Deriving Ionospheric Sporadic E Intensity From FORMOSAT-3/COSMIC and FY-3C Radio Occultation Measurements

The total electron content (TEC) profiles from the FORMOsa SATellite mission-3/Constellation Observing System for the Meteorology, Ionosphere, and Climate (COSMIC) and the Fengyun-3C (FY-3C) radio occultation (RO) missions during 2006 and 2019 are used to establish the models for deriving the ionospheric sporadic E (Es) critical frequency (foEs). S-max is derived as the maximum vertical gradient of TEC disturbance, which is extracted from a RO TEC profile using the singular spectrum analysis method. Three candidate conversion models are obtained by fitting S-max with foEs observed by collocated ionosondes, and the one with the best performance, the COSMIC & FY-3C (CF) model, is applied for deriving foEs (foEs(CF)) from the TEC data of the two missions jointly. For comparison, the C model, which is of similar form as the CF model while being established without using the FY-3C data, is applied for deriving foEs (foEs(C)) from the COSMIC TEC data only. The validations over four independent ionosonde stations demonstrate that mean improvements of 0.11 MHz (32%) in the absolute bias and 0.24 MHz (17%) in the root mean square error are obtained by foEs(CF) compared with foEs(C). The spatiotemporal distributions of Es intensity during 2015 and 2019, presented respectively by foEs(CF) and foEs(C), are further compared. The CF model provides more abundant and more detailed information about Es intensity distribution, which is most distinct at high latitudes where the contribution of FY-3C RO TEC data is significant. Our results reveal that with the aging of the COSMIC satellites, FY-3C TEC data is a valuable supplementary RO data source for Es-related studies. Plain Language SummaryIonospheric sporadic E (Es) layers have significant impacts on radio communication and the propagation of global navigation satellite system signals, and the degrees of the impacts are closed related to the distributions and the intensity of Es layers. The radio occultation (RO) total electron content (TEC) profiles can be used for detecting the existence and the intensity of Es layers due to that the Es layers can bring to sudden enhancements of TEC values. In this study, based on the TEC data from FORMOsa SATellite mission-3/Constellation Observing System for the Meteorology, Ionosphere, and Climate (COSMIC) and Fengyun-3C (FY-3C) RO missions and the collocated ionosonde observations during 2006 and 2019, an optimal model for deriving the Es intensity is promoted, and is compared with another model established using the same method but without using the FY-3C data. The model using the FY-3C data shows a better performance for deriving the Es intensity and provides more abundant and more detailed information about Es intensity distributions, especially in high-latitude regions. The results contribute to understanding the value of FY-3C RO TEC data for Es layer-related studies, which is most significant during the last years of COSMIC mission lifespan, when the aging of the satellites brings to the decline in the COSMIC RO data volume.