Introductory and Background Material - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 20

hold in a tenuous, magnetized plasma, where E is the electric field in the plasma,
V is its flow velocity, and the subscript indicates the component perpendicular

to B (Chen, 1984). These two expressions are, in fact, equivalent, since taking
the cross product of (1.2b) with B and dividing by B 2 yields (1.2a). For our
present purposes we accept these expressions as accurate relationships between
E

⊥

. In the direction parallel to B , charged particles move very freely.
This means that magnetic field lines usually act like perfect electrical conduc-
tors, transmitting perpendicular electric fields and voltages across vast distances
with no change in the potential in the direction parallel to B . Thus, any flowing
magnetized plasma can act as a source of voltage if there is a component of V
perpendicular to B .
These electrical properties become very important factors in the interaction
between the solar wind and the magnetosphere, once we include the interplane-
tarymagnetic field in our solar windmodel. Schematic views of the solar magnetic
field are given in Figs. 1.14 and 1.15. As the solar wind expands, the magnetic
field is stretched into a disklike geometry that has flutes much like a ballerina's
skirt. Consideration of the Maxwell equation

and V

⊥ ⊥

∇× B = μ 0 J

shows that a geometry with adjacent magnetic fields that are antiparallel must
have a current sheet separating them (indicated by the crosshatched surface in
Fig. 1.14). The entire pattern rotates with the 27-day rotation period of the
sun. As the pattern rotates past the earth, the position of the current sheet is
alternately above and below the ecliptic plane. As illustrated in Fig. 1.15, the
interplanetary magnetic field (IMF) can be considered approximately as a series
of spirals emanating from the sun. Near the earth the magnetic field in the spiral
is directed toward or away from the sun depending on whether the current sheet
is above or below the plane. In addition, the inclination of the current sheet with
respect to the ecliptic plane produces a northward or southward component
of the IMF relative to an axis normal to the plane. Considerable small-scale
turbulence structure exists in both the solar wind velocity and magnetic fields,
and the interplanetary medium is also greatly affected by shock waves and solar
flares. Taken together, these properties of the solar wind result in a highly variable
buffeting of the earth's magnetosphere.
It is customary to specify the solar wind parameters in terms of three mutually
perpendicular components with respect to fixed axes. In these geocentric solar
ecliptic (GSE) coordinates, the x -axis points from the center of the earth to the
sun, while the z -axis is positive parallel to the earth's spin axis (to the north)
and is perpendicular to the ecliptic plane. The y -axis makes the third mutually
perpendicular right-handed system. Figure 1.15 shows the situation looking in
the negative z direction down onto the ecliptic plane. The interplanetarymagnetic
field is stretched out by the radially flowing solar wind but remains anchored to
the rotating sun. The resulting geometry is similar to that of the water stream