O+ density profiles provided by the ultraviolet imagers onboard ICON: comparison with radio based observations and role of the equatorial ionization anomaly

The NASA-ICON mission was dedicated to the observation of the terrestrial equatorial ionosphere between November 2019 and November 2022 from a circular orbit at about 600 km altitude. The scientific payload encompasses two ultraviolet imagers: the Far Ultraviolet Imaging Spectrograph (FUV) and the Extreme Ultra Violet (EUV) spectrograph. FUV observes the emission of the atomic oxygen doublet at 135.6 nm as well as the Lyman-Birge-Hopfield (LBH) band of N2 near 157 nm while the EUV spectrograph records daytime limb altitude profiles of terrestrial emissions in the extreme ultraviolet spectrum from 54 to 88 nm. Every 12s, based on the 135.6 nm emission for FUV and on the OII–61.7 nm and 83.4 nm emissions for EUV, both instruments provide O+ density profiles for nighttime and daytime conditions, respectively.
Besides, the GNSS radio-occultation mission COSMIC-2 daily provides, since 2019, several thousands of electron density profiles above low and mid-latitudes, in addition to ground-based ionosondes delivering high-quality observations at a regular cadence. For FUV, the peak density and height are, on average, similar to radio-based observations by about 10% in density and 7 km in altitude. The EUV spectrograph provides peak density values smaller than that from other techniques by 50 to 60%, while the altitude of the peak is retrieved with a slight bias of 10 to 20 km on average. While the equatorial ionization anomaly does not have a significant influence on the EUV comparisons, it is found that the largest density differences between FUV and C2/ionosonde data are related to the ionization crests where their large density gradients and specific geometry break the spherical symmetry assumed by the inverse Abel transform to retrieve the O+ density profile. We perform a dedicated analysis of these particular cases using GNSS-TEC maps to identify the problems arising when considering multi-sensor data fusion at low-latitudes.