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8 High-Latitude Electrodynamics
In this chapter we study the macroscopic motion of the high-latitude ionospheric
plasma in the plane perpendicular to the magnetic field lines. Since the magnetic
field is nearly vertical, this corresponds to the horizontal motion of plasma. At the
large scales
considered here, the electric force in (2.36b) dominates the
pressure gradient and gravitational forces so only an imposed electric field and a
neutral wind need be considered in the perpendicular plasma motion. In order to
understand the characteristics and the sources of the imposed electric field, we first
deal briefly with the relationships between electric fields and currents that exist in
the ionosphere, outer magnetosphere, and solar wind. These fields and currents are
coupled along the earth's magnetic field. Following this, we discuss the observed
characteristics of electric fields and currents in the ionosphere and their relationships
to the magnetic field topology throughout the ionosphere, the magnetosphere, and
the solar wind system.
(>
100 km
)
8.1 Electrical Coupling Between the Ionosphere,
Magnetosphere, and Solar Wind
8.1.1 General Relationships
We begin by considering two regions: one in the ionosphere and one in the
magnetosphere. Below about 200 km the ionosphere is a resistive medium, and
in Chapter 2 we showed that the electric field E and electric current J are related
by the equation
J
= σ · (
E
+
U
×
B
)
(8.1)
where
is the ionospheric conductivity tensor and U and B are the neutral wind
velocity and magnetic field, respectively. In that chapter we also pointed out that
above about 2000 km, the magnetospheric plasma is essentially collisionless and
the equations governing E , J , and the plasma velocity V are
σ
E
+
V
×
B
=
0
(8.2)
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