Abstract
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.
Highlights
A large number of researches have been devoted to the formation of additional layers in the F region of the equatorial ionosphere, first of which has been published in 1940s
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
At the equator the bifurcation of the F2 layer begins to occur at about 12:00 LT and ends at about 19:00 LT according to McClure and Peterson [12] based on the result of the observations made with Thomson scatter radar
Summary
Lynn et al [65] first presented the latitudinal dependence of the F3 layer occurrence at the equatorial latitudes in South East Asia using the observations from a number of oblique and vertical ionosondes They found that the region of maximum F2 layer stratification lay between the magnetic equator and the peak of the southern EIA. Their oblique ionosonde measurements in Southeast Asia show that the transitory layer was seen as an F3 layer close to the magnetic equator but invariably as an F1.5 layer farther from the magnetic equator These observations suggest that the distortion in the equatorial electron density profile associated with a movement toward the base of the F2 layer as magnetic field lines descended with increasing latitude.
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