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60
40
16
16
14
20
16
14
0
12
14
6
10
220
12
8
14
6
8
2
40
8
6
10
260
10
12
12
10
2
80
2
100
2
120
0 0 0 0
Zonal wind (ms
21
)
80
100
120
140
160
Figure 3.31
Position of the tip of the measured wind vector as a function of universal
time for August 21, 1978. The heavy dashed line gives the measured values and the
solid line represents the TGCM calculations. Two other TGCM calculations are also
presented: the fine dashed curve is a numerical experiment that includes tides from the
lower atmosphere and enhanced (60 kV cross-tail potential) magnetospheric convection
at high latitudes; the dotted line further includes enhanced
E
B
drifts representative of
solar maximum conditions. [After Sipler et al. (1983). Reproduced with permission of
the American Geophysical Union.]
×
“moves out of the way,” Second, the downward electric field builds up due to
the F-region dynamo (with little E-region shorting), and ion drag becomes small.
3.6 Mesospheric and Lower Thermospheric Dynamics
3.6.1 Atmospheric Winds in the Mesosphere and Lower Thermosphere
The mesosphere and lower thermosphere are collocated with the E region. Most
of what we know about dynamics comes from rocket and radio wave techniques
but more recently, lidars have become practiced. Meteor radars use the drift
of meteor trails to accumulate line-of-sight velocities over some suitable period
of time and convert them to vector winds. MF (medium frequency) radars use
partial reflections from the D-layer structures seen in Fig. 1.7a, b to determine
the winds. Large incoherent scatter radars can be used in the daytime, but it is
not practical to use this technique for climatological studies.
In Chapter 1 a representative temperature profile was presented for the earth's
atmosphere. Below about 60 km the temperature at a given height stays within
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