Manifestations of short-period electric field fluctuations in the equatorial lower thermosphere

Abstract

Measurements (90h) of phase path, P of lower F-region (~ 200km) reflections at normal incidence over Kodaikanal (dip 3.0°N) revealed the time rate of change of phase path, (Doppler frequency shift, Δf) to undergo quasi-periodic fluctuations quite regularly during the day time. The changes in which have peak-to-peak amplitudes of 6 to 30 ms-1 (0.1-0.5Hz in Δf) exhibit quasi-periods in the band 60-600s, comprising of two overlapping spectral domains: the shorter-period components (T < 240s) and the longer-period components (T > 300s). The spectral content of the variations in is found to depend on the ambient equatorial electrojet strength and associated ionospheric conditions in that, while the longerperiod components persist almost all the time, the shorter-period components tend to cease during the times of disappearance of equatorial sporadic-E (Esq) on ionograms (partial counter-electrojet). Perturbations in the F-region vertical plasma drift, VzF are considered to be primarily responsible for the observed changes in , particularly the longer-period (T > 300s) fluctuations. Correlative and visual studies of the simultaneous data ( 52h) of phase path at Kodaikanal and of ground level geomagnetic micropulsations at Trivandrum (located similar 200km south of Kodaikanal, dip 0.6°S) showed the absence of a persistent and clear-cut association of the fluctuations in with Pc4/Pc5 micropulsations, indicating that hydromagnetic waves are not a primary source of the VzF pulsations. It is suggested that the longer-period (300 < T < 600s) fluctuations in VzF are due to gravity waves related changes in the east-west electric field at E-region altitudes outside the electrojet belt, and mapped along the field lines to F-region levels over Kodaikanal.