It's Not Easy Being Green: Quantitative Modeling of STEVE's Picket Fence Emissions Driven by Local Parallel Electric Fields

Abstract

The vibrant green streaks of the 'picket fence' typically appear below a STEVE arc in the subauroral sky, at lower latitudes than the auroral oval. 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. In this study, we investigate this hypothesis by quantitatively comparing observed picket fence optical spectra with emissions generated in a kinetic model driven by parallel electric fields in a realistic neutral atmosphere. We find that sufficiently large parallel electric fields can reproduce the observed ratio of N2 first positive to oxygen green line emissions, without producing N2+ first negative emissions. We find that, at a typical picket fence altitude of 110 km, parallel electric fields between 40 and 70 Td (~80 to 150 mV/m at 110 km) result in calculated spectral features consistent with observed ones, providing a benchmark for future observational and modeling studies. Additionally, we review studies which have identified similar features to the picket fence in the aurora, suggesting that a similar mechanism may be at work there. Since visible and ultraviolet auroral emissions are increasingly used to infer magnetospheric activity, it is important to better understand and quantify potential sources of emission beyond particle precipitation.