Identification of Auroral Electron Precipitation Mechanism Combinations and Their Relationships to Net Downgoing Energy and Number Flux

We present initial statistical results of a new methodology for identifying electron precipitation mechanisms in Earth's auroral zone. Unlike previous methodologies, it identifies multiple mechanisms observed in the same event, utilizing Fast Auroral Snapshot measurements of upward energy and pitch angle spectra in addition to downward energy spectra. For intense precipitation (peak downgoing differential energy flux > 10(8) eV/cm(2)-s-sr-eV) our method separately identifies the three main precipitation mechanisms: quasi-static potential structure (inverted-V, QSPS) acceleration, Alfvenic acceleration, and wave scattering or other nonaccelerated isotropic (diffuse) precipitation. Intense precipitation (similar to 14% of all Fast Auroral Snapshot coverage) accounts for similar to 80-90% of electron number flux into the ionosphere globally and similar to 65% of the energy flux on the nightside. It is found that two or more different mechanisms occur in the same event similar to 60-75% of the time. Alfvenic and QSPS acceleration and the combination of the two contribute substantially. Each of the three primary precipitation mechanisms (alone or in combination) occur >similar to 35% of the time with QSPS and Alfvenic acceleration observed together being the dominant identifiable energy precipitation mechanism/combination. This combination also significantly contributes to the net number flux. QSPS acceleration is the most prevalently observed mechanism (50-60%). The mechanism inferred from classification by downgoing spectral characteristics alone (i.e., monoenergetic = QSPS, broadband = Alfvenic, and diffuse = nonaccelerated isotropic) is not observed in the classification using our method similar to 20-65% of the time. The results do not confirm and may be inconsistent with wave scattering of electrons (diffuse auroral precipitation) being the dominant mechanism for electron energy and number flux into the ionosphere.