Subauroral Green STEVE Arcs: Evidence for Low-Energy Excitation

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, ) showed that the visible emission in such arcs is mostly OI 557.7 nm with minimal N-2(+) 1N. This finding is consistent with the color ratios found in digital camera photos (Mende & Turner, 2019, ) and is distinct from the ratios in auroral precipitation. The spectrum also contains intense N-2 first positive (1P) emission. The presence of OI 557.7 (similar to 4.19 eV excitation energy) and N-2 1P (similar to 7.35 eV), combined with the lack of N-2(+) first negative (similar to 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, ; Gillies et al., 2019, https://doi.org/10.1029/2019GL082460; Gillies et al., 2019, https://doi.org/10.1029/2019GL083272).).