Physics Department Seminar University of Alaska Fairbanks

J O U R N A L    C L U B


High Speed Sprite Imaging:
A model and its potential application to the aurora.

Hans Nielsen
Geophysical Institute, UAF



Sprites are very dynamic mesospheric features following large cloud-to-ground lightning strikes. To resolve their temporal development requires high speed imaging. Quality recordings have shown details of highly dynamic streamer activity and examples will be presented. But in video (25-30 fps) and photographs the more stationary glows and beads dominate indicating that these features provide the main energy input into the local atmosphere. Glow appears in the onset region, while beads are small isolated optical structures formed in the conducting channel created by downward or upward propagating streamers. Surprisingly, our imaging has shown that the decay of beads and glow within a given streamer channel is similar even over altitude ranges corresponding to several atmospheric scale heights. Modelling suggests coupling through the streamer channel current combined with the electron attachment instability. The attachment instability drives the plasma locally towards one of two states: A highly conducting state resulting in a low E-field and hence, no optical emissions, or, a low conducting state with a high E-field and optical emissions. The current densities involved appear somewhat similar to those in the aurora so, maybe, this could also apply to thin auroral layers and enhanced aurora documented years ago, but never explained.


Friday, 16 September 2016

Globe Room, Elvey Building

3:45 PM