Insights into Nightside Auroral Electron Precipitation from FAST Observations Including Evidence of Alfvénic and Inverted-V Acceleration at Low-Latitudes During Large Storms

We present findings of a study of low latitude (<~60° ILat) nightside auroral electron populations from FAST spacecraft data. Such auroral electron acceleration is of interest due to the insights it can provide to auroral processes and magnetospheric-ionospheric coupling in general, but also because these field lines, particularly during large storms, act as a diagnostic into the region of interaction between plasma sheet, ring current and plasmaspheric particle populations. Our analysis of the 159 large storms (DST <-75nT) during the >12 year life of FAST indicate that significant, non-diffuse auroral, electron precipitation occurs during >23% of the FAST nightside orbits >1° equatorward of the nominal, AL-based, diffuse auroral boundary during such storms. And they are observed >58% of the time in such orbits >1° equatorward of the nominal, AL-based, auroral zone boundary. The vast majority of these events would be considered “broadband” electron precipitation events and would result in what is classified as type-d aurora, with electron energies of 1-1000 eV. Presented detailed analysis of these electron precipitation events indicates that, while the precise prevalence of the various acceleration mechanisms producing these events is not easily determined, 1) the mechanisms responsible are varied, and 2) the features of most of these events are consistent with inverted-V and/or Alfvénic acceleration processes occurring on these field lines. Evidence for these acceleration mechanisms in operation during these events as well as details of the extent of the difficulties in isolating and definitively identifying these mechanisms are presented. Analysis of these events has also led to further understanding of the inverted-V acceleration process, even during non-storm times, and the trends in dependencies of the potential structure characteristics on latitude are also presented.