Far ultraviolet airglow remote sensing measurements on Feng Yun 3D meteorological satellite

The Ionospheric Photometer (IPM) is carried on the Feng Yun 3D (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 night-side thermosphere and 135.6 nm 15 and N2 LBH emissions in the day-side thermosphere that can be used to invert the peak electron density of the F2 layer (NmF2) at night and O/N2 ratio in the daytime, respectively. Preliminary observations show that the IPM could monitor the global structure of the equatorial ionization anomaly (EIA) structure around 2:00 local time using OI 135.6 nm nightglow properly. It could also identify the reduction of O/N2 in the high-latitude region during the geomagnetic storm of Aug. 26, 2018. The IPM derived NmF2 accords well with that observed by 4 ionosonde stations along 120oE with a standard deviation 20 of 26.67%. Initial results demonstrate that the performance of IPM meets the designed requirement and therefore can be used to study the thermosphere and ionosphere in the future.

times more than FUV radiation. The other problem is that ionospheric photometers need to eliminate 130.44nm and shorter 60 wavelengths airglow and collect 135.6 nm airglow emission with high sensitivity.  The IPM instrument is shown in Fig. 1 and includes a telescope, a filter wheel, a detector system, and control electronics cabinet. The telescope has a field-of-view of 3.5° (along orbit)×1.6° (cross orbit). An off-axis aluminum mirror coating 70 MgF 2 is used to collected airglow emission in the telescope. To suppress the longer wavelength radiance, a sunblind PMT (R10825, Hamamatsu) with CsI photocathode is used in the detector system (Fu et al, 2015). The quantum efficiency of the PMT with an effective area of 4 × 9.5 mm, is about 26 % at the wavelength 135.6 nm, 6.17×10 -5 at 254 nm, and 4.06×10 -8 at 514 nm. The PMT has better than 10 −4 rejection at wavelengths longer than 200 nm. IPM monitors 135.6 nm emissions in the nighttime and 135.6 nm and N 2 LBH emissions in the daytime by employing a filter 75 wheel. There are six spots in the filter wheel ( Fig. 1 (c)) corresponding to six channels of IPM: dark count channel, 135.6 nm nightside channel, red-leak nightside channel, red-leak dayside channel, N 2 LBH dayside channel, and 135.6 nm dayside channel. The Channel information of IPM is shown in Table 2. In order to suppress the longer wavelength radiance further, the band-pass filter centred on 135.6 nm is used in the 135.6 nm dayside channel, and the band-pass filter centred on 160 nm is used in the N 2 LBH channel. Besides, IPM specifically adds two red-leak signal channels for daytime and nighttime red-80 leak respectively. Based on the design of dayside or nightside channel, a SiO 2 filter is added in red-leak channels in order to eliminate below 180 nm wavelength airglow. By differencing the measurements of dayglow channels and red-leak dayside channel, dayglow radiations can be detected. And by differencing the measurements of 135.6 nm nightside channel and redleak nightside channel, 135.6 nm radiation in the nighttime can be detected. To exclude radiation shorter than 135.6 nm completely, a 0.5 mm-thin VUV-grade BaF2 flat filter is used and the transmittance at 135.6 nm at room temperature is 0.5 85 (Fu et al., 2015). The emission of wavelengths shorter than 132 nm cannot pass the 0.5 mm-thick BaF 2 filter over a temperature range of 5 °C to 35 °C.

Laboratory Calibration 95
The IPM was calibrated in ground laboratory prior to flight. The optical calibration facility in the ground laboratory has a deuterium lamp, a monochromator, a collimator, a diffuser board, and a NIST standard detector assembled in a modular pattern. The deuterium lamp (L11798) with a MgF 2 window has 150W power and provides a bright, stable source of FUV radiation. The source of FUV radiation is wavelength-selected by the monochromator (234/302) which has a ƒ/4.5 0.2 m Czerny-Turner with a 1200 grooves/mm grating. A collimator ensures that the beam consists of parallel rays. The NIST 100 standard detector (AXUV-100G) provides a reference for calibrating IPM. The entire facility is installed in a vacuum environment which allows the propagation of radiation in the far ultraviolet. light source, the error of the standard detector, the bi-directional reflection distribution function (BRDF) uncertainty of the diffuser board, the non-uniformity of the light source, and so on. The uncertainty of the ground calibration is estimated to reach 11.25%. As a function of wavelength, the responsivity of the 135.6 nm nightside channel from 130 to 200 nm is shown in Figure 2. The responsivity to 135.6 nm radiation at night is about 266.9 counts/s/R near the peak of the responsivity 115 function distribution, and reaches the design requirement of the 135.6 nm nightside channel. The responsivity to 135.6 nm radiation at night provides high sensitivity in observations of OI 135.6 nm radiation at night.

120
As a function of wavelength, the responsivity of the 135.6 nm dayside channel from 130 nm to 200 nm is shown in Figure 3.
The responsivity to the 135.6 nm radiation in daytime is about 23.20 counts/s/R, and also reaches the design requirement of the 135.6 nm dayside channel. The responsivity is much less than the one on the nightside due to the bandpass used in the 135.6 nm dayside channel, which is designed to obtain the radiation of 135.6 nm in daytime and suppress the radiation at 125 wavelengths shorter than 135.6 nm, N 2 LBH and red-leak in daytime. The other bandpass is used in the N 2 LBH day channel in order to obtain the radiation of N 2 LBH and suppress the radiation of 135.6 nm and red-leak in daytime. The responsivity of N 2 LBH channel is shown in Figure 4.   The example of the global count of the 135.6 nm nightside channel is presented in Fig. 6 (a). The red solid line indicates the 150 magnetic dip equator. The data in Fig. 6 are from 7 to 11 December 2017. From 7 to 11 December 2017, Kp index is not more than 4 and the geomagnetic condition kept quiet relatively. As shown in Fig. 6 (a), there is a high-count area near the magnetic dip equator in South America, which shows the contamination in SAA associated with particles impacting the instrument. The example of global brightness of the 135.6 nm nightside channel without red-leak and the effect of dark count is presented in Fig. 6 (b). As shown Fig. 6 (b), there are some brighter areas located oneither side of the magnetic dip equator 155 in South America and Africa, which are the so-called equatorial ionization anomaly (EIA) structure. EIA has been https://doi.org/10.5194/amt-2021-195 Preprint. Discussion started: 30 September 2021 c Author(s) 2021. CC BY 4.0 License. 160 studied extensively by using data from ground-based ionosodes (Moffett and Hanson, 1965;Walker, 1981) and groundbased optical observations (Thuillier et al., 1976). The OI 135.6 nm emission data from GUVI on board TIMED satellite, FUV on board the IMAGE satellite, and the TIP on board the COSMIC satellites have been used in study of the EIA phenomenon (Christensen et al., 2003;Sagawa et al., 2005;Immel et al, 2006 andCoker et al., 2009). The local time of the IPM orbit on the nightside is 2:00 am. The EIA structure which we found at the 2:00 local time is later than other results 165 mentioned earlier, and it need to be studied furtherly.

NmF 2 and TEC
OI 135.6 nm emission is one of the strongest lines in the FUV nightglow at low latitudes and has relatively high transparency in the upper atmosphere. In the nightside ionosphere, there are two primary production mechanisms of OI 135.6 nm emission: (1) Atomic oxygen is excited through the recombination of atomic oxygen ions with electrons and produces OI 135.6 nm 170 emission; (2) Atomic oxygen is excited through the mutual neutralization of O + with O − and produces OI 135.6 nm emission (Meier, 1991). The mutual neutralization has a relatively smaller contribution. The brightness of OI 135.6 nm emission varies with the electron density and the oxygen ion concentration basically. Equivalently, OI 135.6 nm emission is approximately proportional to the square of the electron density in the F-region.
Based on the previous studies of the nighttime OI 135.6 nm airglow using the radiative and emissive model, IRl2000 model, 175 and MSISE90 model, the retrieval algorithm of NmF 2 derived from nighttime OI 135.6 nm emission was presented by Jiang et al. (2018). The brightness of the nighttime OI 135.6 nm emission is used to calculate ionospheric NmF 2 by the ratio https: //doi.org/10.5194/amt-2021-195 Preprint.  between NmF 2 and OI 135.6 nm emission from the retrieval algorithm. We selected the IPM derived NmF 2 data which were near to four IGGCAS ionosonde stations (Sanya (18.3º N,109.6º E), Wuhan (30.5º N,114.4º E),Beijing (40.3º N,116.2 º E), and Mohe (50.2º N,122.5º E)) from November 25, 2017 to May 8, 2018. Their difference in longitude were less than 12º 180 and in latitude were less than 5º . There is a standard deviation of 26.67% between IPM NmF 2 and IGGCAS ionosonde NmF 2 (shown in Fig. 7). between the IPM substellar point and ionosonde stations is less than 12º , and the latitude difference is less than 5º . ) 2014). We further calculated total electron content (TEC) from IPM results and compared with that of MIT TEC data from November 25, 2017 to April 8, 2018. The MIT TEC data (Rideout and Coster, 2006) was obtained from the MIT Haystack Observatory Madrigal database (http://www.openmadrigal.org). There is a standard deviation of 39.41% between IPM TEC 190 (total electron content unit, TECu) and MIT TEC (TECu) (shown in Fig.8). The standard deviation between IPM TEC (TECu) and MIT TEC (TECu) is more than the one between IPM NmF 2 and IGGCAS ionosonde NmF 2 . In the Ionosphere plasmasphere coupled system, the ionosphere in conjugate hemispheres forms a plasmasphere reservoir along the interconnecting flux tube. There is diurnal interchange between the ionosphere and the plasmasphere that the downward diffusion from the plasmasphere helps to maintain the nighttime F 2 -layer. The results of Jason-1, Metop-A, and TerraSAR-X (Yizengawa et al., 2008;Zakharenkova and Cherniak, 2015;Klimenko et al., 2015) show that at day the contribution of the plasmasphere in TEC is less than the one of the ionosphere, whereas at night the contribution of the plasmasphere in TEC is increasing and even more than the one of the ionosphere. Auroral emission can be derived from the 135.6 nm nightside channel. There is obviously a strong auroral emission feature in the Northern Hemisphere in Fig. 6. By the way, the wide-field auroral imager (WAI), one of ten scientific instruments aboard the Feng Yun 3D meteorological satellite, has provided large field of view (FOV), high spatial resolution, and broadband ultraviolet images of the aurora (Zhang et al, 2019).