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| Ring current
shown around the Earth demonstrated by images from
IMAGE/HENA instruments. The lines show the magnetic
field as viewed from nearly above the North Pole. |
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The environment of Earth in space is a complicated
stew of energetic particles such as protons and electrons. Space
scientists have discovered, however, that it is possible to
discern three distinct ingredients to this stew. Like salt and
pepper mixed together, each system of particles has its own
unique properties. The densities of these components are so
low that the particles only occasionally collide with one another
as they travel around Earth at speeds of thousands of kilometers
per hour or more. One of these components, the Van Allen belts,
is a well-known landmark of near-Earth space discovered at the
dawn of the Space Age.
Van Allen Belts
Encircling the equatorial region
of the Earth, and extending to latitudes nearly as far as
the Arctic Circle, the Van Allen belts form two, nested donuts
of particles. The inner belt contains 10 million-volt, high-energy
protons (the stripped nuclei of hydrogen atoms), and is located
between 700 kilometers and 12,000 kilometers from the Earth's
surface. The outer belt contains mostly electrons with energies
higher than one million volts, located between 25,000 and
40,000 kilometers from the Earth’s surface. Both belts
are severe radiation hazards for astronauts, which is why
the orbits of the International Space Station and typical
Space Shuttle missions are well below the inner edges of these
belts. The Van Allen belts are always present in space, like
fixed coral reefs in the ocean, which must be avoided at all
times.
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| The
location of the inner and outer Van Allen radiation
belts.
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The Plasmasphere
A second population of particles,
the plasmasphere, consists of low-energy particles with energies
of a few tens of volts. Unlike the Space Shuttle which orbits
Earth once every 90 minutes under the influence of gravity,
plasmasphere particles are not under the influence of gravity
at all. Earth's magnetic field is so strong that the plasmasphere
particles are actually pulled along with Earth's 24-hour rotation
at a much slower speed than the Space Shuttle. Like the Van
Allen belts, the plasmasphere is a permanent feature of Earth's
environment, though it is not a health hazard for astronauts.
The Ring Current
The third component of near-Earth
space is the ring current which overlaps both the Van Allen
belts and the plasmasphere from 8,000 to 30,000 kilometers
from Earth. Ring current particles carry energies of several
thousand volts, but unlike Saturn's rings, they do not flow
in a complete ring around Earth. It is more prominent on the
night time side of Earth, and it is at its strongest, carrying
the most particles, during severe solar storm events which
shake Earth's magnetosphere. This current of particles carries
enough energy to actually change the strength of Earth's magnetic
field even at ground level, especially in the equatorial regions.
Although Van Allen belt particles are probably captured from
the solar wind, and plasmasphere particles probably come from
the upper atmosphere and ionosphere, the origin of the ring
current particles is something of a scientific mystery. Scientists
don't fully understand where they come from, or how they get
energized to such high voltages within the magnetosphere.
Find out more about the Sun-Earth Connection at the Sun-Earth
Connection Education Forum Web site.
Text adapted from the
Sun-Earth Connection Tutorial courtesy of NASA, originally
written by Dr. Sten Odenwald. Images and videos courtesy of
NASA unless otherwised noted.
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