Wave Ducting Workshop

Mike Hickey, Clemson University, hickey@hubcap.clemson.edu
Jim Hecht, Aerospace Corporation, james.h.hecht@aero.org
NIST 1107, 1:00-3:00 PM
NIST 1103-5, 4:00-? PM
Thursday, June 29, 2000

Airglow imaging observations often reveal wave structure covering a significant fraction of the sky and moving coherently across the entire field of view. The displays usually last for a few hours. Inferred wavelengths, wave periods, and horizontal phase speeds are typically 20 – 50 km, 5 – 15 minutes, and 30 – 80 m/s, respectively. The inferred directions of propagation have a seasonal dependence, being predominantly equator-ward in winter, and pole-ward in summer.

It has been suggested by Walterscheid et al. [1999] that the observations can be explained by the presence of waves ducted or trapped in the thermal duct in the mesopause region. Numerical modeling using a full-wave model substantiates this claim. The effect of mean winds complicates the interpretation of the observations. However, extensive numerical modeling described by Hecht et al. [2000] still supports the wave ducting hypothesis.

In spite of the successful interpretation of the observations in terms of wave ducting, some issues still remain unresolved. Large winds and large wind shears, as inferred from observations of TMA trails, may mitigate ducting. Additionally, given the transient nature of the MLT region, large stable ducts are unlikely to remain coherent over appreciable times and distances. Large wind shears may also generate Kelvin-Helmholtz instabilities, and there is some evidence for KH billows in the airglow. Large temperature gradients may generate convectively unstable region. Theory and the limited number of observations suggests that shorter scale (< 20 km) structure in the airglow may primarily be due to KH billows or convective instabilities, whereas the larger scale structure (> 20 km) may partly be due to trapped or ducted gravity waves. Are there more rigorous ways to differentiate between the two? What observations are needed? Is current theory adequate?

This workshop will bring people together to discuss these issues. In order to maintain an informal workshop format, and to facilitate the open exchange of ideas, the number of viewgraphs will be limited, if possible, to between three (3) and six (6) per person. A brief agenda is presented below. Speakers may be asked questions, or comments may be made from others, at any time. Also note that this workshop will continue after the break in a different room.