Observations and model calculations of theF₃layer in the Southeast Asian equatorial ionosphere

Abstract

[1] 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.

Stratification of the Low-Latitude and Near-EquatorialF2Layer, Topside Ionization Ledge, andF3Layer: What We Know about This? A Review

“…The statistic of the F3 layer occurrence at Chiang Mai (18.8 • N, 98.9 • E; 13.2 • ), Thailand resembles that of Indian result [41]. The statistic of the F3 layer occurrence at Chumphon (10.7 • N, 99.…”

“…Similarly, according to the ground-based data F3 layer is observed rarely at the equator than at low latitudes ±(7∼8 • ) [37,39,41,65]. Recent studies show that the F3 layer near the magnetic equator behaves in a different way than that in the magnetic low-latitude region [41,43,66]. The F3 layer moves rapidly upward near the magnetic equator, while it stays almost at a certain altitude in the magnetic low-latitude region.…”

“…To reproduce the F3 layer occurrence during geomagnetic storms, it is necessary to take into account both the dynamo electric field and the penetration of the magnetospheric convection electric field to lower latitudes [45,46]. Uemoto et al [41] performed model calculations using the SAMI2 code [64] to theoretically discuss the mechanism of occurrence features of the F3 layer obtained through the statistical analysis and to examine the relationship between the F3 layer and the equatorial anomaly. The SUPIM, SAMI2, and GSM TIP models take into account all processes that proposed to explain the equatorial F2 layer stratification and the F3 layer formation, namely, the electromagnetic plasma drifts, neutral wind, thermal plasma diffusion processes along geomagnetic field lines, thermal plasma sources, and losses.…”

“…In addition the hemispheric asymmetry of the seasonal F3 layer occurrence should also be considered. Uemoto et al [41] showed that at Kototabang (0.2…”

“…It is obvious that the equatorial F2 layer stratification and the occurrence of an additional layer in the equatorial ionosphere is the one phenomenon. [32], the subject of the F3 layer received extensive investigations at low and equatorial latitudes concerning its spatial and temporal distribution based on the ionosonde observations of single or several stations distributed at three longitudinal sectors (American, Indian, and Pacific longitudes) [34][35][36][37][38][39][40][41][42][43][44]. From these investigations, it is known that the F3 layer can be observed under geomagnetically quiet conditions.…”