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The OI135.6 nm radiation intensity and the associated change with solar activity are very complex, and this is particularly the case during November 2020. In this paper, we investigated the OI135.6 nm radiation intensity in the low-latitude ionosphere during a quiet geomagnetic period. The Ionospheric Photometer (IPM) instrument onboard the FY-3(D) meteorological satellite was employed to measure the OI135.6 nm night airglow at 02:00 LT (local time) and its response to the solar activity. The results showed there is a statistically significant correlation between the intensity of the equatorial ionospheric anomaly (EIA) and solar activity after midnight. The EIA at 02:00 LT and before midnight shared the same climatological characteristics—strong in equinoxes and weak in solstices. In November 2020, when the F10.7 flux significantly increased, the OI135.6 nm radiation intensity in the EIA region recorded a 100–200% increase compared to the previous month, which was much higher than in the same period in the preceding two years. A similar phenomenon was observed at the same time by the Global-scale Observations of Limb and Disk (GOLD), which makes continuous observations of ionospheric structure variation in global patterns. Data analysis suggests that the EIA at 02:00 LT was due to the attenuation of the EIA before midnight after the disappearance of the eastward electric field. The magnetic latitude of the EIA crest (hereafter denoted by θ m l F 2 ) indicates a range-and-seasonal rule of hemispherical asymmetry: closer to the geomagnetic equator in equinoxes and farther away from the geomagnetic equator in solstices. Further studies are needed. [ABSTRACT FROM AUTHOR]

The Ionospheric Photometer (IPM) is carried on the Feng Yun 3-D (FY3D) meteorological satellite, which allows for the measurement of far-ultraviolet (FUV) airglow radiation in the thermosphere. IPM is a compact and high-sensitivity nadir-viewing FUV remote sensing instrument. It monitors 135.6 nm emission in the nightside thermosphere and 135.6 nm and N 2 Lyman–Birge–Hopfield (LBH) emissions in the dayside thermosphere that can be used to invert the peak electron density of the F 2 layer (NmF 2) at night and the O/N 2 ratio in the daytime, respectively. Preliminary observations show that the IPM could monitor the global structure of the equatorial ionization anomaly (EIA) structure around 02:00 LT using atomic oxygen (OI) 135.6 nm nightglow. It could also identify the reduction of O/N 2 in the high-latitude region during the geomagnetic storm of 26 August 2018. The IPM-derived NmF 2 agrees well with that observed by four ionosonde stations along 120 ∘ E with a standard deviation of 26.67 %. Initial results demonstrate that the performance of IPM meets the design requirements and therefore can be used to study the thermosphere and ionosphere in the future. [ABSTRACT FROM AUTHOR]

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. Key Points: We develop a lookup‐table algorithm for ionosphere retrieval with the FUV spectral imager on the Fengyun‐4C geostationary satelliteAssumptions made in the algorithm lead to only ∼2%–4% uncertainties in the nmF2 and Total Electron Content (TEC) retrievals under the modeled ionospheric conditionsThe algorithm is verified by retrieving nmF2 and TEC from the Global‐Scale Observations of the Limb and Disk observations and comparing with external radio measurements [ABSTRACT FROM AUTHOR]