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

Abstract

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.

Experimental Validation of N2 Emission Ratios in Altitude Profiles of Observed Sprites

“…Below 60 km, surprisingly, AP-measured ratios fell below the predicted ratio 1 E k . We conjectured that the lower emission ratios are contributed from those non-streamer regions (upper propagating streamer, sprite beads, and glow) by Stenbaek-Nielsen et al (2020). In addition, the AP-measured ratios have a lower bound of predicted emission ratios associated with 0.4 E k .…”

“…A measurement of the N 2 emission band ratio in sprites (e.g., 2P/ 1P or 1N/2P) is one of the most common remote optical diagnostic methods used to explore the physical conditions of complex sprite phenomena (Armstrong, 1998(Armstrong, , 2000Morrill et al, 2002;Miyasato et al, 2003;Kuo et al, 2005;Adachi et al, 2006;Adachi et al, 2008;Kuo et al, 2008;Kuo et al, 2009;Stenbaek-Nielsen et al, 2020). Sprite streamers associated with the first positive (1P), second positive (2P), and first negative (1N) band emission of N 2 can be analyzed using the analytical expressions for emission ratios in sprite streamer simulations (Celestin and Pasko, 2010;Pérez-Invernón et al, 2018).…”

“…Previous spectral observations demonstrated two major visual emission bands: N 2 second positive band (2P, C 3 Π u − B 3 Π g ) for blue emission, and N 2 first positive band (1P, B 3 Π g −A 3 Σ + u ) for red emission (Mende et al, 1995;Green et al, 1996;Hampton et al, 1996). One of the minor blue emission bands is the N 2 + first negative band (1N) for ionization associated emission (Armstrong et al, 1998(Armstrong et al, , 2000Morrill et al, 2002;Kanmae et al, 2010aKanmae et al, , 2010bStenbaek-Nielsen et al, 2020). Observational techniques for sprite emission ratios have made significant progress using spectrophotometers (Armstrong et al, 1998;, array photometers (Miyasato et al, 2002;, optical spectra at 10,000 fps (Kanmae et al, 2010a(Kanmae et al, , 2010bStenbaek-Nielsen et al, 2020) in ground campaigns, airplane campaigns (Morrill et al, 2002), and satellite measurements (Kuo et al, 2005;Mende et al, 2005;Adachi et al, 2006;Liu et al, 2006;Adachi et al, 2008;Kuo et al, 2008;Kuo et al, 2009).…”

“…One of the minor blue emission bands is the N 2 + first negative band (1N) for ionization associated emission (Armstrong et al, 1998(Armstrong et al, , 2000Morrill et al, 2002;Kanmae et al, 2010aKanmae et al, , 2010bStenbaek-Nielsen et al, 2020). Observational techniques for sprite emission ratios have made significant progress using spectrophotometers (Armstrong et al, 1998;, array photometers (Miyasato et al, 2002;, optical spectra at 10,000 fps (Kanmae et al, 2010a(Kanmae et al, , 2010bStenbaek-Nielsen et al, 2020) in ground campaigns, airplane campaigns (Morrill et al, 2002), and satellite measurements (Kuo et al, 2005;Mende et al, 2005;Adachi et al, 2006;Liu et al, 2006;Adachi et al, 2008;Kuo et al, 2008;Kuo et al, 2009). The investigation of the electrical discharge phenomena using remote sensing observation will help us study the impact of plasma chemistry and energy deposition by sprites in the middle atmosphere (Gordillo-Vázquez and Pérez-Invernón, 2021).…”

“…The sprite emission ratio of N 2 2P to 1P can indicate the gained energy of accelerated electrons and the applied electric field in the streamer process and stands as a proxy for the electron energy leading to the emissions (Stenbaek-Nielsen et al, 2020). However, an inconsistent interpretation of sprite emission has appeared from past to now.…”