The critical importance of multiple concurrent electron precipitation mechanisms and backscatter to understanding auroral precipitation, a study using FAST satellite data analysis and computer models

Electron precipitation is the dominant particle interactions from an energy and atmospheric effect viewpoint in MIT coupling. Therefore, understanding the details of the mechanisms causing this precipitation, what effects they have in the (IT) atmosphere and with each other is critical to understanding MIT coupling. While the spectral classifications of "monoenergetic", "broadband" and so called "diffuse" have been dominant for over a decade and have been useful in a preliminary way for progress in the field, we now understand that these labels of the downgoing electron spectral characteristics have only a partial relation to the physical mechanisms involved in the precipitation (Dombeck et al, 2018). Multiple electron precipitation mechanisms occurring on the same field lines are the norm, occurring about in about 70% of electron precipitation. Additionally, the downgoing precipitation is only one component of the MIT coupling electron interactions. Backscatter caused by the precipitation causes only part of the energy and particle input to be absorbed into the IT, and it feeds back on and affects the precipitation mechanisms. In particular, any quasi-static potential at all, even a low potential one, which are ubiquitous in precipitation events (of all three types in spectral classifications) cause at least part of the backscatter to be reflected back down. Events without quasi-static potentials allow all backscatter electrons to escape; potentially (likely) re-precipitating in the conjugate hemisphere. We present results of the combination of a detailed study of backscatter escape and re-precipitation, and analysis of multiple mechanism interaction effects from the entire (13 year) FAST mission and insight gained from 2 different computer models. One is an empirical backscatter model derived from the FAST data, and the other is a physical model of the effects quasi-static potential structures of various magnitudes have on electrons in conjunction with other mechanisms. These results indicate, among other things, that the quasi-static mechanism dominates the effects even when the potential drop is well below the temperature (kT) of the source population and substantially modifies and amplifies the effects of Alfvénic acceleration.