Small-scale secondary electron features and FACs in diffuse and pulsating aurora

English Abstract:

Observations of small-scale structures in the secondary and backscattered populations associated with diffuse and pulsating aurora have been observed by several spacecraft including ePOP, REIMEI, and the ROPA sounding rocket. We present in situ observations of such sub-keV electron precipitation mapped into the THEMIS GBO all-sky images, with evidence of field aligned currents and acceleration through parallel electric fields. These observations are consistent with Sato et al. (2002, 2004) who show FAST measurements of sub-keV inverted-V structures collocated with pulsating aurora. In the case of ROPA, the sub-keV electron signatures appear to be the result of secondary / backscattered electrons that are accelerated downward through a potential at an altitude above the spacecraft. Thus the intermittency of the sub-keV signatures seen in the ROPA measurements would reflect structure in the accelerating potential, suggesting small scale, local electric field structure above the spacecraft. A lack of velocity dispersion in these accelerated, secondary electron signatures is consistent with an acceleration region that is not far from the spacecraft, or may suggest a lack of temporal variation in the associated accelerating field structure during the measurement. PFISR observations of similar pulsating aurora show, in some directions, high altitude electron density enhancements that likely correspond to sub-keV electron signatures. The higher altitude density enhancements associated with the sub-keV electron signatures more or less align with gaps between lower altitude density enhancements presumably caused by the pulsating patches. This raises questions as to the role of the ionosphere in the generation of pulsating aurora.