A study of inverted-V auroral acceleration mechanisms using Polar/Fast Auroral Snapshot conjunctions

We present observations of electrons and ions that have been affected by an auroral plasma acceleration mechanism. In the nine events studied, nearly simultaneous and magnetically conjugate measurements were made by instruments on the Fast Auroral Snapshot (FAST) and Polar satellites (1900-4100 and 20000-37000 kin altitude ranges, respectively). FAST sees inverted-V-type electron spectra in five out of nine events, in the remaining four events the precipitation seen at FAST is diffuse rather than inverted-V-type. In those inverted-V cases where the electron distribution at Polar is quasi-Maxwellian (two events), assuming a potential drop of appropriate magnitude between the two spacecraft can well explain the spectra. In more complicated non-Maxwellian cases (three events), a net potential drop between FAST and Polar could explain the total FAST energy flux rather well, but it fails to explain the details of the distribution functions. Thus the "standard" explanation of inverted-V-type electron spectra in terms of potential drop acceleration may work in the quasi-Maxwellian events (which represent the small-energy end of inverted-V spectra, with 1-2 keV energies) but not in the non-Maxwellian events. Measurements of the upward flowing ionospheric ions at Polar also support this view. In the non-Maxwellian events, sporadic very broadband electrostatic wave activity (4-1000 Hz frequency range) temporally correlates well with field-aligned acceleration of the cold (but not the hot) electron population.