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The Question
(Submitted February 28, 1997)
I wonder if you could tell me exactly what the VAN ALLEN BELT is and how
much radiation does it contain, ie how many rems of radiation are there out
there? Plus, what protection would organic life need to be protected from
this radiation?
The Answer
David Stern, a researcher in another lab here at Goddard, has graciously
supplied an answer to your question, given below:
"The radiation belts are regions of high-energy particles, mainly
protons and electrons, held captive by the magnetic influence of the
Earth. They have two main sources. A small but very intense
"inner belt"
(some call it "The Van Allen Belt" because it was discovered
in 1958 by James Van Allen of the University of Iowa) is trapped within
4000 miles or or so of the Earth's surface. It consists mainly a high-energy
protons (10-50 MeV) and is a by-product of the cosmic radiation, a thin drizzle
of very fast protons and nuclei which apparently fill all our galaxy.
" In addition there exist electrons and protons (and also oxygen particles
from the upper atmosphere) given moderate energies (say 1-100 keV; 1 MeV
= 1000 keV) by processes inside the domain of the Earth's magnetic field.
Some of these electrons produce the polar aurora ("northern lights")
when they hit the upper atmosphere, but many get trapped, and among those,
protons and positive particles have most of the energy .
"I looked up a typical satellite passing the radiation belts (elliptic
orbit, 200 miles to 20000 miles) and the radiation dosage per year is
about 2500 rem, assuming one is shielded by 1 gr/cm-square of aluminum
(about 1/8" thick plate) almost all of it while passing the inner belt.
But there is no danger. The way the particles move in the magnetic field prevents
them from hitting the atmosphere, and even if they are scattered so
their orbit does intersect the ground, the atmosphere absorbs them long
before they get very far. Even the space station would be safe, because
the orbits usually stop above it--any particles dipping deeper down
are lost much faster than they can be replenished.
"If all this sounds too technical but you still want to find out--
what ions and magnetic fields and cosmic rays are, etc.--you will find
a long detailed exposition (both without math) on the World Wide Web
at:
http://www.phy6.org/Education/Intro.html
Good luck!
David Stern
Note:
Another point of particular interest to us in high-energy astrophysics is the
South Atlantic Anomaly. This is a region of very high particle flux about
250 km above the Atlantic Ocean off the coast of Brazil and is a result of
the fact that the Earth's rotational and magnetic axes are not aligned (see
http://www.oulu.fi/~spaceweb/textbook/radbelts.html). The particle flux is
so high in this region that often the detectors on our satellites must be
shut off (or at least placed in a "safe" mode) to protect them from the
radiation.
Andy Ptak
for Ask an Astrophysicist
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