F region strata at the magnetic equator

The stratified irregularities which appear to rise throught the equatorial F layer in the daytime have been studied at Sarh (Chad). Diurnal, seasonal and magnetic activity dependence are described on a sample of 28 months quaterhourly ionograms. The characteristics of the rise help to distinguish three subtypes. The occurrence study suggests a new physical interpretation. New and full moon epochs (+- one day) present important maxima. Autocorrelation of the intervals between days of occurrence show a clear periodic lunar background of 14 solar day period. Sharp lines peak out of these lunar maxima, but also a major peak on the solar activity 27 day interval. The luni-solar modulation and the complex variation observed are best explained by the pile-up effect to the upward drift working against a downward drift as might be expected from the polarisation due to the F region westward zonal wind. Thus our African strata occurrence seems primarily modulated by the lunarcontrolled downward drift and its associated gravity wave shears