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valley. The production rate is very low, but neutral atmospheric waves gather
the ions and produce easily observable features in the plasma profile. To this
are added meteoric sources of metallic ions at lower altitudes. There, long-lived
ions are also gathered together by wind and wave patterns to form the ionization
layers observed.
6.5 Horizontal Structure in the Midlatitude Ionosphere
Studies by Bowman (1981, 1985) seem to show clearly that much of midlatitude
spread F as registered on ionosondes is due to altitude modulation of the electron
density in the F layer, most likely due to gravity waves. Some of the more violent
disturbances that include small-scale structure most likely also involve the plasma
instabilities discussed following. Since these features propagate, they are referred
to as traveling ionospheric disturbances or TIDs.
We have concentrated thus far on vertical ionospheric structure, since it is far
easier to observe. Ionosondes usually are directed vertically but can detect hori-
zontal variations due to their broad antenna patterns. Bowman (1981, 1985) has
described and reviewed the results of such observations. Figure 6.13a shows one
interpretation of ionosonde observations of amedium-scale traveling ionospheric
O-Ray ionograms for MS-TIDs and LS-TIDs
50 km
(a)
Travelling ionospheric
disturbance
A
D
C
B
Bribie island
October 24/25, 1990
MS-TID
Periodicity
5 20 min
Significant spread
(traces A, B, C and D)
(67 ionograms used)
(b)
200 km
f 0 F2 Minimum 5 7.5 MHz
B
A
Plasma freq. (MHz)
7.0
No spread
traces A and B
almost coincident
Diagrams not to scale
6.5
LS -TID
Periodicity
5
370
80 min
6.0
(c)
8.0
5.5
4.5
330
330
3.0
290
290
3
8
250
2300
0000
0100
0200
Hours
Figure 6.13 Cartoons (a, b) showing how a corrugated ionosphere leads to ionosonde
spreading. Also, actual ionosonde data (c) showing downward phase progressions for a
large-scale TID. [After Bowman (1981). Reproduced with permission of Elsevier Science.]
 
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