The Earth's Magnetosphere
Earth’s magnetic field has two regions of very energetic
charged particles that encircle the Earth like donuts. These
regions are known as Van Allen radiation belts, named for
the scientist who discovered them. The main constituent of
the inner belt, which extends from an altitude of 2,000 to
5,000 kilometers above the Earth, is high-energy protons (H+),
produced when cosmic ray ions collide with atoms of the atmosphere.
The number of such ions is relatively small, and accumulates
slowly in the inner belt. Because they are trapped in a stable
region near the Earth, rather high intensities are reached,
even though their build-up may take years.
Further out, at an altitude of 15,000 km there is a region
about 6,000 kilometers thick populated mostly by ions and
electrons of much lower energy, produced by cosmic rays and
magnetsopheric acceleration processes. The most energetic
among these are also known as the "outer radiation belt."
Included in this mixture are "alpha particles"
(He2+), which are found in the solar wind, and
a small percentage of O+ ions. The most energetic
of these ions, most of which are electrons, are also known
as te "outer radiation belt." Unlike the inner belt,
this population fluctuates widely.
The Earth's magnetic field has two poles, north and south,
but the fields' pull extends far beyond the surface of the
Earth. An average of 50 tons of plasma per day in this field
flows against the gravitational pull of the Earth, much as
the solar wind flows away from the sun. This plasma contains
hydrogen, helium, oxygen and nitrogen atoms and ions.
Earth's magnetosphere and the solar wind do not interact smoothly.
When the solar wind plasma flows past the Earth, it has difficulty
penetrating the planet's magnetic field, leading to the creation
of a huge bow-shaped shock wave. The bow shock, which marks
the limit of the Earth's magnetic influence, occurs where
the velocity of solar wind particles decreases from supersonic
to subsonic speeds.
The solar wind plasma comes closest to the center of the
Earth at the stagnation point, where the planet's magnetic
field repulsive force equals the solar wind's pressure. The
position of the stagnation point depends on the solar-wind
pressure, but it is usually about 64,000 kilometers (10 times
the Earth's radius) out from the side of the planet facing
For a more technical description, take a Closer
The Structured Sun and Solar Max: At the Core of the Matter.