Publication Date:
2019
Description:
Abstract
Photos of a spectacular optical phenomenon, nicknamed “STEVE” show finely structured, purple colored, east‐west arcs spanning the sky. These purple Subauroral Arc Emissions (SAE‐s) 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 (RGB) 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), the O(1S) 557.7 nm and N2+1N bands, were cleanly separated in the RGB 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 〈150km.
Print ISSN:
2169-9380
Electronic ISSN:
2169-9402
Topics:
Geosciences
,
Physics
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