A distinctive emission feature, Strong Thermal Emission Velocity Enhancement (STEVE), was recently reported by the scientific community (MacDonald et al., 2018) with the help of citizen scientists via Aurorasaurus (MacDonald et al., 2015). However, STEVE may have been reported decades ago (Bailey et al., 2018; Hunnekuhl & MacDonald, 2020). STEVE occurs in the subauroral region equatorward of the auroral oval. It is narrow in latitude and wide in longitude (Gallardo-Lacourt, Nishimura, et al., 2018). In addition, STEVE may include green picket-fence features that propagate westward along with a purplish emission. The green picket-fence may not always be present during STEVE events. In the ionosphere, STEVE is accompanied by increased electron temperature, plasma density gradient, westward ion velocity, and weak magnetic perturbations indicative of field-aligned currents (FACs), which are the ionospheric features of a subauroral ion drift (SAID) (MacDonald et al., 2018). A statistical study shows that STEVE occurred only for the most intense SAIDs (Archer, Gallardo-Lacourt, et al., 2019). The spectral continuum purplish STEVE emissions show multiwavelength features captured by all-sky cameras measuring different wavelengths (Chu et al., 2019). Its spectrum consists of enhanced 630.0-nm OI redline emission and a spectral continuum enhancement spanning the visible range of 400-730 nm (Gillies et al., 2019). The green picket fence has enhanced OI 557.7-nm emission, which was similar to the typical green aurora. However, Mende et al. (2019) pointed out that the spectrum of the green picket fence emission from (Gillies et al., 2019) has OI 557.7-nm (∼4.19 eV excitation energy) and N 2 first positive emissions (∼7.35 eV), but shows lack of N 2 + first negative emissions (∼18.75 eV). It is considered strong evidence against the precipitation hypothesis of energetic electrons.
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