Geoscience Reference
In-Depth Information
Spatial resonance could thus occur with a zonally eastward-propagating wave,
since it is well known that the plasma also drifts in the eastward direction (see
Chapter 3). Furthermore, the magnitudes of the typical eastward and downward
plasma drifts are roughly comparable to the typical phase velocities of large-scale
gravity waves in the upper atmosphere.
Gravity wave-induced electric fields have now been observed (Kudeki et al.,
1999; Varney et al., 2009). Figure 4.13b shows the classic downward-phase
progression of the vertical velocity over Arecibo. If we assume a nonconducting
E region and a gravity wave at F-region heights, then, as previously argued, the
divergence of the wind-driven current will set up polarization charges and an
electric field such that
δ
J
=
σ
P
(δ
E
+
δ
U
×
B
)
=
0
and thus,
δ
E
=−
(δ
U
×
B
)
where
σ
P
is the Pedersen conductivity,
δ
E
is the perturbation electric field in
the earth-fixed frame,
U
is the perturbation wind velocity due to the gravity
wave, and
B
is the magnetic field. The net effect of the dynamo is to cause the
plasma and the neutral gas to move together in the gravity wave wind field,
since
δ
B
2
U
. To determine quantitatively the effect of such a
perturbation, we first consider the case of perfect spatial resonance in which the
mean unperturbed plasma is at rest in a fixed phase front pattern associated
with the gravity wave; that is, the plasma drift matches the wave phase velocity
δ
V
=
δ
E
×
B
/
=
δ
09/29/94
20
10
0
10
2
20
2
0
1
19
20
21
22
23
LT (hours)
Figure 4.13b
Stack plot of the vertical drift velocities at individual heights. The lowest
height is 225 km and each subsequent line is 15 km higher and has been shifted by 2m
s
upward. The red portions of the plot are points inferred by interpolation. The slanted
black lines are identical and have been aligned with peaks in the data to illustrate the
downward-phase velocity. See Color Plate 5.
/
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