The Earth’s Magnetic Field - Ultra and Extremely Low Frequency Electromagnetic Fields - page 24

In summary, an important aspect of the interaction between the solar wind and the
Earth magnetic field follows from the fact that the solar wind plasma is frozen to the
interplanetary magnetic field whereas the terrestrial/ionospheric plasma is frozen to
the Earth's field. These plasmas form distinct regions separated by a thin boundary,
that is the magnetopause, and they practically do not mix.

1.2.3 Structure of the Earth Magnetosphere

Although the solar wind plasma is frozen to the large-scale interplanetary magnetic
field, the “frozen-in” picture can break down at the magnetopause boundary, where
high current densities occur in the plasma. As a result the interplanetary magnetic
field diffuses through the plasma in the magnetopause. In such a case the inter-
planetary and terrestrial field lines will connect through the dayside magnetopause,
asshowninFig. 1.9 a with line 3. Following Dungey ( 1961 ) this process has been
termed magnetic reconnection. The essential breakdown of frozen-in flow occurs
not only at the dayside magnetopause but also in the magnetotail that results in the
appearance of reconnection in that region. The concept of magnetic reconnection
is a relatively new phenomenon, which frequently occurs in the magnetospheres of
planets, stars, and other cosmic objects. The origin of this phenomenon has been the
subject of a great deal of research during recent years.
A cutaway of an actual Earth magnetosphere illustrating the solar wind/
magnetosphere interaction is shown in Fig. 1.9 b. The dipole magnetic field
dominates only in the inner magnetosphere in the region with radius about 3R e .
Across the magnetopause the magnetic field usually undergoes a sharp change in
both strength and direction. An energy and momentum transfer from the solar wind
into magnetosphere is basically due to reconnection and quasi-viscous interaction
between the magnetic field frozen in the solar wind and the Earth's magnetic field
(e.g., Mishin and Bazarzhapov 2002 ). The solar wind plasma is deflected at the
bow shock, flows along the magnetopause, pulling the terrestrial magnetic field
into a long magnetospheric tail/magnetotail on the night side thereby providing
the magnetotail with antiparallel magnetic fields in North and South parts of the
magnetotail. The magnetotail extends several hundred Earth radii in the antisunward
direction. The diameter of the tail is about 40R e .
The solar wind flows around the Earth into the magnetic tail and is then injected
back toward the Earth within the region called the plasma sheet, as shown in
Fig. 1.9 b. The plasma sheet is sandwiched between two bundles of magnetic field
lines. One bundle, to the north of the equator, consists of field lines that enter the
north magnetic pole, and the other bundle, to the south of the equator, contains field
lines directed out of the southern polar cap. There are open field lines that cross the
magnetopause and then connect interplanetary magnetic fields to the Earth's field.
The northern and southern polar cusps are narrow funnel-shaped regions of
recently “opened” or merged magnetic field lines connected with those of the
interplanetary magnetic field rather than the magnetosheath magnetic field. The hot