Spencer Hatch, PhD Candidate,

Dept. of Physics and Astronomy, Dartmouth

Extremely Nonthermal Monoenergetic

Precipitation in the Auroral Acceleration

Region: In Situ Observations

We present FAST satellite observations of discrete, field-aligned

electron precipitation with differential number flux spectra

exhibiting remarkably high fluxes at energies well above the

energy of the observed flux peak, which indicate a

magnetospheric electron source population in a stationary state

far from thermal equilibrium. We show that observed

distributions are much better described by a kappa distribution

with  ≃ 2 than by a Maxwellian distribution. As a second,

independent estimate of the observed kappa values we show

that the field-aligned current densities and voltages within these

auroral arcs appear to be described by the current-voltage

relationship predicted under the assumption of a nonthermal

magnetospheric source population [Dors and Kletzing, 1999] with

≃ 1.5, but markedly deviate from the current-density

relationproposed by Knight [1973], which assumes a source

population in thermal equilibrium. These results represent the first

observations of  ≃ 1.5 within the auroral acceleration region, and

the first direct demonstration of which we are aware that the

current-voltage relationship obtained by assuming a

magnetospheric source population in thermal equilibrium cannot

describe the current densities