Magnetosphere-Ionosphere coupling processes shape precipitating electron distribution functions within the aurora

Auroral electron precipitation is a major form of magnetospheric energy input to the upper atmosphere at high latitudes. The energy distribution of the precipitating electrons can have a significant effect on how and where energy from the precipitating electrons impacts the ionized and neutral populations of the upper atmosphere. Recent theoretical work by Khazanov et al., [2016, 2017] used the SuperThermal Electron Transport (STET) code to show that precipitating electrons of magnetospheric origin can be reflected back into the magnetosphere at the two magnetically conjugate atmospheres, leading to a series of multiple reflections that can greatly alter the initial precipitation flux distribution. The resultant population of secondary and primary electrons cascade toward lower energies during the multiple reflections in the conjugate ionospheres. This was observationally demonstrated recently by Wing et al. [2019] using precipitating electron observations from the Defense Meteorological Satellite Platform (DMSP) satellites, who showed electron distributions functions with significantly enhanced low energy tails within moderate auroral activity. Wing et al. used the STET code to explain such distributions. Further examination of one of the substorm periods analyzed by Wing et al [2019] revealed that often the precipitating electron distributions with strong low-energy tails are surprisingly narrow and/or short-lived, lasting for no more than 1 sec; and they also tend to bound inverted V accelerated populations. The inference is that these short-lived distributions are either evidence that the build-up of low energy tails by multiple reflections happens in time scales much faster than theoretically predicted, or that they occur over very narrow spatial scales. We expand this study here and examine several periods of mild, moderate, and intense auroral activity to identify distributions with enhanced low energy tails. We use DMSP observations of precipitating electrons at 1 Hz temporal resolutions and REMEI satellite observations of precipitating electrons at 25­­­ Hz temporal resolution. We seek to understand through observational analysis and modeling firstly, when they occur and secondly, how they form and how they are structured.