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2222222 2222222
2
2
B
0
E
E
E
E
z
v
v
v
v
=n E
0
V
d
J
e
x
J
J
J
J
1111111 1111111
1
1
Figure 4.39
Schematic representation of wave-driven currents in the equatorial electro-
jet. [After Oppenheim (1997). Reproduced with permission of the American Geophysical
Union.]
between the wave and the electron drift direction as seen in Woodman and Chau
(2002), Shume et al. (2005), and Bahcivan et al. (2005).
On the left of Fig. 4.39, we show the vertical electrojet electric field,
E
0
, the
geomagnetic field,
B
0
, the plasma density gradient,
n
, and the electrojet electron
drift direction,
V
d
.If
V
d
exceeds a threshold then compressional plasma waves
develop as shown by the varying shades of grey, darkest where the waves enhance
the plasma density and lightest where they reduce it. At the density maxima and
minima, we show the direction of the perturbed electric field,
∇
δ
E
, the direction in
which the electrons drift in response to
δ
E
,
δ
v
, and the resulting electron current,
δ
v
. This current is larger where the plasma density is enhanced than
where it is reduced. On the right, we show the direction of the net, wave-driven,
vertical, electron current,
J
=
n
δ
J
e
. An identical mechanism generates wave-driven
currents in the auroral electrojet when
δ
∇
n
=
0,
E
0
is horizontal, and
B
0
is
vertical (Oppenheim, 1997).
The simulations also show that FB waves nonlinearly drive a large-scale (dc)
current in the E-region ionosphere, as shown in Fig. 4.39 (Oppenheim, 1997).
This current flows parallel to the fundamental Pedersen current and with a
comparable magnitude. These currents can restructure the electrojet as shown
in Fig. 4.40. Also, by effectively increasing the Pedersen currents, wave-driven
currents reduce the electrojet charge and polarization field,
E
0
, responsible for
driving FB waves. This makes the linear phase velocity drop toward the acoustic
speed and may play an important role in sound speed saturation of type 1 waves.
A wave-driven current results from two fundamental features of E-region
plasma waves. First, electrons travel mostly perpendicular to the electric fields
due to the geomagnetic field while ions travel mostly parallel to the fields
because ion-neutral collisions make magnetic field effects inconsequential. Sec-
ond, gradient-drift and two-stream instabilities cause compressional waves
where the plasma density enhancements and the perturbed electric fields remain
largely in phase. At the plasma density maxima of the propagating wave fronts,
electrons move perpendicular to the wave direction and the geomagnetic field.
At the density minima, electrons move in the opposite direction with an equal
velocity. However, more electrons exist at the maxima than at the minima caus-
ing a greater current in one direction than the other, resulting in a net (direct)
current.
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