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A magnetosphere is the volume of space around an
astronomical object that is controlled by that object's
magnetic field. The Earth's magnetosphere
is the cavity
formed by the Earth's magnetic field in the flow of
plasma
from the Sun known as the
solar wind. The interaction with
the solar wind
deforms the Earth's basically dipolar
magnetic field, compressing the field lines on the day side
and stretching them out to form a long comet-like tail (the
magnetotail) on the night side. On the day side, the
magnetosphere extends out to a distance of approximately
10
Earth radii (under quiet conditions), while the
magnetotail extends several hundred Earth radii in the
antisunward direction. The magnetosphere contains various
large-scale regions, which vary in terms of the composition,
energies, and densities of the plasmas that occupy them. The
sources of the plasmas that populate these regions are the
solar wind and the Earth's
ionosphere; the relative
contributions of these two sources to the magnetospheric
plasma vary according to the level of geomagnetic activity.
(See the glossary entry on the DST index,
which is one of the measures of geomagnetic activity.)
The Earth's magnetosphere is a highly dynamic structure
that responds quite dramatically to changes in the dynamic
pressure of the solar wind and the orientation of the
interplanetary magnetic field (IMF). Its ultimate source of
energy is the interaction with the solar wind. Some of the
energy extracted from this interaction goes directly into
driving various magnetospheric processes, while some is
stored in the magnetotail, to be released later in
substorms.
The principal means by which energy is transferred from the
solar wind to the magnetosphere is a process known as
"reconnection," which occurs when the IMF is oriented
antiparallel to the orientation of the Earth's field lines. This
orientation allows interplanetary and geomagnetic field lines
to merge, resulting in the transfer of energy, mass, and
momentum from the solar wind to the magnetosphere. The
viscous interaction of the solar wind and the magnetosphere
also plays a role in solar wind/magnetosphere coupling, but
is of secondary importance compared with reconnection.
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