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

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. ©2020. American Geophysical Union. All Rights Reserved.We gratefully acknowledge the support of the NSF/NCAR High Performance Instrumented Airborne Platform for Environmental Research (HIAPER) project, as well as the pilots and technical staff that made the HIAPER Gulfstream V missions possible. We also acknowledge the contribution by T. Kanmae who did the early analysis of the spectra, and discussions with N. Liu, F. J. Pérez‐Invernón, and A. Malagón‐Romero. The research has been supported in part by National Science Foundation grants 1104441 to the University of Alaska Fairbanks and 1201683 to the US Air Force Academy. A. Luque was supported by the European Research Council (ERC) under the European Union H2020 programme/ERC grant agreement 681257.Peer reviewe