Preliminary Verification of NmF2/hmF2 predicted by IRI-2016 model with the ionospheric occultation data

To comprehensively validate the accuracy of the latest IRI-2016 model, statistical and climatological analysis of NmF2 and hmF2 predicted by IRI-2016 were conducted with the ionospheric radio occultation (IRO) data during the recent low solar activity (LSA) period between 2017.035 and 2019.035. The IRO data with advantages of high-precision, long-time stabilization and global coverage is a decent data set for the verification of the IRI-2016 model. As the performance of the Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) declines, IRO data of FengYun-3C/Global Navigation Satellite System Occultation Sounder (FY3C/GNOS) was added to the validation data set, which is the first GNOS payload compatible with both Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS) in the world. It is worth noting that there is only one NmF2 prediction option of IRI-2016, which is based on the ionosonde measurements. While there are currently two newly added hmF2 prediction options (AMTB and SDMF2) of IRI-2016, in which the AMTB is based on ionosonde data, and the SDMF2 option assimilates the ionosonde measurements as well as the IRO data including COSMIC. To eliminate the correlation of hmF2 between IRI-2016 predictions and the COSMIC, we excluded COSMIC from IRO validation data set and only used the third party independent IRO data of FY3C-GNOS in the hmF2 evaluation. So, we compared the NmF2 prediction of IRI-2016 with IRO data set including COSMIC and FY3C, and compared hmF2 predictions by AMTB and SDMF2 options of IRI-2016 with the hmF2 measurements of FY3C only. The results of NmF2 statistical analysis indicate the 10% systematic offset in NmF2 between IRI-2016 predictions and the IRO data set. The global ionospheric behaviors of NmF2 predicted by IRI-2016 are highly consistent with that of IRO NmF2, such as equatorial ionospheric anomaly (EIA), semi-annual anomaly, annual anomaly, winter anomaly, although exceptions do exist in IRI-2016 like the absence of the four-peaked longitude structures in the nighttime sector of March equinox season. Due to the NmF2 overestimation of IRI-2016 concentrated on the marine area, there are magnitude discrepancies in the Weddell Sea Anomaly (WSA) between IRI-2016 and IRO data set. Thus, the global distributed IRO data may shed light on the IRI-2016 NmF2 prediction in the marine region. In the statistical analysis of hmF2 between FY3C-GNOS and AMTB/SDMF2 options of IRI-2016, hmF2 predicted by the SDMF2 option is reduced by about 15km and 7km, respectively, in bias and standard deviation(std) compared with the AMTB option. The SDMF2 option primarily based on COSMIC IRO and ionosonde measurements improves the prediction accuracy of IRI-2016 compared to AMTB option based only on the ionosonde data. In hmF2 climatological analysis, the hmF2 predictions of AMTB and SDMF2 both show hemispheric asymmetry like the IRO data set. Nevertheless, under the strong modulation of the geomagnetic field, hmF2 predicted by AMTB option does not show some common characteristics shared by SDMF2 and IRO data, more ionosonde/digisonde data in solar minimum may be required for its further improvement. The verification of the IRI-2016 model with IRO data provides the scientific reference for its application in ionospheric climatology from another perspective.