An Improved Method for Function-Based Ionospheric Tomography With Plasmasphere Correction and Virtual Ray Augmentation

The ionosphere is a crucial component of the Earth’s atmosphere and a focal point for various space application activities. This study proposes an improved method for function-based ionospheric tomography with plasmasphere correction and virtual ray augmentation. First, a cubic polynomial is used to fit the electron content variations in the plasmasphere above 2000-km concerning time, latitude, and longitude. The contribution of the plasmasphere to total electron content (TEC) is then corrected rather than being treated as a constant. The results indicate that the electron content above 2000 km varies from a maximum of 1.6 TECU to a minimum of 0.75 TECU. Second, spherical harmonics and empirical orthogonal functions (EOFs) are employed to describe the horizontal and vertical distribution of ionospheric electron density (IED). To address the insufficient distribution of ground-based observation stations, a method is proposed to calculate the vertical TEC of virtual rays by the IRI-plas model with the global ionosphere map (GIM) assimilation, thereby enhancing ray coverage. In the study area, the performance of the tomography model is improved with the increase in the order of the spherical harmonic function, while the improvements are not significant for orders above 8. Finally, the inverted slant TEC (STEC) is compared to the original STEC observations to extract the modeling residuals. Corrections are made to the inverted IED profiles based on the STEC residuals. The modeling utilizes data from 243 global navigation satellite system (GNSS) stations located in the China region. The daily average internal accuracy of the function-based tomography model ranges from a minimum of 1.15 TECU to a maximum of 1.60 TECU, with an overall 30-day average internal accuracy of 1.34 TECU. The external accuracy has a maximum of 1.85 TECU, a minimum of 1.4 TECU, and a 30-day average of 1.55 TECU. A comparison with ionosonde data shows that the RMSE at different altitudes averaged $7.36\times 10^{10}$ el/m3 for the first-step model and $6.82\times 10^{10}$ el/m3 for the second-step model. Additionally, a comparison with 30-day radio occultation data yielded average RMSEs of $6.09\times 10^{10}$ and $5.40\times 10^{10}$ el/m3, confirming improved accuracy in regions with limited ray penetration.