Stratification of the F 2 layer observed in Southeast Asia

This paper describes the latitudinal variation in F2 stratification [Balan and Bailey, 1995] as observed by a number of oblique and vertical ionosondes operating in Southeast Asia during 1997. Stratification of the F2 layer was seen at dip latitudes from 4°S to 18°S on the southern side of the magnetic equator but did not occur at the closest reflection point to the magnetic equator (dip latitude = 2.3°N). The observed transient cusp (vertical ionosonde) or additional nose (oblique ionosonde) was defined as an F3 layer or an F1.5 layer depending on whether it occurred above or below the layer which maintained continuity with the normal F2 layer peak. Within the zone of occurrence, the transient layer was commonly seen as an F3 layer at reflection points closest to the magnetic equator but invariably as an F1.5 layer at reflection points farther from the magnetic equator. These observations suggest that the distortion in the equatorial electron density profile associated with the phenomenon moved toward the base of the F2 layer as magnetic field lines descended with increasing latitude. Stratification of the F2 layer commenced at the same local time (e.g., 0845 LT in November 1997) throughout the longitudinal range of coverage and was associated with a rapid rise in F2 layer height following sunrise. The stratification ended at times varying from 1300 LT to sunset and was associated with a fall in the height of F2 peak electron density. The region of maximum F2 layer stratification lay between the magnetic equator and the peak of the southern equatorial anomaly.