Calculation of photoelectron induced UV emission with application to the SMILE mission

The SMILE mission, supported by the European Space Agency and the Chinese Academy of Sciences, is scheduled for launch in 2024. The mission is augmented by a substantial ground-based network of optical ASI's. Here, we report  progress in developing a numerical model of UV emissions to aid interpretation of images collected by the SMILE UVI.  The model calculates UV emissions produced by suprathermal electrons, accounting for prominent UV auroral and dayglow emission lines and bands, including OI 130.4/135.6nm, Lyman-Birge-Hopfield (LBH) and Vegard-Kaplan (VK) bands. It also calculates line-of-sight absorption and the integrated UV photon flux spectrum reaching each UVI-imager pixel. Photoelectron energy spectra for the UV emission module are generated using a Monte Carlo model of photoelectron propagation. This model accounts for 52 kinds of electron-neutral collisions as well as Coulomb collisions. Considering closed geomagnetic field lines in the night sector, and depending on Earth's position relative to the Sun, the model predicts the appearance of energetic photoelectrons coming from the day sector. Coulomb scattering prevents pthese hotoelectrons from reaching the opposite ionosphere [c.f., Khazanov et al, 1994]. To reveal the importance of Coulomb collisions, model photoelectron fluxes and related UV emissions were calculated with Coulomb collisions included and omitted for five locations along the orbit of the DMSP F16 satellite on UT0800 January 1, 2017, which observed anomalous UV emission induced by conjugate photoelectrons [Kil et al., 2020]. Omitting Coulomb collisions overestimates the photoelectron flux and the intensity of UV emission by up to 50% with the effect being more pronounced on longer field lines. Solar EUV photons also produce energetic photoelectrons which ionize neutrals, heat ambient electrons, and cause UV emission. These photoelectrons can penetrate into the nightside even when connected to the day sector by geomagnetic field lines. UV emission caused by such photoelectrons in the night sector is called anomalous UV emission. knowlege of which is important for the analysis of data from the SMILE UVI. The model development for SMILE includes a module that calculates propagation of photoelectrons and related UV emission. Results from the model are benchmarked against observations by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) of the DMSP F16 spacecraft. The spacecraft was in Earths shadow, and traveling towards the equatorial plane. The observed anomalous UV emission rapidly decreases as the spacecraft approaches lower latitudes where field lines are shorter and almost completely in the shadow. Values of the UV emission at wavelengths of 135.6 nm and 130.4 nm were calculated from the model at several locations along the spacecraft orbit. Calculations performed with a tilted dipole geomagnetic field gave values that were significantly larger than the observed ones. Calculations using the International Reference Geomagnetic Field (IGRF) provided much improved agreement between the model and the observation because the IGRF places the southern ends of geomagnetic field lines farther from the sunlit hemisphere. The improved agreement suggests the model development related to the SMILE mission will aid interpretation of the data.