March 1991 (A(p) = 161; SIGMAK(p) = 54) revealed quasi-periodic variations i n the time rate of change of phase path or Doppler velocity, V(D), of large amplitude (peak-to-peak amplitudes 18-66 m s-1) over the period 1730-1845 IST, concurrent with ground-level geomagnetic pulsations. Th e spectral content of the pulsations in H-field at Trivandrum (dip 0.6 -degrees-S, 76-degrees-28'E) and Alibag (dip 24.5-degrees-N, 72-degree s-52'E) as well as in V(D) at Kodaikanal is confined to the period ban d 6.4-10.7 min, with dominant periods around 8 min. Though such quasi- periodic variations in V(D) are not uncommon at electrojet locations d uring daytime and the dusk period, the variance (sigma2) of fluctuatio ns in V(D) in the period range 6.4-10.7 min (computed from V(D) time s eries synthesized through inverse FFT, in the chosen frequency band, o f the FFT of original V(D) time series) on 24 March is higher by a fac tor of 4.28 than the average for 19 other days in the same month when geomagnetic pulsations were absent over the interval 1734-1837 IST. Th e observations strongly indicate an association of the geomagnetic and the ionospheric Doppler velocity oscillations. The variations in V(D) at Kodaikanal in the vicinity of the dip equator are attributed to fl uctuations in the F-region vertical plasma drift caused by the zonal c omponent of time-varying ionospheric electric fields (peak-to-peak amp litude 0.25-1.3 mV/m) associated with the ULF geomagnetic pulsation ac tivity.