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 N2+ 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 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,; Gillies et al., 2019,

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
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