The term "killer electron" was first used in an article by Graham (1994) to describe highly energetic (relativistic) radiation belt electrons that can seriously damage orbiting satellites. Since the 1990s, spacecraft safety has been a pressing concern of space weather science, and a major challenge is forecasting radiation belt conditions (Baker, 2002;Horne et al., 2013;Webb & Allen, 2004). There have been many studies related to the generation and dynamics of relativistic (>1 MeV) radiation belt electrons (e.g.,