Journal of Geophysical Research: Space Physics

Presunrise spread F at Fortaleza


  • J. W. MacDougall,

  • M. A. Abdu,

  • P. T. Jayachandran,

  • J.-F. Cecile,

  • I. S. Batista


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.