Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 504

convect through regions of varying E-region conductivity. The resulting ampli-
tude distribution depended on season and local time because of the peculiar
trajectories of some flux tubes (see Chapter 8).
Further advances in this area required a self-consistent calculation of the
ambipolar electric field because, unlike the simplification used by Vickrey and
Kelley (1982), which is valid only for large structures, the electric field mapping
process is scale size dependent (see Chapter 2). In addition, the E region is not
a passive medium but rather one in which structures can form that mirror the
F region irregularities. These “images” were first pointed out in the context of
barium cloud striation physics by Goldman et al. (1976).
In the remainder of this section we follow a unified description of diffusion
that includes F and E-region coupling (Heelis et al., 1985). First, it should be
emphasized that the electrical conductivity along the magnetic field lines is not
infinite but rather is a finite, usually large, quantity. The mapping characteristics
of electric fields can easily be deduced by considering the current continuity equa-
tion and employing analysis methods such as those discussed by Farley (1959)
and presented in Chapter 2. Figure 10.18 shows some solutions for different
scale size electric field structures applied at 500 km altitude. The ionospheric
plasma concentration profile used in the calculation is shown in the right panel.
As expected, we see that a 10 km structure maps almost unattenuated throughout
the E and F regions. At scale sizes below 5 km, however, significant attenuation
occurs and can render the electric field essentially zero below 200 km for fields
with scale sizes of 1 km or less that are applied at high altitudes.

500

400

300

200

.3

.5

1.

2.

7.

3.

4.

10.

100
0.0

0.5
Normalized potential

1.0

10 2 10 3 10 4 10 5 10 6

Ion concentration (cm 2 3 )

Figure 10.18 The variation in electrostatic potential from different scale size electric
fields is shown in the left panel. The right panel shows the ionospheric ion concentration
profile. The potentials were applied at 500 km altitude. (Figure courtesy of R. Heelis.)