High-Speed Observations of Sprite Streamers

Abstract

Sprites are optical emissions in the mesosphere mainly at altitudes 50-90 km. They are caused by the sudden redistribution of charge due to lightning in the troposphere which can produce electric fields in the mesosphere in excess of the local breakdown field. The resulting optical displays can be spectacular and this has led to research into the physics and chemistry involved. Imaging at faster than 5,000 frames per second has revealed streamer discharges to be an important and very dynamic part of sprites, and this paper will review high-speed observations of sprite streamers. Streamers are initiated in the 65-85 km altitude range and observed to propagate both down and up at velocities normally in the 10 6-5 9 10 7 m/s range. Sprite streamer heads are small, typically less than a few hundreds of meters, but very bright and appear in images much like stars with signals up to that expected of a magnitude-6 star. Many details of streamer formation have been modeled and successfully compared with observations. Streamers frequently split into multiple sub-streamers. The splitting is very fast. To resolve details will require framing rates higher than the maximum 32,000 fps used so far. Sprite streamers are similar to streamers observed in the laboratory and, although many features appear to obey simple scaling laws, recent work indicates that there are limits to the scaling.

Preface to the Special Issue on Thunderstorm Effects in the Atmosphere–Ionosphere System

“…Price (2013) also discusses the connection between severe weather phenomena (tornadoes, hailstorms, tropical storms, hurricanes or rainfall) and noticeable changes in key lightning features such as their number (more frequent) and polarity (increasing numbers of ?CG discharges, even though there are still more -CG discharges that ?CG ones), and on the importance of lightning observations as a valuable tool to monitor and predict severe weather scenarios. Finally, Price (2013) comments on how the aerosol loading in the atmosphere influences the occurrence of lightning.A comprehensive overview of the key features of sprite streamers as derived from highspeed imaging observations is presented by Stenbaek-Nielsen et al (2013) in the fourth article of this Special Issue. This paper goes through the different findings gained by the authors during recent years from several high-speed sprite imaging campaigns using equipment with frame rates of up to 32,000 fps, and with 30-50 cm focal length lenses reaching spatial resolutions down to 15 m. Such observations have allowed StenbaekNielsen et al (2013) to investigate the morphology and brightness of sprite streamers and to perform detailed analysis of the sprite streamer splitting dynamics from their high-speed 698 Surv Geophys (2013) 34:697-700 123 imaging data.…”

“…A rich discussion is also presented on the limits of applicability of streamer scaling laws when comparing some streamer features (velocity/diameter/brightness) in sprites and laboratory discharges under, respectively, varying and constant density conditions. Finally, the work by Stenbaek-Nielsen et al (2013) poses several future challenges to be explored with next generation high-speed imaging systems. In particular, they point out five aspects worthy of future investigations, such as the double-headed features predicted by some models of streamers but not observed in high-frame-rate images, the relationship between velocity and width of sprite streamers, the general relationship between sprites and the driving lightning electric field produced by tropospheric thunderstorm activity, the observation of minimum size sprite streamers (that would require images with spatial resolutions below 10 m) and further studies on the streamer splitting dynamics that would require the cameras' digital technology to go beyond their present limits.…”

“…Finally, the work by Stenbaek-Nielsen et al (2013) poses several future challenges to be explored with next generation high-speed imaging systems. In particular, they point out five aspects worthy of future investigations, such as the double-headed features predicted by some models of streamers but not observed in high-frame-rate images, the relationship between velocity and width of sprite streamers, the general relationship between sprites and the driving lightning electric field produced by tropospheric thunderstorm activity, the observation of minimum size sprite streamers (that would require images with spatial resolutions below 10 m) and further studies on the streamer splitting dynamics that would require the cameras' digital technology to go beyond their present limits.The observations reviewed by Stenbaek-Nielsen et al (2013) are taken into account for the development of sprite and halo models. These models are reviewed in the last paper of this Special Issue, by Pasko et al (2013).…”

“…A comprehensive overview of the key features of sprite streamers as derived from highspeed imaging observations is presented by Stenbaek-Nielsen et al (2013) in the fourth article of this Special Issue. This paper goes through the different findings gained by the authors during recent years from several high-speed sprite imaging campaigns using equipment with frame rates of up to 32,000 fps, and with 30-50 cm focal length lenses reaching spatial resolutions down to 15 m. Such observations have allowed StenbaekNielsen et al (2013) to investigate the morphology and brightness of sprite streamers and to perform detailed analysis of the sprite streamer splitting dynamics from their high-speed 698 Surv Geophys (2013) 34:697-700 123 imaging data.…”

“…The observations reviewed by Stenbaek-Nielsen et al (2013) are taken into account for the development of sprite and halo models. These models are reviewed in the last paper of this Special Issue, by Pasko et al (2013).…”