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, Ṗ (Doppler frequency shift, Δf) to undergo quasi-periodic fluctuations quite regularly during the day time. The changes in Ṗ 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 Ṗ 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 Ṗ, 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 Ṗ 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.