Satgen 511  Radiation Belts   by GM4IHJ  (BID SGEN511)   1999-01-09

Forty years ago, James Van Allens Explorer satellite gave the first
warning that approximately 1200 kms or so above our heads there was a
source of dangerous radiation. Trapped by the earths magnetic field in
a zone bounded by 1200 and 41000 kms altitude, and, latitudes 60 north
and 60 south, high levels of radiation produced two "belts" where
humans and satellite electronics could suffer serious damage. Since
then books on Space and Satellites have featured illustrations of the
contours of two sharply defined permanent radiation belts separated by
a small altitude gap.

The first clue that this was an over simplification, came in the 1980s
, when satellites over Brazil and the South Atlantic encountered high
levels of radiation, at altitudes far below the anticipated lower
limit of the Van Allen belts. At which point it became clear that the
belts of radiation were displaced with respect to the earth because
the earths magnetic field was displaced with respect to the earths
rotation axis. A situation which called for a rapid redesign of a
number of satellites planned for what had previously been assumed to
be radiation benign altitudes ,where their increasingly numerous
complements of micro chips would be subject to an otherwise
overwhelming number of "soft" radiation hits.

But it was not until the 1990s that an even greater change in the
description of the belts began to become necessary, as the anticipated
theoretical effects of the radiation belts on satellites which had to
go to high altitude , was seen to be markedly different at times from
the actual practical results. Whereby the satellites concerned were
simply not experiencing the levels of radiation damaged that had been
predicted.

The first real test of this situation featured two small British
satellites STRVI a and b ( 1994 34B and 34C). Launched into
geostationary transfer orbits these satellites went from a perigee low
altitudes of about 282 kms up to apogee highs of 35822 kms approx,
every 634 minutes. Passing right through the belts nearly twice each
day they revealed that the radiation levels were not consistent day to
day, and the belt contours were highly variable.

Since then data from 2 dozen satellites has shown that these effects
are not due to the Solar storms ( which provide the actual particles
themselves). Rather it is the solar storms which push and squeeze the
earths magnetic field, pumping up the trapped particle energies. Such
that energy levels can vary by a factor of one thousand over a period
of a few minutes. In addition , while the two main belts of radiation
are densest between 7000 and 41000 kms altitude, there is a gap
between them which comes and goes. Where at times, a third belt
occupies the gap. An extra belt which arises as a consequence of large
infrequent magnetic storms , and which can survive for several weeks.

So the problem now is to get more real time measurements in order to
test out theoretical models. So that perhaps we can restore some order
to what we once thought, we thoroughly understood. But clearly did
not.