Fundamentals of Atmospheric, Ionospheric, and Magnetospheric Plasma Dynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 69

Information is transmitted from one region to the other in the solar wind-
magnetosphere-ionosphere system by waves of various types. For example, the
solar wind generates Alfvén waves, which travel down magnetic field lines carry-
ing energy and momentum. This system can be described as a transmission line
with the ionosphere as the load. If the load impedance (the field line-integrated
Pedersen conductivity ( η A ),

reflection will occur, but after a few bounce times this electrodynamic descrip-
tion may be replaced by what is termed electroquasistatics. Most of ionospheric
physics is carried out in this context.
Vasyliunas and Song (2005) have argued that the use of quasistatics hides
the fundamental physics and that electric fields cannot be thought of as act-
ing on the plasma. Rather, only the plasma velocity matters, and the electric
field is an illusion of one's frame of reference. In this approach, the waves put
the ionosphere in motion and the electric field follows. However, decades of
successful application of electroquasistatics in the ionosphere should not be
replaced when it is applicable. This approach is valid on time scales longer
than the scale of the system divided by the Alfvén speed or equivalently, for
the Alfvén wavelength larger than the system scale. We argue that if you live
on a resistor—for example, the ionosphere—then the electric field and Joule
heating are very real and can be used as a proper language. In this text we
use electroquasistatics whenever possible. At times we do use the full electrody-
namic approach, such as for the solar wind-generated Poynting flux input to the
polar cap.

P )) does not match the transmission line impedance (

2.7 Coordinate Systems

In standard meteorological practice, a local coordinate system has the x -axis
eastward, the y -axis to the north, and the z -axis vertically upward. The three
components of the neutral wind vector U are usually denoted by ( u , v , w )in
those coordinates. We will use this notation here as well.
Some complication arises in ionospheric plasma studies due to the importance
of the magnetic field direction and the fact that it varies from horizontal at the
magnetic equator to vertical at the poles. The reader should be alert to this and
to the fact that the magnetic coordinate systems vary somewhat in the text. For
example, in Chapters 3 and 4 we use a coordinate system in which the magnetic
field is in the direction of the y -axis. The conductivity tensor does not, then, have
the form given in (2.38). In deriving (2.38) and in Chapters 5-10 we have taken
B parallel to the z -axis. The reader should be aware of the possible confusion
caused by the use of different coordinate systems. Furthermore, in the Northern
Hemisphere the “ z -axis” associated with meteorology is nearly antiparallel to
the magnetic field-aligned z -axis. In addition, researchers sometimes define a

z

ˆ

axis antiparallel to B in the Northern Hemisphere.