Equatorial spread F instability conditions as determined from ionograms

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Abstract
Conditions of plasma irregularity (or Spread F) generation under the Rayleigh‐Taylor mechanism in the nighttime equatorial ionosphere are examined from ionogram data registered over Fortaleza (4°S, 38°W, mag. dip lat. −1.7°) Brazil for a one year period. The results show that local ionospheric conditions such as the bottomside F‐region electron density scale length and the ion‐neutral collision frequency at the base of the layer, at sunset hours, determine the generation of the spread F irregularities over the equator in the post sunset hours.