Do Electron Holes Emit VLF Saucers in the Auroral Region? Theory Versus FAST Observations.

The ability of electron holes to act as a source for VLF saucers is a critical question for understanding their generation, their stability, and the dissipation of the huge energy their carry from the low to the high altitude auroral region where they have been observed in the past few years by the FAST and POLAR spacecraft. We start with the data and first determine the properties of VLF saucers from the interferometric analysis of a set of electric signals recorded by the FAST spacecraft in the low-altitude auroral region. This analysis allows us to get the first direct estimation of the parallel phase velocity of these electrostatic waves in these regions. Then, we compare this estimate with the velocity of the electron beams which are observed in the source region of VLF saucers. In past studies, these electron beams have been considered as the most likely candidate for the emission of VLF saucers. In light of the interferometric analysis, we investigate an alternate generation mechanism of VLF saucers by the small-scale electron holes observed in association with electron beams. Using an analytical kinetic model of these electron holes, we derive the growth/damping rate of the VLF waves through their resonant interaction with the electrons trapped in the potential structure of the electron hole. We conclude on the characteristics of electron holes which may and may not emit VLF saucers in the auroral region.