Characterizing the ionization anomaly crests using the Far Ultraviolet imager onboard ICON

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. Onboard, the FarUltraViolet imager 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. The latter operates daytime only and is used for the monitoring of the thermospheric [O/N2] ratio. During nighttime, the 135.6 nm channel provides six brightness profiles every 12s with a vertical resolution of a few km, which once inverted, give O+ density profiles.
The inverse Abel transform procedure, used to retrieve the O+ density profile, generally assumes a spherically symmetric ionosphere, which is a reasonable hypothesis over mid-latitudes or during deep night conditions. However, the presence of important asymmetries such as those due to the presence of the ionization crests violate this assumption and distort the retrieved profiles, which can strongly differ from profiles derived from radio-occultation and ionosonde data. Remote sensing of the equatorial ionization anomaly crests in 3D is therefore an important challenge not only for constraining the inversion procedure, but also for modeling and forecasting purposes. More specifically, their rapid morphological changes and the rise of associated irregularities require to observe and characterize them at rather short timescales. The day-to-day variability of the crests, both in amplitude and in spatial extent, is particularly difficult to predict and remains a source of discrepancies between models.
In this study, we use COSMIC-2 radio-occultation data, ionosonde measurements and FUV remote sensing observations to analyze coincidental simultaneous observations from different origins, and identify the problems arising when considering multi-sensor data fusion at low-latitudes. We then deduce a novel method for monitoring the morphology of the equatorial ionization crests using the high cadence and high-resolution of the ICON-FUV instrument during nighttime.