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to a very slight initial charge imbalance. This keeps the plasma charge neutral
and allows the plasma to build up into the layers cited above. This mechanism
is thus most efficient at low and middle latitudes. The wind shear process can
be considered as an example of the compressibility of the ionospheric plasma in
the direction perpendicular to
B
when
1. As noted earlier, F-region perpen-
dicular plasma flow is virtually incompressible
κ
i
≈
0 but in the lower F
and E regions this is not the case. If we consider a meridional wind, the Lorentz
force only deflects ions east-west and thus has no effect on producing vertical
layering, but it could create horizontal patchiness in the layers if the winds are
periodic, as in gravity waves.
The wind shear theories outlined here yield a very nice dynamical explanation
for ionospheric layering. The downward progression of the layers is accounted
for, since gravity wave and tidal theories both predict downward phase propa-
gation for an upward group velocity (expected for lower atmospheric sources).
The theory initially ran into quantitative difficulty, however, since the standard
recombination rate at the heights of interest was too large to support the observed
layer densities (up to 10
6
cm
−
3
). But rocket data show that metallic ions such
as Mg
+
,Si
+
, and Fe
+
due to meteoric sources proved to be the dominant ions
in these layers (Herrmann et al., 1978). This fact removed objections to the
wind shear theory, since such ions have very long lifetimes. An example of the
ion composition measured in a sporadic E layer during a daytime rocket flight
is presented in Fig. 6.9c. The
(
∇
⊥
·
V
⊥
)
=
M
+
curve shows all the metal ions that track
the peaks in the electron density quite well and constitute more than half of all
the ions, even during the daytime. Such data strongly support the notion that
metallic ions are responsible for long-lived intense sporadic E layers. The inter-
mediate layers remain a problem, however, since they do not contain metallic
ions. One resolution of this problem, discussed in some detail in the next section,
involves additional ionizing radiation at midlatitudes over and above the usual
photoionization sources.
Neglecting ionization and recombination, and assuming that all variables
are functions of
z
only, the relation between the altitude profile of plasma
density produced by the neutral motion and
dN
dz
has been calculated by
/
Gershman (1974). Assuming
ν
in
, as in Gershman's
derivation, the inverse vertical scale length of the plasma density gradient is
given by
e
sin
I
ν
en
and
i
sin
I
N
u
i
v
in
v
z
)
dz
dN
(
z
)
=
i
cos
I
v
in
D
A
v
en
e
sin
I
L
−
1
=
(
−
sin
I
+
(6.14)
where
u
e
and
v
n
correspond to the maximum zonal (positive eastward) and
meridional (positive northward) winds about the shear point.
D
A
is the ambipo-
lar diffusion coefficient. Substituting reasonable values for these parameters,
we find
L
<
100m, which is smaller than observed. Kagan and Kelley (1998)
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