A large number of researches have been devoted to the formation of additional layers in theFregion of the equatorial ionosphere, first of which has been published in 1940s. Originally the occurrence of such layer was named “stratification of equatorialF2layer.” The additional layer was later named as theF3layer. The theoretical researches have shown that theF3layer is formed by zonal component of electric field with assistance of meridional component of thermospheric wind and field-aligned plasma diffusion. The physical mechanism of theF3layer formation is clearly formulated for the morning-noon period, although theF3layer is also observed at other hours. This paper presents a brief review into the history of the additional layer researches, describes the current progress of these researches, and identifies the most important problems in this field of the ionospheric physics.
Physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere