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vertically upward field below the F layer. This in turn will cause a westward
plasma drift. However, the observed westward plasma drift velocity is higher
than was expected for a wind-driven process in the 1980s. For example, the
weak westward plasma drift at the base of the F layer in the calculation shown
in Fig. 3.11 (indicated by the change in sign of the vertical electric field) was due
to the theoretical E-region dynamo. The reversed field is much lower than the
measured values, however. New measurements of neutral winds in the E region
(see Chapter 5) show that high velocities do occur contrary to tidal theory. Thus,
the E-region dynamo is likely much stronger than previously thought. Improved
programs at Jicamarca have allowed direct ISR observations of the postsunset
equatorial zone. An example of the data is presented in Fig. 3.13. A remarkable
vortex is seen in the postsunset region, in agreement with the barium cloud and
JRO/d960917/p_w0
SNR +1 in dB 0.0
9.8
19.6
600
400
200
19
20
21
Local time (hr)
22
23
Figure 3.13
A combination of irregularity and
E
B
drifts and a backscattered power
map for September 17, 1996 (sunset time 18:03 LT, Kp
×
=
3
−
). A data gap exists at
∼
20:00 LT. [After Kudeki and Bhattacharyya (1999). Reproduced with permission of
the American Geophysical Union.] See Color Plate 1.
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