Simultaneous observations of mesospheric gravity waves and sprites generated by a midwestern thunderstorm

The present report investigates using simultaneous observations of coincident gravity waves and sprites to establish an upper limit on sprite-associated thermal energy deposition in the mesosphere. The University of Alaska operated a variety of optical imagers and photometers at two ground sites in support of the NASA Sprites99 balloon campaign. One site was atop a US Forest Service lookout tower on Bear Mt. in the Black Hills, in western South Dakota. On the night of 18 August 1999 we obtained from this site simultaneous images of sprites and OH airglow modulated by gravity waves emanating from a very active sprite producing thunderstorm over Nebraska, to the Southeast of Bear Mt. Using 25 s exposures with a bare CCD camera equipped with a red filter, we were able to coincidentally record both short duration (\u3c10 ms) but bright (\u3e3 MR) N2 1PG red emissions from sprites and much weaker (~1 kR), but persistent, OH Meinel nightglow emissions. A time lapse movie created from images revealed short period, complete 360° concentric wave structures emanating radially outward from a central excitation region directly above the storm. During the initial stages of the storm outwardly expanding waves possessed a period of τ≈10 min and wavelength λ≈50 km. Over a 1 h interval the waves gradually changed to longer period τ≈11 min and shorter wavelength λ≈40 km. Over the full 2 h observation time, about two dozen bright sprites generated by the underlying thunderstorm were recorded near the center of the outwardly radiating gravity wave pattern. No distinctive OH brightness signatures uniquely associated with the sprites were detected at the level of 2% of the ambient background brightness, establishing an associated upper limit of approximately ΔT ≤ 0.5 K for a neutral temperature perturbation over the volume of the sprites. The corresponding total thermal energy deposited by the sprite is bounded by these measurements to be less than ~1 GJ. This value is well above the total energy deposited into the medium by the sprite, estimated by several independent methods to be on the order of ~1–10 MJ