Variability of an additional layer in the equatorial ionosphere over Fortaleza

The day-to-day variations (or the weather) of an additional layer, called t he F-3 layer, that has been predicted to exist at altitudes above the F-2 p eak in the equatorial ionosphere are studied through ionosonde observations and theoretical modeling. The ionograms recorded in 1995 at the equatorial station Fortaleza (4 degrees S, 38 degrees W; dip angle 9 degrees S) in Br azil show the occurrence of the F3 layer during daytime from 0800 to 1630 L T, with the duration of occurrence ranging from 15 min to 6 hours. Although the layer occurs most frequently (75% of the days) in local summer as prev iously predicted, there are consecutive and individual magnetically quiet a nd disturbed days when the layer does not occur. There are also days when t he layer reoccurs. The model results, obtained using the Sheffield Universi ty plasmasphere-ionosphere model, show that the day-to-day variations of th e F-3 layer arise from the corresponding variations of the vertical plasma velocity. The layer occurs when the time-cumulative vertical velocity displ aces the daytime F-2 peak to high altitudes, to form the F-3 layer, while t he normal F-2 layer develops at low altitudes. Sudden displacements result in more distinct F-3 layers than gradual displacements. Model results also show that the plasma temperature within the F-3 layer decreases as the plas ma density increases, and, like the plasma density, the plasma temperature also undergoes large day-to-day variations.