HR: 11:50h
AN: AE42A-07 INVITED    [Abstracts]
TI: High time-resolution sprite observations
AU: * Stenbaek-Nielsen, H C
EM: hnielsen@gi.alaska.edu
AF: Geophysical Institute, University of Alaska, 903 Koyukuk Drive, Fairbanks, AK 99775, United States
AU: McHarg, G G
EM: Matthew.Mcharg@usafa.edu
AF: Dept of Physics, US Air Force Academy, Colorado Springs, CO 80840, United States
AB: Imaging sprites at 10,000 fps have revealed new details about their temporal development. TV observations show a highly structured central body with downward tendrils and upward branches. But rather than being leaders, as suggested by the long streaks in the TV recordings, tendrils and branches are actually formed by spatially compact streamer heads moving at velocities up to 0.3 c. In an individual sprite event the downward moving streamer heads start first forming the tendrils; later, and from a lower altitude and from existing luminous sprite structures, upward moving streamer heads may appear to form the branches. If there are no upward moving streamer heads the event would be classified as a C-sprite, otherwise it would be a carrot sprite. Following the streamer head activity we see afterglow in which little or no temporal and spatial activity is present. The streamer heads are very bright and they appear to be point sources, i.e. their spatial dimensions are less than our 100-200 m image resolution. Streamer head modeling indicates a scale size of ~25 m in which case the brightness would be in the range 1-100 GR. Other models predict volume emission rates leading to a streamer head spatial scale size in the 10 to 100 m range. Our observations conclusively show the downward and upward propagating streamer heads to be separated in time and space. This is in contrast to a number of models in which both down and up going streamer heads emanates from the origin of the process. We frequently see old sprites re-appear in response to new activity suggesting that sprite activity leaves some imprint on the background atmosphere. Given the very large brightness of the streamer heads it would not be surprising if sprite activity initiates chemical processes that could locally affect the composition of the atmosphere, but whether this affects the mesosphere on a larger scale remains uncertain.
DE: 0649 Optics (4264)
DE: 3324 Lightning
DE: 3332 Mesospheric dynamics
SC: Atmospheric and Space Electricity [AE]
MN: 2007 Fall Meeting