Optical Spectra of Small‐Scale Sprite Features Observed at 10,000 fps

• Published in: Journal of geophysical research. Atmospheres 2020-10, Vol.125 (20), p.n/a
• Main Authors: Stenbaek‐Nielsen, H. C., McHarg, M. G., Haaland, R., Luque, A.
• Format: Article
• Language: English
• Subjects: Altitude; Beads; Electron energy; Emissions; Frames per second; Geophysics; Nitrogen; Nitrogen emissions; Nitrogen ions; Spectra; sprite; sprite spectra; Sprites; Streamers; Time series; TLE
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2020JD033170

Spectra of small‐scale sprite structures, downward and upward propagating streamers, glow, and beads, were recorded with a slitless spectrograph at 10,000 frames per second (fps) from aircraft missions in 2009 and 2013. The spectra are dominated by emissions from molecular nitrogen, the first positive band in the red, and in the blue the second positive band plus the first negative band of molecular nitrogen ions. The excitation threshold for the blue emissions is higher than for the red emissions, so the blue/red ratio can, in principle, be used as a proxy for the electron energy leading to the emissions. We extracted for analysis time series of spectra from 11 sprites: 18 time series from downward propagating streamers, 6 from upward propagating streamers, 14 from glow, and 12 from beads. The total number of spectra in the 50 time series is 953. Blue emissions are almost exclusively associated with streamers indicating the more energetic nature of streamers compared with glow and beads. Both downward and upward propagating streamers start and end with low blue emissions indicating time variations in the associated processes. Because the red and blue nitrogen emissions are significantly affected by quenching, which is altitude dependent, and we do not have sufficiently accurate altitudes, the observed spectral blue/red ratios cannot be directly applied to sprite models. Key Points Spectra of small‐scale sprite features (downward and upward streamers, beads, and glow) have been recorded at 10,000 spectra per second The spectra are dominated by molecular nitrogen emissions; the relative blue and red emission rates can be used to assess the processes leading to the emissions Blue emissions are almost exclusively associated with sprite streamers indicating more energetic processes compared to those associated with beads and glow