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 additional layer, called the F-3 layer, that exists in the equatorial ionosphere are presented. A statistical analysis of the occurrence of the layer recorded at the equatorial station Fortaleza (4 degrees S, 38 degrees W; dip 9 degrees 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 upward ExB drift and neutral wind provides a vertically upward plasma drift velocity 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 develops at lower altitudes through the usual photochemical and dynamical effects of the equatorial region. The peak electron density of the F-3 layer can 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 activity and to become less distinct as the solar activity increases. Ionograms 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 longer compared to the other seasons; on the average, the layer occurs at around 0930 LT and lasts for about 3 hours. The altitude of the layer is also high 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 layer (f(o)F(2)) by the largest amounts in winter and equinox; f(o)F(3) exceeds f(o)F(2) by a yearly average of about 1.3 MHz.