Geoscience Reference
In-Depth Information
B
U
U
3
B
11
1
1
2
2
E
p
E
b
E
p
1
E
b
Longitude
Figure 5.24b
Cartoon showing how the tidal winds create zonal electric fields.[After Kil
et al. (2007). Reproduced with permission of the American Geophysical Union.]
5.3.2 Wind Profiles
Over the last few decades, nearly a hundred trimethyl aluminum (TMA) trails
have been laid behind sounding rockets at all latitudes. Many of winds deduced
from these trails are superimposed in Fig. 5.25a. The peak observed winds are
much larger than predicted by tidal theory. On the other hand, they do have tidal
properties. For example, the wind vector rotates with altitude as predicted for
the tides. Another example is presented in Fig. 5.25b. Here a chemiluminescent
meteor trail is shown 82 seconds after a Leonid meteor hit the atmosphere.
A nearly perfect circle (corresponding to a helix-like structure) is seen on the
image (Drummond et al., 2001), suggesting that a large amplitude inertiogravity
wave is the source.
Lasers can also be used to study winds and thus have become a powerful diag-
nostic. If tuned to the 589 nm sodium line, a resonance fluorescent interaction
increases the cross section enormously, making the sodium layer visible. The
straight line from the middle of Fig. 5.25b to the right-hand side was generated
by a sodium lidar. Created by meteor ablation, the sodium layer density can
exceed 10
4
cm
−
3
. One event in which TMA and lidar methods were used simul-
taneously is shown in Fig. 5.26a and b. Here, lidar sodium echoes were obtained
on a night when several rockets were fired. We have used the wind profile to
generate the neutral atmospheric Richardson number,
R
i
, for the event where
2
b
ω
R
i
=
(5.26)
d
dz
2
|
U
|
/
and
4 the medium is unstable to the
Kelvin-Helmholtz instability of the atmosphere and the wind shears are strong
enough to create unstable neutral layers. The KHI is most likely responsible for
ω
b
is the Brunt-Vaisala frequency. If
R
i
<
1
/
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