Oscillations in F-region Doppler velocity in the dip equatorial region associated with ULF geomagnetic pulsations

Abstract

Measurements of phase path, P, of F-region reflections at vertical incidence over Kodaikanal (dip 3°N, 77°28′E) on 24 March 1991 ( A p = 161; ΣK p = 54) revealed quasi-periodic variations in 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. The spectral content of the pulsations in H-field at Trivandrum (dip 0.6°S, 76°28'E) and Alibag (dip 24.5°N, 72°52′E) as well as in V D at Kodaikanal is confined to the period band 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 during daytime and the dusk period, the variance ( σ 2) of fluctuations in V D in the period range 6.4–10.7 min (computed from V D time series synthesized through inverse FFT, in the chosen frequency band, of the FFT of original V D time series) on 24 March is higher by a factor 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. The 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 fluctuations in the F-region vertical plasma drift caused by the zonal component of time-varying ionospheric electric fields (peak-to-peak amplitude 0.25–1.3 mV/m) associated with the ULF geomagnetic pulsation activity.