Abstract
Ionosonde observations over Cachoeira Paulista (22°S, 45°W), a low-latitude station, show distorted h(ƒ) traces in the ionograms due to passage of TIDs. The distortions of the traces, at times, take the form of an F-region bifurcated into two distinct layers. These effects are observed mostly during presunrise hours. One specific event is studied in greater detail using true height analysis of the quarter hourly ionograms. The characteristics of the electron density and height oscillations for this case are compared with the results obtained, from the simulation of the same, based on linear theory of gravity wave induced ionization perturbations. The results show that gravity wave winds of unrealistically large magnitudes are required to explain the observation. The results suggest further that the inclusion of a source of ion production in the presunrise period, perhaps, produced by precipitation of low energy (< 10 keV) electrons in the South Atlantic anomaly, together with the ionization loss process, might lead to a better agreement between the observation and the theory.
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