Comparison of the Ionospheric F2 Region Critical Frequency between Ionosonde Measurements and IRI-2016 Predictions over Sanya

The ionospheric F
₂
region critical frequency (foF2) data collected by Sanya ionosonde (18.3°N, 109.6°E; geomagnetic: 8.39°N, 181.97°E) in 2014 have been analyzed and compared with IRI-2016 predictions using CCIR and URSI options under low geomagnetic activity conditions. We found that the peak duration of the observed foF2 values was longer in spring and autumn than that in summer and winter. Usually, the maximum of the observed foF2 appeared near 8 UT (i.e., 15 LT), and the observed foF2 had two peaks in spring and autumn. The observed foF2 values in summer were generally smaller than those in winter. Moreover, the observed foF2 values in winter were smaller than those in the equinox months. That is to say, the presence of semiannual and winter anomaly in the observed foF2 was found over Sanya in 2014. The prediction accuracy of the IRI-2016 predictions with the CCIR option and that with the URSI option are basically the same. The foF2 values predicted by IRI-2016 model are close to the observed values during daytime. However, the deviation (i.e., the absolute deviation and the relative deviation) of the IRI-2016 predicted foF2 values with respect to the observed foF2 values during nighttime is much larger than that during daytime. During nighttime, the IRI-2016 predicted foF2 values with the CCIR or URSI option underestimate or overestimate the observed foF2 values more than 50%. In general, the IRI-2016 predicted foF2 values with the CCIR or URSI option underestimate the observed foF2 values in the equinox months and overestimate the observed foF2 values in January, June and July. Therefore, our findings show that IRI-2016 model cannot predict foF2 over Sanya in 2014 very well.