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 >108 eV/cm2-s-sr-eV) our method separately identifies the three main precipitation mechanisms: quasi-static potential structure (inverted-V, QSPS) acceleration, Alfvénic acceleration, and wave scattering or other nonaccelerated isotropic (diffuse) precipitation. Intense precipitation (~14% of all Fast Auroral Snapshot coverage) accounts for ~80–90% of electron number flux into the ionosphere globally and ~65% of the energy flux on the nightside. It is found that two or more different mechanisms occur in the same event ~60–75% of the time. Alfvénic and QSPS acceleration and the combination of the two contribute substantially. Each of the three primary precipitation mechanisms (alone or in combination) occur >~35% of the time with QSPS and Alfvénic 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 = Alfvénic, and diffuse = nonaccelerated isotropic) is not observed in the classification using our method ~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.
Bibliographical noteFunding Information:
The authors would like to thank Jack Vernetti and Tim Quinn of the Space Sciences Laboratory at the University of California at Berkeley for their efforts in providing assistance with FAST data access. The data used for the study herein is publically available at http:// sprg.ssl.berkeley.edu/fast/. The study presented herein was supported by NASA grants NNX10AL03G, NNX14AF32G, and NNX16AG67G.