Stratification of the F-2 layer observed in Southeast Asia

This paper describes the latitudinal variation in F-2 stratification [Balan and Bailey, 1995] as observed by a number of oblique and vertical ionosond es operating in Southeast Asia during 1997. Stratification of the F-2 layer was seen at dip latitudes from 4 degreesS to 18 degreesS on the southern s ide of the magnetic equator but did not occur at the closest reflection poi nt to the magnetic equator (dip latitude = 2.3 degreesN). The observed tran sient cusp (vertical ionosonde) or additional nose (oblique ionosonde) was defined as an F-3 layer or an F-1.5 layer depending on whether it occurred above or below the layer which maintained continuity with the normal F-2 la yer peak. Within the zone of occurrence, the transient layer was commonly s een as an F-3 layer at reflection points closest to the magnetic equator bu t invariably as an F-1.5 layer at reflection points farther from the magnet ic equator. These observations suggest that the distortion in the equatoria l electron density profile associated with the phenomenon moved toward the base of the F-2 layer as magnetic field lines descended with increasing lat itude. Stratification of the F-2 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 F-2 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 F-2 peak electron density. The regi on of maximum F-2 layer stratification lay between the magnetic equator and the peak of the southern equatorial anomaly.