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

Abstract

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.

Temporal Evolution of Substorm‐Driven Global Alfvén Wave Power Above the Auroral Acceleration Region

“…Such analysis will need to be done in future. And finally, some caution is advised as not all data points in our wave band necessarily represent Alfvén waves (Keiling et al, 2019a), and there is the possibility that the number flux of Alfvénic electrons, and thus their total energy flux, was overestimated by Newell & Gjerloev (2011), which was recently suggested by Dombeck et al (2018). In spite of these uncertainties, which cannot be resolved here, we still find our results mostly consistent, while providing, for the first time, global power estimates for Alfvén waves during different substorm phases above the AAR.…”

“…For example, Newell et al (2009), using only low‐altitude electron measurements (from DMSP data) to separate auroral precipitation types, suggested that up to 13% of the auroral power (as determined from the entire hemisphere) is likely associated with Alfvén wave acceleration processes during active periods (strong solar wind driving) and up to 6% during quiet conditions (weak solar wind driving). Allowing for the recent objections by Dombeck et al (2018), these values could be higher, because it was argued that the diffuse aurora was potentially significantly overestimated, which would have the effect of lowering the contributions due to the discrete aurora, both quasistatic aurora and Alfvénic aurora. In contrast, Chaston et al (2007), using both Alfvén wave and electron measurements (from FAST data), inferred that with increasing auroral activity, 25% to 39% of electron energy deposition into the hemispheric auroral zone ionosphere, and up to 50% in the premidnight region alone, may be attributed to the driving of Alfvén waves.…”

Assessing the global Alfvén wave power flow into and out of the auroral acceleration region during geomagnetic storms

“…It has been a challenge to determine the global contributions of each type to the aurora. Under various geomagnetic conditions (but not storms per se), estimates of Alfvénic aurora range between 6 and 50% of the total aurora ( 33 – 35 ), with the lower estimate believed to be too low ( 36 ). To what extent the storm aurora is Alfvénic is not known as of yet, and its determination, is outside the scope of this study.…”

Global Alfvén Wave Power in the Auroral Zone in Relation to the AE Index

“…In another study, using only low‐altitude (below the AAR) electron measurements, it was suggested that up to 13% of the auroral power is likely associated with Alfvén wave acceleration processes during active periods (Newell et al, ). However, this estimate could be higher if one accounts for the objections brought forth by Dombeck et al (). Morphological similarities between global Alfvén wave maps and Alfvénic electron maps, as shown in the above studies, further corroborate the Alfvénic coupling scenario.…”