Nonthermal Limit of Monoenergetic Precipitation in the Auroral Acceleration Region

Abstract

We report statistics from FAST satellite observations revealing the prevalence of nonthermal monoenergetic electron distributions within and below the auroral acceleration region (AAR), and over 21–24 magnetic local time. The energy spectra of intra-AAR auroral primaries reveal a probability distribution of κ (“kappa”) values that is “piled up” against a lower bound, κ ≳ κt ≃ 2.45. The measurements are recorded in the lower reaches of the AAR, as determined from simultaneous upward ion beams, and are therefore unaffected by processes at lower altitudes that may alter auroral primary distributions. We show evidence that the κ values below the AAR tend to be greater than those within the AAR, which suggests that auroral primaries evolve toward thermal equilibrium as they move toward the ionosphere. Only for ∼4% of the reported events corresponding to κ ≤ κt does a kappa current density-voltage relationship differ appreciably from the classical Maxwellian current density-voltage relationship.