When sunlight shines upon planetary atmospheres, energetic photoelectrons are excited and ejected from neutral atoms by the absorption of extreme ultraviolet solar radiation. Photoelectrons play a key role in many aspects of atmospheric physics; they are an important source of heat for the ionosphere and upper thermosphere (Shea et al., 1968); they strengthen the ambipolar electric potential drop generated by planetary ionospheres (Khazanov et al., 1997;Lemaire, 1972) which is critical to understanding the quiet time polar wind (Banks & Holzer, 1968) and its global structure (Glocer et al., 2017); photoelectrons are also important in magnetospheric physics, enabling field line tracing (