Simultaneous near-infrared and visible observations of sprites and acoustic-gravity waves during the EXL98 campaign

This paper reports the first images of Near-InfraRed (NIR) 900 to 1700 nm sprite emissions obtained during the Energetics of Upper Atmospheric Excitation by Lightning 1998 (EXL98) airborne observing mission. Results include the first NIR observations of sprites and a correlation between nighttime hydroxyl (OH) airglow, acoustic-gravity waves, and sprite location. A NIR camera monitored both OH airglow and N-2(B) First Positive (1PG) emissions in the NIR by sprites. OH airglow is used as a tracer of acoustic-gravity waves and density structures in the neutral atmosphere in the 80-95 km altitude region. The visible-light imager also observed faint hydroxyl airglow, at times, allowing convenient comparison of the gravity waves and the sprites without image processing. On only one night of observations was a clear correlation observed between the OH airglow and the sprites, with tops of extended lines of sprites appearing to align with the acoustic-gravity wave troughs in the 80 to 95 km altitude region. This investigation shows that a proper viewing geometry is needed to detect such a correlation and requires a distance of about 400-800 km from the sprites. Comparison of sprite visible and NIR emissions show that the NIR is brightest in the central body, likely dimmer at the tops, and rarely above sensitivity in the tendril regions. The observed NIR sprite emissions are thought to be primarily 1PG from the (1,0) and (0,0) vibration transitions near 888 and 1050 nm respectively. The study indicates a need for future NIR spectrographic measurement.