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 occurrence is observed during sunspot minimum and, in particular, during December solstice. The spread F takes the form of patches of irregularities that are convecting eastwards at ∼50 m s −1 . Most of the patches are collocated with bottomside bulges of the ionosphere. Our measurements indicate that these bottomside bulges are unstable due to a gradient‐drift instability that is slowly growing and produces the spread F . The bulges themselves seem to be evidence of a Rayleigh‐Taylor instability process.