Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 467

10 Instabilities and Structure in the

High-Latitude Ionosphere

The high-latitude sector is extremely rich in plasma instabilities and other processes
that act to create structure in the ionosphere. Our primary interest lies in the hor-
izontal variation of plasma density and electric fields. Since the magnetic field is
nearly vertical, the horizontal structure is equivalent to variations perpendicular to
the magnetic field. We use the generic term structure , since terms such as waves or
plasma density irregularities conjure up specific sources. In fact, a very long list of
processes contributes to the generation of horizontal structures in the plasma den-
sity and velocity field of the high-latitude ionosphere. It is essentially impossible to
treat the topic in its entirety, since our understanding is developing very quickly.
Our hope instead is to give a reasonable hint at the breadth of the phenomena
involved and, within the various subtopics, to treat a few important processes in
some detail. As was the case in earlier chapters, we start with the F region and
follow with the E region in the second portion of the chapter.

10.1 Planetary and Large-Scale Structures in the
High-Latitude F Region

In this section we review the physical processes that create horizontal variations
in the high-latitude plasma density at the largest scales. We somewhat arbitrarily
define the planetary scale to be larger than 500 km and large-scale processes
to have perpendicular wavelengths in the range 30 km 500 km. In these

regimes production, loss, and transport dominate the ionospheric processes.
Ionospheric plasma instabilities are not very important in this scale size
regime (although some local ionospheric processes do contribute at the lower
end). Of course, this is not to say that magnetospheric plasma instabilities are
unimportant, since they control some of the precipitation and turbulence in
the flow patterns which are impressed on the ionosphere. Detailed study of
the associated magnetospheric physics is beyond the scope of the present text,
although we do refer in passing to some of the most important processes that
occur.

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