Coincident Imaging and Spectrometric Observations of Zenith OH Nightglow Structure
During the ALOHA-90 campaign a novel comparative study was made between near infrared wave structure imaged in the zenith using a CCD camera and that detected at infrared wavelengths by a Fourier Transform Spectrometer. Coincident measurements were made briefly on several occasions and for an extended period on 31 March. The temporal variations imaged in the near infrared structure during this night almost completely matched those detected in the OH (3,1) band spectrometer data when similar viewing fields were compared. However, the image data also displayed small scale wave forms that were not resolved by the larger field instrument. These structures exhibited significant changes in brightness and position on a time scale much shorter than the local Brunt-Vaisala period indicating that very high resolution measurements are necessary to investigate short period (<20 min) upper atmospheric wave motions.
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 <lter, we were able to coincidentally record both short duration (¡10 ms) but bright (¿3 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 ≈10 min and wavelength ≈50 km. Over a 1 h interval the waves gradually changed to longer period ≈11 min and shorter wavelength ≈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 AT . 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. c © 2003 Elsevier Science Ltd. All rights reserved.
Gravity Wave Studies at Polar Latitudes
Images of structure in the OH nightglow emission have been used to establish the presence of short period gravity waves in the winter polar mesosphere. The observations, which were made from the auroral station at Longyearbyen, Svalbard (78°N), are currently being used to investigate the generation of small scale gravity waves by auroral processes. Results so far show that the airglow wave patterns appear similar in both morphology and dynamics to those commonly seen at mid-latitudes indicating tropospheric sources for the waves. However, on one occasion there is good evidence for the generation of gravity waves by an auroral electric field that exhibited an intrinsic periodicity close to that observed in the airglow wave pattern Further studies are in hand.