First observations of large‐scale wave structure (LSWS) and the subsequent development of equatorial spread F (ESF), using total electron content (TEC) derived from the ground based reception of beacon signals from the CERTO (Coherent Electromagnetic Radio Tomography) radio beacon on board C/NOFS (Communications/Navigation Outage Forecasting System) satellite, are presented. Selected examples of TEC variations, using measurements made during January 2009 from Bac Lieu, Vietnam (9.2°N, 105.6°E geographic, 1.7°N magnetic dip latitude) are presented to illustrate two key findings: (1) LSWS appears to play a more important role in the development of ESF than the post‐sunset rise (PSSR) of the F‐layer, and (2) LSWS can appear well before E region sunset. Other findings, that LSWS does not have significant zonal drift in the initial stages of growth, and can have zonal wavelengths of several hundred kilometers, corroborate earlier reports.

[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.

Results from the meridional ionosonde network located in Southeast Asia (SEALION) demonstrate the interesting nature of the F3 layer, showing its generation mechanism. Ionograms obtained on 16 November 2004 and 31 March 2005 at Chiang Mai (CMU; geographic latitude 18.8°N, geographic longitude 98.9°E, and magnetic latitude 13.2°N), Chumphon (CPN; 10.7°N, 99.4°E, and 3.2°N) and Kototabang (KTB; 0.2°S, 100.3°E, and 10.1°S) showed significant differences between CPN near the magnetic equator, and CMU and KTB in the magnetic low‐latitude region. The simultaneous magnetic conjugate observations of the F3 layer achieved using the SEALION ionosonde network data showed clear dependences of the F3 layer on the magnetic latitude. It is suggested that these magnetic latitude dependences of the F3 layer can be explained by considering the plasma diffusion effects along the magnetic field lines in the magnetic low‐latitude region.

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Abstract. The virtual height of the bottom side F-region (h F ) and equatorial spread-F (ESF) onsets at Chumphon (10.7 • N, 99.4 • E; 3.3 • N magnetic latitude) were compared with the behaviour of equatorial electrojet (EEJ) ground strength at Phuket (8.1 • N, 98.3 • E; 0.1 • N magnetic latitude) during the period from November 2007 to October 2008. Increase in the F-layer height and ESF onsets during the evening hours were well connected with the EEJ ground strength before sunset, namely, both the height increase and ESF onsets were suppressed when the integrated EEJ ground strength for the period from 1 to 2 h prior to sunset was negative. The finding suggests observationally that the pre-sunset E-region dynamo current and/or electric field are related to the F-region dynamics and ESF onsets around sunset.