Abstract

Photos of a spectacular optical phenomenon, nicknamed STEVE, show finely structured, purple-colored, east-west arcs spanning the sky. These purple Sub-auroral Arc Emissions are associated with Sub-Auroral Ion Drifts, often accompanied by separate green arcs frequently displaying magnetic field aligned rays suggesting charge particle excitation. Both types of these arcs and polar auroras appear in some photos. Splitting the images into red, green, and blue channels allowed comparison of color ratios of the three phenomena. Wavelength calibration of the camera verified that the dominant atmospheric auroral emissions, 630.0 nm O(¹D), O(¹S) 557.7 nm, and N₂⁺1N bands, were cleanly separated in the red, green, and blue channels of the camera. In the absence of a spectrogram the ratios between the color channels were interpreted in terms of possible excitation mechanisms. The purple arcs contained an excess of blue, presumably N₂⁺1N intensity. This excess production could be due to the excitation of N₂⁺ ions that were ionized through charge exchange with O⁺. The green companion arcs appear to be purely green (557.7) with almost no blue and minimal red suggesting excitation by low-energy electrons excitation at altitudes >100 and <150 km.

Key Points

• The color ratios of purple and green subauroral arcs were compared to normal aurora using there red, green and blue intensisities
• In purple arcs blue emission is enhanced, perhaps due to soft electron acting on charge exchanged N₂⁺, and red due to 630 nm from O
• Green companion arcs are likely to be soft electron excited green line at low altitudes with no blue. Source of soft electrons is unknown