It's Not Easy Being Green: Kinetic Modeling of the Emission Spectrum Observed in STEVE's Picket Fence

Recent studies suggest that, despite its aurora-like appearance, the picket fence may not be driven by magnetospheric particle precipitation but instead by local electric fields parallel to Earth's magnetic field. Here, we evaluate the parallel electric fields hypothesis by quantitatively comparing picket fence spectra with the emissions generated in a kinetic model driven by local parallel electric fields energizing ambient electrons in a realistic neutral atmosphere. We find that, at a typical picket fence altitude of 110 km, parallel electric fields between 40 and 70 Td (similar to 80-150 mV/m at 110 km) energize ambient electrons sufficiently so that, when they collide with neutrals, they reproduce the observed ratio of N2 first positive to atomic oxygen green line emissions, without producing N2+ ${\mathrm{N}}_{2}<^>{+}$ first negative emissions. These findings establish a quantitative connection between ionospheric electrodynamics and observable picket fence emissions, offering verifiable targets for future models and experiments. The "picket fence" is a captivating visual phenomenon featuring vibrant green streaks often observed with and at lower altitudes than the rare purpleish-white arc called STEVE (Strong Thermal Emission Velocity Enhancement). It occurs in the subauroral sky, at lower latitudes than the auroral oval, raising questions about whether it is a type of aurora or a separate phenomenon. A recent hypothesis proposes that electric fields aligned with Earth's magnetic field in the dense part of the atmosphere where the picket fence forms might energize local electrons, which collide with the neutral atmosphere to create picket fence emissions. This distinguishes the picket fence from traditional auroras caused by energetic particles accelerated higher up in space which stream down and collide with the upper atmosphere. In this study, we compare optical observations of the picket fence to a detailed calculation of the emissions produced by ambient electrons energized by parallel electric fields in the upper atmosphere. The results show that large parallel electric fields can indeed replicate the observed picket fence phenomenon. These findings offer important targets for future picket fence models and experiments. This research demonstrates that the picket fence serves as a valuable testing ground for understanding the chemistry and electrodynamics of Earth's upper atmosphere. Observations of picket fence spectra differ quantitatively from green aurora spectral observations, suggesting different originsKinetic modeling driven by local parallel electric fields replicates picket fence spectra without requiring particle precipitationAt 110 km, parallel electric fields between 40 and 70 Townsend (similar to 80-150 mV/m at 110 km) reproduce observed picket fence spectra