Physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere

A physical mechanism and the location and latitudinal extent of an addition al layer, called the F-3 layer, that exists in the equatorial ionosphere ar e presented. A statistical analysis of the occurrence of the layer recorded at the equatorial station Fortaleza (4 degrees S, 38 degrees W; dip 9 degr ees S) in Brazil is also presented. The F-3 layer forms during the morning- noon period in that; equatorial region where the combined effect of the upw ard ExB drift and neutral wind provides a vertically upward plasma drift ve locity at altitudes near and above the F-2 peak. This velocity causes the F -2 peak to drift upward and form the F-3 layer while the normal Fa layer de velops at lower altitudes through the usual photochemical and dynamical eff ects of the equatorial region. The peak electron density of the F-3 layer c an exceed that of the F-2 layer. The F-3 layer is predicted to be distinct on the summer side of the geomagnetic equator during periods of low solar a ctivity and to become less distinct as the solar activity increases. Ionogr ams recorded at Fortaleza in 1995 show the existence of an F-3 layer on 49% of the days, with the occurrence being most frequent (75%) and distinct in summer, as expected. During summer the layer occurs earlier and lasts long er compared to the other seasons; on the average, the layer occurs at aroun d 0930 LT and lasts for about 3 hours. The altitude of the layer is also hi gh in summer, with the mean peak virtual height being about 570 km. However , the critical frequency of the layer (f(o)F(3)) exceeds that of the F-2 la yer (f(o)F(2)) by the largest amounts in winter and equinox; f(o)F(3) excee ds f(o)F(2) by a yearly average of about 1.3 MHz.