Dynamics and Electrodynamics of the Mesosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 367

the intensity. In both emissions, a series of waves followed in the wake of the
front. The waves themselves were 180 ◦ out of phase between the two emissions.
The feature eventually covered the entire sky, from horizon to horizon in every
direction. As many as 10 crests can be identified. Very similar events have been
reported subsequently (Garcia, 1999), one of which was visible to the naked eye
in the OH emission (Smith et al., 2003).
These features have been called sharp fronts, wall waves, and mesospheric
bore waves. The former two designators refer to a guided nonlinear step or
solitary wave. The theory is best developed for mesospheric bores and will be
emphasized here.
7.9.2 Analogy to a Hydraulic Jump
A bore, or hydraulic jump, in an open channel is an abrupt change in the depth of
the channel fluid in response to a transition from supercritical to subcritical flow
and is analogous to the shock that forms in a gas in the transition from supersonic
to subsonic flow. Dewan and Picard (1998) observed that the airglow image data
of the sharp front in the Aloha campaign showed remarkable similarities to river
channel bores, an example of which is shown in Fig. 7.23. They have advanced
a theory to try to attribute the sharp fronts to “mesospheric bores.”
Bores in an open channel have been well studied. Lighthill (1979) provides a
good description of open channel bore theory. The problem in relating atmo-
spheric bores to open channel bores is that there is no channel in the atmosphere,
so what is the guiding mechanism? Dewan and Picard (1998) cite examples
(e.g., see Smith, 1988) of bores in the troposphere to show that internal bores
Figure 7.23 A tidal bore on the River Mersey from the air (Dewan and Picard, 1998)
from (Tricker, 1965). [After Dewan and Pickard (1998). Reproduced with permission of
the American Geophysical Union.]