Geoscience Reference
In-Depth Information
38.0
568
LINDAU - TSUMEB
36.0
45
8
March 1, 1971 18.21 MHz
ms
46
8
34.0
408
West
33
8
40
8
32.0
35
8
Great circle deviation
5
24
8
t
9
30.0
288
148
Great circle propagation
27.8
t
9
2
20
8
30.0
2
27
8
2308
2268
East
32.0
2
30
8
Scattered path
Discrete path
Scatter background
2328
34.0
2
34
8
2
33
8
ms
2398
36.0
C
C
1700
1800
1900
2000
2100
2200
t
UT
Figure 4.12
Forward scatter measurements in the African sector showing regular wave-
like regions of enhanced plasma density. [After Röttger (1973). Reproduced with permis-
sion of Pergamon Press.]
capabilities: if an internal gravity wave (see Chapters 5 and 6) organizes equa-
torial plasma into high- and low-altitude contours with the same horizontal
wavelength as the gravity wave. The generalized RT instability can take over
and cause the oscillation to grow. In fact, the generalized R-T growth rate is so
small that seeding may be essential, either by gravity waves or by the collisional
Kelvin-Helmholtz process discussed following. Such seeding can greatly decrease
the time needed to develop a large-amplitude disturbance. Strong evidence for
the organization of bottomside plasma into structures with scales of several hun-
dred kilometers is given in Fig. 4.12 (see also Figs. 4.1 and 4.5b). The data are
from a transequatorial HF radio propagation experiment. If the ionosphere were
uniform, refraction would yield a single “great circle” path with some minimum
time delay. However, when the plasma density has an east-west structure, addi-
tional, larger time delays occur, which can be used to characterize the structure.
The data show that paths other than the great circle route occur in regular inter-
vals, which are found to be typical of internal gravity wave wavelengths. One
explanation for this effect, proposed by Beer (1973), was that the temperature
variation in the gravity wave would change the recombination rate of the iono-
spheric plasma and thereby create a density modulation. The effect would be
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