Geoscience Reference
In-Depth Information
90
80
70
Inertial
60
50
40
Viscous
Buoyancy
30
20
10
0
10
24
10
23
10
22
10
21
1 0
10
2
10
3
10
4
Scale (m)
Figure 3.36
Various subranges in the earth's atmosphere.
(Figure courtesy of
W. Hocking.)
Since atmospheric radars can detect scatter from clear air turbulence, Fig. 3.36
shows why a given radar can only see up to a certain height. If the radar Bragg
scale is too deeply into the viscous subrange, no echo can be detected. In the meso-
sphere it is necessary for electrons to be mixed by the turbulence to get a signal
because the air is so tenuous. Thus, large long-wavelength (e.g., 50MHz) radar
systems can obtain mesospheric echoes in daytime and M-S-T (Mesosphere-
Stratosphere-Troposphere) radars can be built to study virtually the entire atmo-
sphere up to 100 km.
In summary:
1. Below 110 km or so, on average, the atmosphere is mixed by turbulence, which feeds
off buoyancy or shear forces. Such structures are roughly isotropic.
2. Above this height the atmospheric constituents begin to separate according to multi-
component molecular diffusion (Vlasov and Davydov, 1982).
3. Above the turbopause the neutral atmosphere no longer has a turbulent structure,
and any radar scatter must be due to plasma instabilities or incoherent scatter. The
former is highly anisotropic.
References
Anderson, D. N., andMendillo, M. (1983). Ionospheric conditions affecting the evolution
of equatorial plasma depletions.
Geophys. Res. Lett.
10
, 541.
Anderson, D. N., and Roble, R. G. (1974). The effect of vertical
E
B
ionospheric drifts
on F-region neutral winds in the low-latitude thermosphere.
J. Geophys. Res.
79
,
5231.
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