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electric field, the layer is stable on the side where
E
is parallel to
n
but stable on
the other side. Likewise, as shown in the bottom panel of Fig. 6.37b, a uniform
zonal neutral wind is unstable where
U
∇
n
. But in both cases, a
layer is unstable on one side but stable on the other. At first glance this seems to
preclude any instability at all when the growth rate is integrated over the layer,
a fact that makes midlatitudes as interesting as they are. In the next sections
we explore how additional complexities in the midlatitude ionosphere can be
used to help explain midlatitude E-region radar echoes. For reference, and due
to the potential importance of neutral winds, we show the conditions for the
×
B
is parallel to
∇
V
5
E
3
B
0
B
0
V
d
N . 0
V
N , 0
N . 0
N , 0
N . 0
V
V
E
0
=
N
E
E
2
2
2
1
1
1
1
1
2
2
2
1
1
1
2
2
2
2
2
2
E
E
E
V
V
V
(a)
V 5
E 3 B
0
B
0
u
n
N
.
0
N
,
0
N
.
0
N
,
0
N
.
0
V
V
V
=N
1
1
2
E
2
E
2
2
2
2
1
1
1
1
2
2
2
1
1
2
2
2
E
E
E
V
V
V
(b)
Figure 6.37b
A schematic diagram for gradient drift instability driven by (a) an electric
field or (b) a neutral wind. Dark regions are high density and light regions are low density.
In (a) the classic Hall current is to the right (carried by electrons
E
0
×
B
drifting to the
left). A small; perturbation then grows due to the
B
drift. In (b) the neutral wind
(
u
n
) simply drags the ions with it to the right with the same effect. (Figure courtesy of
L. Kagan.)
δ
E
×
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