Geoscience Reference
In-Depth Information
Log
10
of N
e
(cm
23
)
Arecibo: Oct 1-2, 2002
6.1
500
4.8
400
300
3.5
200
100
1
0.8
0.6
0.4
450
400
350
300
0
0
0
3
0
2
1
2
3
4
5
6
7
8
9
10
11
12
Time (UT, LT14)
Figure 3.30b
Periodic oscillations of the ionosphere over Arecibo with the downward
phase progression typical of gravity waves are shown in the top panel. One of these
oscillations triggered a turbulent upwelling over Jicamarca near dawn. [After Nicolls
et al. (2004). Reproduced with permission of the American Geophysical Union.] See
Color Plate 2.
A thermospheric model has been run by Sipler et al. (1983), who used it as a
diagnostic tool in a numerical experiment to determine which processes dominate
the wind variability. The measurements and model calculations of the wind are
plotted in Fig. 3.31. In this study, the thermospheric general circulation model
(TGCM) was run for August 21, 1978, and compared to the measured winds
indicated by the heavy dashed line in Fig. 3.31. Then the lower atmospheric tides
were added along with the high-latitude influence (light dashed line). Finally,
ionospheric electric fields were added (dotted line). The conclusion was that the
day-to-day variability was tied most closely to the tidal and electric field effects
but that even in relatively quiet conditions the high-latitude plasma circulation
played a role in the equatorial winds.
Finally, we return to the curious fact that the earth's upper atmosphere super-
rotates—that is, the mean zonal wind in the rotating frame is eastward in the
equatorial zone. A number of explanations have been put forth concerning this
effect (e.g., the review by Rishbeth, 1972). Since a high eastward velocity is
most common in the sunset-to-midnight period, theories that control the ther-
mospheric winds via the ionospheric drag effect look very promising, since in
this period the winds are the least opposed by the ionosphere. This happens for
two reasons. First, due to the prereversal enhancement, the ionosphere rises and
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