Thailand low and equatorial F2-layer peak electron density and comparison with IRI-2007 model

Ionosonde measurements obtained at two Thailand ionospheric stations, namely Chumphon (10.72◦N, 99.37◦E, dip 3.0◦N) and Chiang Mai (18.76◦N, 98.93◦E, dip 12.7◦N) are used to examine the variation of the F2-layer peak electron density (Nm F2) which is derived from the F2-layer critical frequency, fo F2. Measured data from September 2004 to August 2005 (a period of low solar activity) are analyzed based on the diurnal and seasonal variation and then compared with IRI-2007 model predictions. Our results show that, in general, the diurnal and seasonal variations of the Nm F2 predicted by the IRI (URSI and CCIR options) model show a feature generally similar to the observed Nm F2. Underestimation mostly occurs in all seasons except during the September equinox and the December solstice at Chumphon, and the September equinox and the March equinox at Chiang Mai, when they overestimate those measured. The best agreement between observation and prediction occurs during the pre-sunrise to post-sunrise hours. The best agreement of the %PD values of both the options occurs during the March equinox, while the agreement is the worst during the September equinox. The Nm F2 values predicted by the CCIR option show a smaller range of deviation than the Nm F2 values predicted by the URSI option. During post-sunset to morning hours (around 21:00–09:00 LT), the observed Nm F2 at both stations are almost identical for the periods of low solar activity. However, during daytime, the observed Nm F2 at Chumphon is lower than that at Chiang Mai. The difference between these two stations can be explained by the equatorial ionospheric anomaly (EIA). These results are important for future improvements of the IRI model for Nm F2 over Southeast Asia, especially for the areas covered by Chumphon and Chiang Mai stations.