PRESUNRISE SPREAD-F AT FORTALEZA

At Fortaleza, Brazil, in the equatorial zone about 400 km south of the magnetic equator a presunrise (secondary) maximum of spread F occurre nce is observed during sunspot minimum and, in particular, during Dece mber solstice, The spread F takes the form of patches of irregularitie s that are convecting eastwards at similar to 50 m s(-1). Most of the patches are collocated with bottomside bulges of the ionosphere. Our m easurements indicate that these bottomside bulges are unstable due to a gradient-drift instability that is slowly growing and produces the s pread F. The bulges themselves seem to be evidence of a Rayleigh-Taylo r instability process.