Subauroral emissions known as STEVEs (Strong Thermal Emission Velocity Enhancements) are sometimes accompanied by green arcs containing magnetic field-aligned “picket fence” structures. In a newly published spectrum of a green picket fence arc Gillies et al. (2019, https://doi.org/10.1029/2019GL083272) showed that the visible emission in such arcs is mostly OI 557.7 nm with minimal N₂⁺ 1N. This finding is consistent with the color ratios found in digital camera photos (Mende & Turner, 2019, https://doi.org/10.1029/2019JA026851) and is distinct from the ratios in auroral precipitation. The spectrum also contains intense N₂ first positive (1P) emission. The presence of OI 557.7 (~4.19 eV excitation energy) and N₂ 1P (~7.35 eV), combined with the lack of N₂⁺ first negative (~18.75 eV) commonly seen in the aurora, suggests that the particles exciting the emission have energy <18.75 eV. This is strong evidence against the precipitation hypothesis recently put forth (Nishimura et al., 2019, https://doi.org/10.1029/2019GL082460; Gillies et al., 2019, https://doi.org/10.1029/2019GL083272).

Key Points

• Recent spectra of a green STEVE arc shows intense 557.7 nm OI and N₂ 1P nitrogen emission in the IR but minimal N₂⁺ 1N in the blue
• Color ratios in photos of the green “picket fence” arcs are consistent with the spectrospcopic results
• Spectra and the color ratios show low energy excitation which is inconsistent with auroral source from the magnetosphere