Polar Ionospheric TEC Inversion Based on Spaceborne Fully Polarimetric SAR and Its Response to Aurora

Auroral activity can enhance ionospheric Total Electron Content (TEC) at high latitudes and thereby affect the propagation of L-band electromagnetic waves transmitted by spaceborne fully polarimetric Synthetic Aperture Radar (SAR) systems. These ionospheric disturbances introduce additional phase delay and polarization distortion, which may reduce SAR data quality and influence the reliability of quantitative interpretation. Because the Faraday Rotation Angle (FRA) derived from fully polarimetric SAR observations is sensitive to the ionospheric electron content along the propagation path, it can be used to retrieve TEC with relatively high spatial resolution. In this study, 32 fully polarimetric SAR scenes were used to examine the performance of four FRA-based inversion algorithms for polar TEC retrieval under auroral conditions. The retrieved TEC was compared with colocated ground-based GNSS-TEC observations in order to evaluate the applicability of different algorithms in the polar ionosphere. In addition, two revisit SAR cases were selected to investigate how TEC enhancements related to auroral activity are reflected in SAR signal characteristics. This analysis makes it possible to assess both the relative performance of the four inversion methods and the response of polarimetric SAR observables to auroral ionospheric disturbances. Results show clear differences among the four algorithms. Of the tested methods, the Bickel and Bates (B&B) algorithm gives TEC estimates that agree best with the ground-based GNSS-TEC measurements, suggesting that it is more suitable for polar TEC retrieval. The revisit cases further show that TEC enhancements associated with auroral arcs can produce distinct changes in SAR observations. In regions affected by enhanced ionization, FRA increases noticeably, whereas the cross-circular polarization product decreases. These changes indicate a deterioration in SAR signal quality, especially a reduction in signal-to-noise ratio. Overall, the results indicate that fully polarimetric SAR, combined with an appropriate FRA inversion method, can provide useful information on polar ionospheric TEC and its disturbance characteristics during auroral activity. This approach offers a practical way to support high-latitude ionospheric studies and to examine the relationship between auroral processes and TEC variability.