Observations and model calculations of the F 3 layer in the Southeast Asian equatorial ionosphere

To clarify the characteristics of the F 3 layer with a focus on magnetic latitude dependence and the relationship to the equatorial anomaly, we performed statistical analysis of F 3 layer occurrences using the ionosonde chain data in a magnetic meridional plane in Southeast Asia and performed model calculations. From comparison of the observational and model calculation results, it was found that the field‐aligned diffusion of plasma acts to make the F 3 layer prominent in the magnetic low‐latitude region while acting to decrease the peak density of the F 3 layer near the magnetic equator. The magnetic latitude dependence of the F 3 layer formation comes not only from the meridional neutral wind effect but also from the field‐aligned diffusion effect. The model calculations revealed that the F 3 peak corresponds to the electron density–enhanced region associated with the equatorial anomaly. This relationship is consistent with the suggestion that the field‐aligned diffusion acts to make the F 3 layer prominent in the magnetic low‐latitude region since the fundamental factors for generation of the equatorial anomaly are also E × B drift and field‐aligned downward diffusion. It is suggested that the local time and magnetic latitudinal variations of the F 3 layer result from those of the electron density–enhanced region associated with the equatorial anomaly.