Fast auroral snapshot satellite observations of very low frequency saucers

Abstract

Wave and charged particle observations of quasielectrostatic whistler emissions known as “VLF saucers” (very low frequency, or kilohertz radio range) from the Fast Auroral SnapshoT (FAST) satellite demonstrate that the majority (∼85%) of VLF saucer emissions are generated on flux tubes that carry antiearthward, energetic (>10 eV) electrons in the downward current region of the aurora. In most cases, the VLF saucers are nested, that is, they have two or more clearly discernible “arms” that indicate several distinct source regions at differing altitudes. These observations verify previous interpretations that the individual source regions are highly localized both in latitude and altitude. In some cases, the individual source regions are localized in three dimensions. An important new finding is that the FAST satellite frequently detects solitary structures identified as electron phase-space holes at the vertex of VLF saucers. Electron phase-space holes were identified in ∼79% of the VLF saucer events in the downward current region. This finding implies either a common energy source or a direct association between the two phenomena. Furthermore, the observations now show a direct association with the VLF saucer source flux tube and diverging dc electric field structures. An interpretation is put forth that the VLF saucer source region is in or near parallel electric fields in the downward current region of the aurora.