Geoscience Reference
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Since in the F region
σ
iP >> σ
eP , this reduces to
2
n
/∂
t
=
D e (
x n
)
In this case therefore the velocity and associated decay rate of the structure
will be the minimum possible under the circumstances considered and will be
characterized by the electron diffusion coefficient D e
.
From the previous discussion it should be clear that the decay rate of an F-
region irregularity depends not only on its scale size and the local ionospheric
conditions but also on the mapping properties of the electric field and on the
conductivity of the regions throughout which the field maps. In our description
we considered two extreme examples where the electric field was easily deter-
mined. In practice, however, the electric field will be determined by applying the
current continuity condition on an intermediate state. This yields slightly more
complicated equations but with essentially the same characteristics.
One further point to consider is the effect that the current produced by a
structure in the F region might have in the E region. This can be an important
consideration because the relatively high collision frequencies in the E region
mean that the plasma is compressible. If this compressional force can overcome
the horizontal diffusion forces that oppose it in the E region, then a structure
can form there that mimics the one in the F region. In order to be significant, this
structure formation process in the E region must also overcome the high rate of
dissociative recombination that exists at such altitudes. We now investigate the
conditions under which such an “image” structure can be formed.
The image formation process can be understood from Fig. 10.17a for a pure
decaying cylindrical irregularity in which the only electric field is due to the
ambipolar effect. For a large enough structure, the E field maps as shown and E-
region ions are gathered together in the center region because of their Pedersen
drifts. The requirement of charge neutrality is met by electrons flowing down
B to the E region. The net result is that the E-region plasma density (ions and
electrons) increases at the center. That is, an image high-density region forms in
the center and two low-density regions form outside. The image amplitude will
eventually be limited by recombination in the E region, which is proportional
to n 2 . Notice that the plasma-gathering process can be discussed in terms of
compressibility of plasma in the E region in response to the applied electric field.
The F region plasma, on the other hand, is virtually incompressible perpendicular
to B .
Although interesting in their own right, we have not explained the role images
play in F-region diffusion. The E-region plasma density gradient due to an image
drives an ion current opposite in direction to the mapped electric field. The net
field-aligned current from the F region is therefore decreased, and the F-region
diffusion coefficient is lowered, tending more toward the low value of D
which
pertains for an insulating E region. Now, since the mapping process is itself
scale size dependent, we conclude that the effective diffusion coefficient of high-
latitude plasma irregularities may also be scale size dependent if the E-region
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