This current flows at a distance of more than 10 Earth radii (RE) from the Earth's surface and is
the outermost boundary of the terrestrial environment. Here ionised particles flowing
from the Sun - the solar wind - encounter the Earth's magnetic field and are deflected
by it. Ions are deflected one way, electrons the other. As a result, a current flows. The current
is such as to confine the Earth's magnetic field within the current sheet boundary in a region known
as the magnetosphere. The magnetospheric magnetic field inside the boundary exerts a
pressure on the boundary which must be in balance with that exerted by the solar wind plasma outside.
This requirement determines the equilibrium shape of the boundary - roughly an ellipsoidal shell
within about 30 RE of the Earth. The magnetopause surface is shown in the figure. Also shown are
the current streamlines within it.
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