Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 469

In this way, planetary scale structuring of the plasma occurs which is much more
complex and interesting than the simple terminator effect would be.
A more subtle process can create planetary scale depletions of plasma as well.
Since the dipole magnetic axis of the earth is offset by 11 ◦ from the rotation axis
and the plasma flow is organized by the magnetic geometry, in the winter time
some convection patterns have flux tubes that are never illuminated by sunlight.
Then, very deep plasma depletions can occur due to recombination, yielding peak
plasma densities as low as 10 3 cm − 3 with He + the dominant ion (see Fig. 9.1).
The two-cell convection pattern is associated with another planetary scale
plasma source: impact ionization by particle precipitation in the auroral oval.
From a visual perspective, this band of light around the polar region expands and
thickens with increasing B z southward and shrinks when B z is northward. Much
of the plasma in this oval is created so low in the atmosphere (

200 km) that
it is short-lived. Nonetheless, the lowest energy precipitating particles produce
plasma high enough in altitude to create an important F-layer plasma source,
particularly in winter.

≤

10.1.2 Some Effects of Plasma Transport and Loss on the
Large-Scale Horizontal Structure of the Ionosphere
One of the most fascinating aspects of the high-latitude ionosphere is its interac-
tion with the various magnetospheric regions to which it is connected by mag-
netic field lines. In portions of the ionosphere which are not sunlit, the influx of
precipitating particles is one of the dominant sources of the ionospheric plasma;
the other major source is transport from either a sunlit region or a region where
particles are precipitating. One might at first suspect that solar-produced plasma
would display very little structure in the F region, since the ionization is long
lived and the source is smoothly varying. However, since the flow field varies
drastically in time and space, even solar-produced plasma may become horizon-
tally structured. It is, in fact, difficult to separate particle precipitation zones from
electric field patterns, since they are intimately related. We concentrate on the
latter in this section and then follow with some comment on precipitation in the
next section. As discussed in Chapters 8 and 9, perpendicular electric fields occur
throughout the auroral zone and polar cap. Their existence results in transport
of F region plasma from production zones to areas where one might not expect
to find much plasma at all. A good example is found in the winter polar cap.
F region plasma is not produced by sunlight at all in this region and there is only
a very weak particle input, called “polar rain,” when B z is northward. However,
observations show considerable structured plasma in the polar cap. The red light
emission of atomic oxygen at 630 nm due to recombination is sufficiently large
to make this plasma visible to image-intensified camera systems. In Chapter 4
we showed that localized low-density regions could be observed as an absence
of these emissions and pointed out that these airglow depletions were due to
equatorial spread F wedges or bubbles. At high latitudes it is the enhancements