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Electrojet current
20
Legend
With NL effects
Without NL effects
Wave threshold
With GD NL effects
15
10
5
0
80
100
120
140
Height (km)
Figure 4.40
Electrojet current density as a function of height. The dashed line shows
the current without nonlinear effects, the solid line shows the current with effects of
a two-stream, wave-driven nonlinear current; the dashed-dotted line shows the current
with effects of a gradient-drift (GD), wave-driven current, and the dotted line shows
the minimum current necessary for initiating two-stream waves and the resulting wave-
driven current. [After Oppenheim (1997). Reproduced with permission of the American
Geophysical Union.]
10
5
0
2
5
2
10
0
50
100
150
Distance (m)
Figure 4.41
The electric field from a one-dimensional, simulated gradient-drift wave
system modified by secondary two-stream waves driving a nonlinear current. Note the
similarity to Fig. 4.29. [After Oppenheim (1997). Reproduced with permission of the
American Geophysical Union.]
The electric fields measured by rockets passing through gradient-drift waves
appear as irregular square waves (see Fig. 4.29). Wave-driven electron currents
can cause these squared-off electric fields through a two-step process. First,
the perturbed electric field of a gradient-drift wave must exceed the threshold
necessary to initiate two-stream waves (Sudan et al., 1973). Second, these sec-
ondary two-streamwaves generate wave-driven electron currents that modify the
original gradient-drift waves. This effect has been estimated numerically in
Oppenheim (1997) and is shown in Fig. 4.41.
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