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are thus consistent with echoing regions and large-scale electrostatic waves orga-
nized horizontally with about a 15 km separation traveling toward the south-
west. It appears clear then based on the Hysell et al. (2004) interferometry that
Q-P echoes should not be considered as altitude changes but rather as horizontal
echoing regions making an angle to the radar beam and moving through it.
6.7.3 Midlatitude E-Region Instabilities: Difficulties with
Simple Explanations
Consider first the instabilities generated by global scale dynamo electric fields
and global scale neutral winds (tides) interacting with vertically stratified plasma
layers. Both the two-stream and gradient drift instabilities have difficulties in this
case. As discussed in Chapter 4, the condition under which primary two-stream
waves occur is given by
V
D
E
0
/
B
>(
1
+
0
)
C
s
(6.27)
where
0
=
ν
i
ν
e
/
i
e
,
C
s
is the acoustic speed, and
E
0
is the dc electric field. The
magnetic field increases quickly with latitude, while the midlatitude electric field
is actually smaller than the field at the equator. Thus, (6.27) is difficult to satisfy
and seems to preclude primary two-stream waves at midlatitudes. However, in
the next section it is argued that if the layers are patchy horizontally, a two-
stream instability can be driven by polarization of the patches.
The problem with the classic electric field-driven gradient drift instability
source is that the layers are not nicely perpendicular to the magnetic field. As illus-
trated in Fig. 6.37a, when a vertically stratified layer is subject to a perpendicular
E
0
n
U n s t a b l e
S t a b l e
n
E
0
B
0
u ( w e s t )
n
U n s t a b l e
S t a b l e
n
u ( w e s t )
B
0
Figure 6.37a
Geometry for stability of a vertically stratified layer in the presence of a
southward electric field (which must map along
B
) and a uniform westward wind (which
does not, however, have to be uniform with height).
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