Color Ratios of Subauroral (STEVE) Arcs

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.

Subauroral Green STEVE Arcs: Evidence for Low‐Energy Excitation

“…The spectrum of the green picket fence arc measured by Gillies et al () has an intense component of the OI 557.7 nm green line with minimal contribution by the N 2 + first negative band, which is a major spectral component in most auroras. This finding was corroborated with DSLR color photography, which showed that the green to blue ratios in picket fence images were much larger than in auroras (Mende & Turner, ). The lack of the N 2 + first negative emission could imply the lack of atmospheric N 2 in the regions where the picket fence arcs are produced.…”

“…We discuss the implications of this finding and show that it is inconsistent with the picket fence being caused by precipitation from the magnetosphere. Second, we revisit the image analyzed by Mende and Turner () to show that the color ratios in photographs of the green picket fence structures are consistent with the Gillies et al, spectrum and that they are almost purely green with very little other colors in the visible spectral range compared to normal auroras. The presence of the low excitation energy emissions such as the OI green 557.7 line and the N 2 first positive and the lack of high excitation energy emissions commonly seen in aurora (e.g., N 2 + 427.8 nm) would be strong evidence for the presence of superthermal electrons and against the energetic particle precipitation hypothesis for the green picket fence structures.…”

“…The exposure time was 20 s at F/2.8, and the equivalent focal length was 15 mm with ISO speed of 5000. In this work, we expand on the analysis by Mende and Turner () by using the raw data files (.NEF format), which provide more physically accurate color intensities than the white‐balanced and gamma‐corrected files produced by the camera software (.TIFF format).…”

“…They concluded that the green picket fence arcs are likely caused by precipitation, while the purple/white arcs are not. However, in a contemporaneous article, Mende and Turner (2019) analyzed a color image of the STEVE picket fence, finding that these arcs appeared to be mostly green with minimal blue and red. Wavelength calibration of the camera used in taking the STEVE image verified that the dominant auroral emissions features, O( 1 D) 630.0 nm, O( 1 S) 557.7 nm, and N 2 Gillies et al (2019) presented true spectral observations of a STEVE event, capturing both the purple/white component and the green picket fence component.…”

Optical Spectra and Emission Altitudes of Double‐Layer STEVE: A Case Study

“…The STEVE AGC may also partly explain the result in Mende and Turner (2019). Since the beginning of the STEVE research, there has been curiosity about the similarity between STEVE and SAR.…”