Introductory and Background Material - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 18

Dawn Dusk

Ecliptic plane

o

Earth's magnetic
field (out of paper)

e 2 H 1

Solar wind ion-Electron pair

Solar wind

Figure 1.12 Schematic diagram showing deflection of solar wind particles by the earth's
magnetic field. The view is in the ecliptic plane.

approach the earth. Once deflected, they will spiral around the magnetic field
lines and drift around the earth. Since the magnetic field strength increases as the
plasma approaches the earth, a distance is eventually reached where the force
is sufficient to keep the particles from penetrating any farther, and they flow
around the obstacle. Notice that since ions are deflected toward dusk and elec-
trons toward dawn, a net duskward current exists in a thin sheet (extending also
out of the plane of the diagram) where the force balance occurs. The secondary
magnetic field generated by this current sheet is parallel to the earth's field in the
region between the earth and the current sheet and is antiparallel to the earth's
magnetic field in the solar wind. This magnetic field cancels the earth's field on
the sunward side of the boundary and increases the value of the magnetic field
inside the current sheet. If this were the entire interaction, the resulting configu-
ration of flow and field would look something like the sketch in Fig. 1.13a. The
volume inside the elongated region is termed the magnetosphere—the region
dominated by the earth's magnetic field. Experiments show that the magneto-
sphere is indeed terminated by a very sharp boundary on the sunward side, which
is called the magnetopause, but is very much more elongated in the antisunward
direction than the sketch shows. (We return to this point later when we include
the interplanetary magnetic field in the discussion.) A configuration such as that
in Fig. 1.13a is called a closed magnetosphere, since there is almost no direct