Update of ICON-FUV hmF2 and NmF2 Comparison with External Radio Observations

The Far Ultraviolet Imaging Spectrograph (FUV) onboard the NASA-ICON spacecraft has been providing nighttime O⁺ density profiles over mid- and low-latitude since December 2019. These profiles are compared to electron density profiles provided by GNSS radio-occultations and ground-based ionosondes, mainly at the F-peak level where both density and height are compared. This work is an important update of the earlier study published by Wautelet et al. (J. Geophys. Res. Space Phys. 126(11):e2021JA029360, 2021) for two reasons: First, several methodological improvements have been implemented at the calibration and inversion levels. Second, the present work relies on an extended time range, ranging from December 2019 to August 2022, covering therefore periods of increased solar activity, which was not the case for the previous work. It is found that the peak density and height are, on average, similar to radio-based observations by about 10% in density and 7 km in height, meaning that FUV provides peak characteristics compatible with existing ionospheric datasets based on radio signals. However, comparisons of FUV and radio-occultation profiles have to be considered very carefully due to the potentially large difference in the observation geometry, which can account for large density discrepancies even between profiles being closely located and mostly simultaneous. This is particularly important around the crests of the equatorial anomaly where the largest density discrepancies have been observed. In addition, this study highlights the variability of the FUV profiles at the bottomside level, with the analysis of cases where rather large density values were observed while small density values are expected. The latter observation does nevertheless not impact the statistics concerning the F-peak characteristics, which show that FUV reliably monitors the peak density and height with an accuracy compatible to that of external data sources.