Validation of FORMOSAT-7/COSMIC2 IVM ion density and TGRS orbit electron density

• Published in: TAO : Terrestrial, atmospheric, and oceanic sciences atmospheric, and oceanic sciences, 2021-12, Vol.32 (6I), p.939-951
• Main Authors: Chou, Min-Yang, Braun, John J., Wu, Qian, Heelis, Roderick A., Zakharenkova, Irina, Cherniak, Iurii, Pedatella, Nicholas M., Stoneback, Russell A.
• Format: Article
• Language: English
• Subjects: Accuracy; Coefficients; Comparative analysis; Correlation coefficient; Correlation coefficients; Density; Electron density; Electrons; Equatorial ionosphere; Equatorial regions; Errors; Evaluation; Global navigation satellite system; Ion density; Ion density (concentration); Ion velocity; Ionosphere; Morphology; Orbits; Payloads; Satellites; Temporal variations
• ISSN: 1017-0839; 2311-7680
• DOI: 10.3319/TAO.2021.06.22.01

In this study, the comparison between the orbit electron density and total ion density measured by payloads of Tri-GNSS Radio-Occultation System (TGRS) and Ion Velocity Meter (IVM) onboard the FORMOSAT-7/COMSIC2 (F7/C2) satellites is presented. The collocated TGRS and IVM observations for each of the F7/C2 satellites at ~715 and ~540 km are evaluated during the whole year of 2020. Comparative analysis reveals that the TGRS and IVM density observations have high correlation coefficients of 0.92 - 0.96 for each of the F7/C2 satellites. The mean differences are around -0.03 × 10^4 to 0.02 × 10^4 cm^(-3) with standard deviations ranging from ~0.91 × 10^4 to 2.18 × 10^4 cm^(-3), demonstrating a good agreement between the independent TGRS and IVM observations. Furthermore, the collocated observations are utilized to examine the global spatial and temporal variations of the topside ionosphere. The results show that the morphologies of the topside ionosphere in TGRS orbit electron density are nearly identical to the IVM ion density, suggesting that both F7/C2 payloads reliably produce accurate topside ionosphere observations. We found that the TGRS orbit electron density tends to be 10 - 30% smaller than the IVM ion density in the daytime equatorial ionosphere region at both high and low orbits, but the density differences decrease to approximately -30 - 0% during the nighttime. These density differences could be due to the error caused by the spherical symmetry assumption in TGRS orbit electron density estimation. Observing system simulation experiments are further performed to evaluate the relative errors between the TGRS and IVM density observations at high and low orbits.