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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 kappa ("kappa") values that is "piled up" against a lower bound, kappa greater than or similar to kappa(t) similar or equal to 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 kappa 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 similar to 4% of the reported events corresponding to kappa <= kappa(t) does a kappa current density-voltage relationship differ appreciably from the classical Maxwellian current density-voltage relationship.

Plain Language Summary Many natural plasmas are extremely rarefied (i.e., collisions between individual charged particles are infrequent) and the motion of the charged particles that comprise them may be highly ordered or correlated. Since one definition of thermal equilibrium is perfectly uncorrelated motion, such ordered motion demonstrates that these plasmas are not strictly in thermal equilibrium. The degree of nonthermality is indicated by the kappa ("kappa") index in space physics and is important because it modifies, for example, plasma resonant frequencies and how/where the energy of a precipitating plasma is deposited in the ionosphere. Satellites and ionospheric sounding rockets that measure properties of magnetospheric plasmas have observed kappa indices indicating that magnetospheric plasmas are out of thermal equilibrium. However, measurements of kappa in the ionosphere are uncertain because magnetospheric precipitation collides with dense ionospheric plasmas. From observations made by the Fast Auroral SnapshoT Explorer (FAST) satellite above the ionosphere, which are free from the effects of low-altitude collisions, we report statistics of the.. index evincing a "pile-up" near kappa = 2.45, which is strong evidence of a presently unknown mechanism for driving magnetospheric precipitation out of thermal equilibrium. These results therefore expose an important deficiency in current understanding of the auroral acceleration process.