Geoscience Reference
In-Depth Information
0508
10
6
0308
10
5
0108
10
4
2308
10
3
1974 April 16-17
100
120
140 160 200
Height (km)
300
400
Figure 5.21
Electron density profiles for the night of April 16-17, 1974. This night was
extremely quiet magnetically. [After Shen et al. (1976). Reproduced with permission of
the American Geophysical Union.]
The altitude of the peak in the plasma density for a number of low-altitude
layers has been determined in a study that includes daytime hours. The results
are plotted in Fig. 5.23 for 78 consecutive hours. A well-defined motion is seen in
the central part of the time period (January 4) in which a layer starts descending
from 150 km at 0600 LT, reaching 90 km at about midnight. Parts of the same
pattern can be discerned on the previous day as well as on the following day,
which suggests that a solar diurnal mode (S1) exists. However a similar pattern
occurs 12 hours before and 12 hours later so the tide could be semidiurnal (S2)
or an in-phase combination of S1 and S2. Furthermore, on January 5 there is
a prominent six-hour tide (S4). A study of tides near the ground using surface
pressure and GPS data shows that S1 through S6 are all energized (Humphreys
et al., 2005). Higher-frequency motions are also clearly indicated in the figure.
When discussing atmospheric waves, as the frequency increases above the tidal
range, it is no longer necessary to discuss the atmospheric winds in terms of tidal
modes because the horizontal wavelengths become much smaller than the radius
of the earth. In this case the appropriate modes are termed internal inertiogravity
waves, a topic we take up in some detail in the next chapter. The high-frequency
features of the data in Fig. 5.23 are thus geophysical in origin and are probably
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