Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 488

We conclude that production and heating of plasma by long-lived soft particle
precipitation creates much of the F-region horizontal structure in the auroral
oval at scales from 30 to 100 km. Larger-scale features are more likely due to a
combination of solar production, chemical losses, and transport by large-scale
electric field patterns. These concepts have been verified in a series of computer
model calculations by Schunk and Sojka (1987).

10.1.6 Summary
We agree with Tsunoda (1988) that ionospheric plasma instabilities per se have
very little to do with ionospheric structure at large to planetary scales. Con-
vection, production, and loss are sufficient to create a marvelously complex
ionosphere.

10.2 Intermediate-Scale Structure in the High-Latitude
F Region

The ordering of high-latitude plasma at scales above 30 km has been described
in detail in the previous part of this chapter. Here we look into the processes
which create structure in the range 0.1-30 km. There is a close analogy here to
equatorial spread F phenomena in the following sense. At the largest scales in the
equatorial case aeronomical processes such as production, recombination, and
electrodynamic transport act in concert with neutral wind phenomena such as
gravity waves to create the large-scale patterns. The intermediate-scale structur-
ing then proceeds primarily via the generalized Rayleigh-Taylor instability. At
high latitudes, we have seen that aeronomical processes are further supplemented
by structured particle precipitation and structured large-scale flow patterns. It
should not be surprising then that intermediate-scale structuring in the auroral
zone and polar cap is also more complicated than at the magnetic equator. In
a turbulent neutral fluid, viscosity eventually limits the scale sizes where struc-
ture occurs to values larger than some wavelength corresponding to the so-called
viscous cutoff. In a plasma there are many more degrees of freedom in the sys-
tem and new sources of structure may arise at various wavelength scales in the
medium. In Sections 10.2.1 and 10.2.2 we concentrate on mechanisms that gen-
erate structure at intermediate scales, scales that lie between the size of the auroral
oval and the size of auroral arcs. Since the linear theory for plasma instabilities
in this range appeals to diffusive damping as a limiting mechanism, we spend
some time discussing cross-field plasma diffusion and images in Section 10.2.3.

10.2.1 The Generalized E B Instability at High Latitudes

Before delving into the differences, some similarities to equatorial spread F are to
be noted. Figure 10.13 shows three sets of plasma density patterns detected along

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