Dynamics of atmospheric gravity waves and ripples in OH airglow images at Maui, HI

Airglow imaging provides a unique means by which to study many wave-related phenomena in the 80 to 100 km altitude regime. Observations reveal quasi-monochromatic disturbances associated with atmospheric gravity waves (AGWs) as well as small-scale ripple instabilities. Ripples are wavelike features that resemble AGWs in appearance, but have short horizontal wavelengths (<15 km) and short lifetimes (a few tens of minutes). A review by Hecht (2004) concluded that ripples are not AGWs but are in fact the manifestation of atmospheric instabilities. Decay of ripples is expected to result in increased turbulence, leading to increased eddy diffusion. Thus, ripple observations may help refine our understanding of the frequency of turbulence in the upper Mesosphere and Lower Thermosphere. The Aerospace Corporation’s Infrared Camera deployed at Maui, HI collected more than 700 nights of airglow images from 2002-2005. The camera measures the OH Meinel (4,2) emission at 1.6 μm using a 1 second exposure at a 3 second cadence, which allows finely detailed study of AGW and ripple features over very short temporal and spatial scales. Determination of AGW and ripple characteristics for this extensive dataset requires an automated wave detection system. Here we describe a 2-D wavelet analysis used to determine wave and ripple characteristics, including horizontal wavelength, propagation direction, period, and phase velocity. The wavelet technique also allows identification and tracking of ripple trains through sequential images, which can be used to study the motion of ripple patches with respect to AGWs and background winds (provided by the University of Illinois Maui Meteor Radar). We present an overview of AGW and ripple activity at Maui, provide examples of ripple tracking using the wavelet technique, and discuss preliminary results from the statistical analysis of AGW and ripple propagation at Maui. Hecht, J. H. (2004), Instability layers and airglow imaging, Rev. Geophys., 42, RG1001, doi:10.1029/2003RG000131.