The figures of this set show not only latitude-longitude cross-section of the ionosphere state, but also the vertical variability. The red isosurface covers the ionospheric regions where the electron density is higher than 8.5 • 1011 electron per m3. This means that the isosurface covers the E and F layers in the mostly lightened regions of the globe, while during the increased solar activity it also covers the higher regions.
The borders of the isosurface represent the day/night border. In some periods of time the isosurface borders follow the meridian line, while in the others they look as two decreasing tales by each side of the magnetic equator. This and many other effects are caused by a wide variety of factors that influence the ionospheric behavior. Most of these factors are accounted for in the data assimilation ionosphere model presented at this website.