Fundamentals of Atmospheric, Ionospheric, and Magnetospheric Plasma Dynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 27

2 Fundamentals of Atmospheric,

Ionospheric, and Magnetospheric
Plasma Dynamics

In this chapter we model the ionospheric plasma as three interpenetrating
fluids, with the electron and ion fluids immersed in the neutral gas. At all heights
of interest in the study of ionospheric phenomena, the neutral gas density exceeds
that of the plasma. In fact, the plasma density does not become comparable to that
of the neutrals until several thousand kilometers in altitude. The primary difference
between ionospheric plasma dynamics and thermospheric neutral gas dynamics is
the effect of electromagnetic forces. The various forces acting on charged parti-
cles drive electric currents that in turn create electric fields that modify the plasma
dynamics. The electrical conductivity of the medium is thus extremely important
and is derived in this chapter. We discuss briefly the generation of electric fields
in the ionosphere and the transmission of electric fields along magnetic field lines
between the ionosphere and the magnetosphere. At middle and low latitudes, the
electric field is generated primarily by the neutral wind field. In later chapters electric
fields impressed on the ionosphere by solar wind and magnetospheric processes
will be taken into account, as will their occasional penetration into the middle-
and low-latitude sectors. In the analysis that follows, we first obtain the equations
for a neutral fluid and then extend them to ionized gases. Finally, we develop the
equations needed to describe collisionless plasmas in the absence of a neutral fluid,
which is the appropriate approximation for the magnetosphere. Although not the
primary topic of this text, such a development is necessary, since we must be able
to describe certain key magnetospheric phenomena in some detail.

2.1 The Basic Fluid Equations

The ions, electrons, and neutrals can be considered as three interpenetrating flu-
ids coupled by collisions and, in the case of ions and electrons, their self-generated
electric and magnetic fields. The gas of ions and electrons, taken together, is often
referred to as a plasma in the text. As usual in a fluid description, we assume
that an element small enough to be treated as a differential volume in the math-
ematical sense still contains a sufficient number of atoms, ions, electrons, and/or