Geofísica Internacional (Jan 2000)
F3 layer observations at low and equatorial latitudes in Brazil
Abstract
Recent studies using model calculation and ionospheric observations have revealed the existence of an additional layer in the topside equatorial ionosphere, the F3 layer. In this work we analyze ionosonde data from two low/equatorial latitude stations in Brazil, during the year 1995, a low solar activity period, in order to study the F3 layer at those locations. At the magnetic station São Luís (2.3° S, 44° W, dip angle –0.5°) the occurrence of the F3 layer is much less evident than at the low latitude station Fortaleza (4° S, 38° W, dip angle –9°). The ionograms recorded at Fortaleza in 1995 show the existence of the layer on 50% of the days, with the occurrence being more frequent (75%) and distinct in summer. The layer appears on 66% of the days in winter and 28% in equinoxes. The layer starts to appear around 0930 LT and lasts from a few minutes to several hours. Its mean height varies from 570 km in summer to 440 km in winter, although on a day to day basis it can vary from as low as 375 km to 775 km. The ionospheric model SUPIM (Sheffield University Plasmasphere-Ionosphere Model) explains well the occurrence of the layer in summer, but not in winter, unless some key input parameters, such as electric field and winds are modified. This suggests that the existing models for electric fields and winds are not representative for the Brazilian low latitude region during winter.
