HR: 08:45h
AN: AE11A-04 INVITED     [Abstracts]
TI: Streamer formation in sprites
AU: * McHarg, M G
EM: matthew.mcharg@usafa.af.mil
AF: United States Air Force Academy, 2354 Fairchild Drive Suite 2A31, USAF Academy, CO 80840 United States
AU: Kammae, T
EM: tkammae@gi.alaska.edu
AF: University of Alaska, Fairbanks, PO Box 757320, Fairbanks, AK 99775 United States
AU: Nielsen, H C
EM: hnielsen@gi.alaska.edu
AF: University of Alaska, Fairbanks, PO Box 757320, Fairbanks, AK 99775 United States
AB: Models of sprite formation for positive cloud-to-ground lightning strokes predict both downward (positive), and upward (negative) propagating streamers. Previous high speed camera observations of sprites are generally consistent with these predictions, but have been unable to resolve the temporal formation of the streamers due to frame rates limited to a few thousand frames per second. We report observations made during the evening of 9 July 2005 at 10,000 frames per second, with the image intensifier gated to 50 microseconds per frame. These observations often show the streamer head to be a bead-like structure propagating downward at approximately 7x10⁶ m/s for 1,500 microseconds. The bead is followed by a dark region, and the main emissions from the sprite column are delayed ~800 microseconds after the passage of the streamer head. There are also "beads" which clearly propagate upward. Some events appear to be very similar to laboratory images of time resolved streamer zones. We interpret these observations in terms of positive/negative streamers. We see evidence for branching of the streamer tips in several cases, as well as evidence of upward propagating streamers transitioning into a more diffuse emission. Previous work (Pasko and Stenbaek-Nielsen, GRL 29(10), 2002) indicates this transition region has a lower border at an altitude when the dielectric relaxation time scale equals the time scale for an individual electron to develop into a streamer, and an upper border when the dielectric relaxation time scale roughly equals the dissociative attachment time scale. The present observations appear to be broadly consistent with this interpretation.
DE: 0342 Middle atmosphere: energy deposition (3334)
DE: 0649 Optics (4264)
DE: 0654 Plasmas
SC: Atmospheric and Space Electricity [AE]
MN: Fall Meeting 2005