Effect of Coulomb Scattering on Propagation of Photoelectrons Between Conjugate Ionospheric Points

Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) is a future spacecraft mission supported by the European Space Agency and the Chinese Academy of Science to be launched in 2024. The mission will target the magnetopause, cusp, and bow shock regions. In order to study magnetosphere-ionosphere coupling, the spacecraft will take auroral images using an ultraviolet imager (UVI) being developed at the University of Calgary in collaboration with European and Chinese scientists. For scientific support and analysis of data collected by the UVI, a numerical model of ionospheric UV emission is being developed in the University of Calgary and the University of Alberta. The model has two modules. One module calculates UV emissions caused by suprathermal electrons. It includes all the prominent UV auroral/dayglow emission lines and band systems such as OI 130.4/135.6nm, Lyman-Birge-Hopfield (LBH) and Vegard-Kaplan (VK) bands. It accounts for absorption along the line-of-sight and calculates the integrated UV photon flux spectrum reaching each UVI pixel. Photoelectron energy spectra for the UV emission module are provided by a Monte Carlo model of photoelectron propagation in the Earth's ionosphere and magnetosphere. The model accounts for 52 kinds of electron-neutral collisions and Coulomb collisions. On closed geomagnetic field lines in the night sector, depending on the Earth's position relative to the Sun, the model predicts appearance of energetic photoelectrons coming from the day sector. Coulomb scattering prevents photoelectrons from reaching the opposite ionosphere [Khazanov et al, 1994]. To demonstrate the importance of Coulomb collisions, photoelectron fluxes and related UV emissions are calculated with the Coulomb collisions turned on and off for 5 points along the orbit of DMSP F16 satellite around UT0800 January 1, 2017, where anomalous UV emission induced by conjugate photoelectrons was observed [Kil et al., 2020]. Omitting the Coulomb collisions overestimates the photoelectron flux and the intensity of UV emission by up to 50%. The effect is more pronounced on longer field lines. Khazanov, Neubert, and Gefan (1994), IEEE Trans. Plasma Sci. 22, 187. Kil, Schaefer, Paxton, and Jee (2020), GRL 47, e2019GL086383.