Satgen496  Magnetism Pt3   by GM4IHJ   (BID SGEN496)  1998-09-26

In the early days of satellites, the Russians launched a probe, which
went much further out in space than previous primitive orbiters. The
probes telemetry was monitored by the Jodrell Bank Observatory in
England. But the Russians did not release the telemetry decode. So the
records of what would have proved quite sensational at the time were
never reported. Meanwhile the Americans began to get into the act,
with James Van Allen's Explorer team leading the way. Their first
flight also went quite high and they immediately saw the surprise
readings in the telemetry whereby the level of radiation hitting the
probe got greater and greater and then suddenly stopped. Which could
only mean one of two things. Either the radiation counter had busted ,
or, it had gone off scale high. The Americans went for the second
possibility and the world was soon informed that far above the
equatorial and temperate latitudes of the earth , the magnetic field
traps incoming radiation in what are best described as magnetic
bottles, regions where the level of trapped radiation is very high.

Further Explorer satellites soon confirmed this picture. The earth is
surrounded by inner and  outer, torus (doughnut shaped) belts of
trapped radiation. Radiation levels in the lower belt begin to get
extreme above an average altitude of 1200 kms with peak radiation
levels occuring around 2500 kms altitude. While the outer belt
radiation peaks around an average altitude of about 16000 kms then
graduallly decreases as you get out to 20000 kms.  These Van Allen
Belts are constrained by the downward curving magnetic field lines
either side of the equator. Above latitude 60 north or south however
the field lines are open to space so there is no bottled radiation 
above the poles or sub polar regions.

Human beings transitting through the Van Allen belts in space vehicles
must do so quickly . They cannot linger there. Nor indeed can
electronics in space crafts remain in the belts for long, before their
electronic chips and solar panels begin to deteriorate because of the
high number of radiation hits suffered. So, although early satellites
( Oscar 6, 7, RS 1 to 8 ) all orbited above 1500 kms) the lesson that
this was dangerous was soon brought home to Amsat groups and since
then launches have rarely exceeded 1200 kms , and in the case of the
electronically susceptible packsats, never exceeded 800 kms. Whilst
manned orbiters have all stayed below 450 kms.

These low altitudes have not been set to give least possible average 
damage. They have been dictated by the fact that the altitudes quoted
above for the radiation belts,are average figures. Average because the
magnetic axis of the earth is 11 degrees away from the rotation axis ,
and the centre of the earths magnetic fiels is not the centre of the
earth . It is 600 kms nearer Singapore than it is Brazil. A feature
revealed when satellites began to encounter the South Atlantic
Radiation anomally whereby Uosats overhead near Brazil were suffering
high levels of soft radiation hits even though they were below 700 kms
altitude - as a result of the magnetic field being displaced with
respect to the earth and hence, the radiation belts being much lower
near Brazil than any other part of the world.