SPRITES-II

The scientific objective of SPRITE SPECTRA-II was to determine the physical processes involved in sprites, and in particular the ionization of molecules in the blue part of the light spectrum. The blue light cannot be adequately observed from ground-based measurements, or by viewing through the normal GV windows as was done in the 2009 SPRITE SPECTRA flights. Sprites typically start 70-80 km above thunderstorms and move downwards over a 50 km altitude range.

SPRITE SPECTRA-II used the NSF/NCAR GV to fly as high as possible at about 200 km distance from thunderstorms and target Mesoscale Convective Systems, MCS, due to their high number of lightning strikes. This project used high-speed imagers to record sprites with high temporal and spatial resolution. They used slit-less spectroscopy to obtain sprite spectra with ~500 m altitude, 0.1 ms temporal and 10 nm spectral (400-800 nm) resolution. For high spatial resolution they used a telescope with 3 m spatial and <100 ms temporal resolution.

 The instrument package consisted of two cameras mounted with the same viewing axis. One (the imager) was mounted inside a normal GV window, the other (the spectral instrument) was mounted to look through a new side-looking fused silica optical window panes, thus extending the transmissivity down to about 275 nm. The fused silica window panes have almost 100% transmissivity well below the atmospheric limit of 360-370 nm for severe absorbtion.