Inferring the Ionospheric State With the Far Ultraviolet Imager on the Fengyun‐4C Geostationary Satellite: Retrieval Algorithm and Verification

Abstract

The Multiband Ultraviolet Spectrum Imager (MUSI) is an optical remote sensing instrument scheduled to launch on the Fengyun‐4C meteorological satellite in 2024. MUSI is designed to measure the airglow emissions between ∼120 and 160 nm above the East Asia and Pacific region from a geostationary orbit to infer ionospheric parameters. Here we develop a lookup‐table algorithm for retrieving the peak electron density (nmF2) and the Total Electron Content (TEC) from nighttime observations of the OI 135.6 nm emission. The algorithm takes into account the north‐south asymmetry of the equatorial ionization anomaly and the atmospheric variations with solar activity and local time. Analysis of synthetic observations shows that the assumptions made in the algorithm only lead to ∼2%–4% uncertainties in the retrievals under the modeled ionospheric conditions. The algorithm is verified by retrieving nmF2 and TEC from the Global‐Scale Observations of the Limb and Disk (GOLD) mission and comparing with coincided radio measurements. The comparison indicates almost negligible systematic differences (typically less than ∼3%) between the GOLD and the ionosonde nmF2, which are within the error limit of the GOLD retrievals. The accuracy of the GOLD TEC retrievals is shown to be comparable to (or in some sense even better than) that of the measurements from the Global Navigation Satellite System. Several simplified yet robust retrieval methods are also developed, which can be readily implemented for both existing and upcoming missions. Our results promote far ultraviolet (FUV) remote sensing to be a key technology for accurate determination of the ionospheric state.

Resolving Some Longstanding Issues in Far Ultraviolet Remote Sensing: A Self‐Consistent Analysis of the GUVI and SEE Observations on the TIMED Mission

“…For this reason, there are usually no "ground-truth" measurements that the physical retrievals from FUV remote sensing can be compared with for validation. One exception is the ionospheric FUV remote sensing, for which the retrieved O + density can be validated by comparison with measurements from ionosondes and incoherent scattered radars (Dymond et al, 2019;Qin et al, 2023;Stephan, 2016;Wautelet et al, 2021). Second, quantification of the uncertainties in the retrievals from FUV remote sensing is extremely difficult due to the complexities in the forward model, the systematic uncertainties in the instrument calibration, and the statistical uncertainties in the optical measurements (e.g., Bhattacharyya et al, 2017;Chaufray et al, 2008;Meier et al, 2015;Qin, 2021;Qin et al, 2017).…”

“…To date, a fundamental challenge remains, that is to develop useful methods for examining and improving the accuracy of FUV remote sensing data products. While ionospheric FUV remote sensing can be examined by comparison with ground-based measurements from ionosondes and incoherent scattered radars (Dymond et al, 2019;Qin et al, 2023;Stephan, 2016;Wautelet et al, 2021), no such "ground-truth" measurements exist for thermospheric FUV remote sensing to compare with.…”

Mars Hot Oxygen Density and Effective Temperature Derived From the MAVEN IUVS Observations

“…For this purpose we modify the one-population radiative transfer model that was recently developed by Qin and Harding (2020) and Qin (2020b) for the studies of the cold oxygen coronae in the terrestrial and the Martian atmospheres (e.g., Qin, 2020a;Qin et al, 2023;Yin et al, 2023). That model was developed based on the Monte Carlo algorithm of Meier and Lee (1982), which uses an angle-dependent partial frequency redistribution function.…”

A Comparative Analysis of the Solar Ultraviolet Spectral Irradiance Measured from Earth and Mars: Toward a General Empirical Model for the Study of Planetary Aeronomy

“…Solar VUV radiation can be separated into X-ray ultraviolet (XUV, 0.1-10 nm), extreme ultraviolet (EUV, 10-120 nm), and farultraviolet (FUV, 120-200 nm) radiation, which can vary drastically by 1 or more orders of magnitude on timescales of minutes (e.g., due to eruptive solar flares), days (e.g., due to solar rotation with periods of ∼27 days), and years (e.g., due to solar cycle variations with periods of ∼11 yr), as well as on geometric scales (e.g., due to the varying distances of the Sun to Earth, Mars, and other planets) (Woods & Eparvier 2006;Woods 2008). Accurate estimation of solar VUV irradiance at various locations and times in the solar system is of critical importance for the study of planetary aeronomy, such as for the study of thermospheric and ionospheric variations (Haider et al 2002;Liu et al 2011;Zhang et al 2015), for the development of global circulation models (Solomon & Qian 2005;Qian et al 2008;Deng et al 2012), for the modeling of airglow emissions (Meier et al 2015;Solomon 2017;Qin 2020Qin , 2021Wan et al 2022;Qin et al 2023;Yin et al 2023;Qin et al 2024), for the prediction of space weather effects (Lathuillere et al 2002;Lilensten et al 2008), and for the study of climate evolution (Lilensten et al 2008;Persson et al 2020).…”