Non-thermal limit of mono-energetic electron precipitation in the terrestrial auroral acceleration region

We report statistics from Fast Auroral Snapshot (FAST) satellite observations revealing the prevalence of non-thermal mono-energetic electron distributions within and below the auroral acceleration region (AAR), over the magnetic local time range 21-24 hours. When fitted to kappa-distributions, the energy spectra of intra-AAR auroral primary electrons reveal a probability distribution of kappa values that is "piles up" against a lower bound of K=2.45. The measurements are made in the lower reaches of the AAR, as determined from the simultaneous upgoing ion beams, and are therefore unaffected by processes at lower altitudes that may alter auroral primary distributions. There is evidence in the altitude variations of the kappa values that distributions below the AAR tend to be characterized by higher kappa values than are those measured within the AAR, which suggests that the auroral primary electrons evolve toward thermal equilibrium as the move downward toward the ionosphere. Only for about four percent of the measured distributions, corresponding to those with kappa less than 2.45, does the current density-voltage relationship modified for kappa distribution differ appreciably from the classic Maxwellian current density-voltage relationship.