Satgen 497  Magnetism Pt4  by GM4IHJ   (BID SGEN497)  1998-10-03

Satgen496 described the way radiation belts are present where the
earths magnetic field "bottles up" radiation overhead equatorial and
temperate regions. Because of the misalignment of the field relative
to the earths rotation axis, radiation over the South Atlantic near
Brazil is much more of a problem than elsewhere.

Just how serious this is, was revealed when the Uosat series of
Amateur radio satellites were launched to orbits around 700kms mean
altitude, carrying large complements of semiconductors and microchips
to support their the experimental "store and forward" message
capability. The computers aboard the satellites carried  hardware and
software to correct errors in memory caused by radiation
induced flips of single bits of data. Soon after launch , it became
clear that the error correcting system had to be turned up to its
maximum design speed to ensure that the memory was WASHED/REFRESHED
and ERROR CORRECTED fast enough to keep the system viable .

The culprit was the South Atlantic Radiation Anomaly - the name given
to the abnormally low radiation belt alignment over and near Brazil.
In this situation satellites in 700 km altitude orbits clock up a
radiation dose of about 1 rad per day. Almost all of which is due to
their 5 or 6 orbits which take them through the anomaly. Humans die if
they acquire a cumulative dose of somewhere between 350 and 600 rads.
Hence the much lower orbit of Mir, never going above 400 kms with its
human cargo. Most commercial semiconductor junctions made today can
survive about 10k rad before expiring - a level reached by a satellite
orbiting at around 1300 kms in about 5 or 6 years.

So contrary to, much common knowledge ? The low earth orbit is not a
benign , safe, place. Indeed it is actually safer to be out in the
geostationary orbit far beyond the Van Allen Radiation Belts. Where,
geosats orbit inside the protection of the earths magnetic field for
most of the time except when strong solar storms throw out a
powerfully augmented solar wind which temporarily pushes in the
magnetic field exposing geosats on the sunward side of the earth to
the solar bombardment. Fortunately this solar wind is mostly light
weight particles - protons and electrons unlike some of the cosmic ray
induced heavier particles which can be trapped in the radiation belts

Other planets generally have magnetic fields commensurate with their
size. Planets smaller than Earth are not big enough to support a
central dynamo in a molten core . So their magnetism is usually only a
weak relic of the general field in which they cooled at their birth (
known as Remanent Magnetism).So these planets have no radiation belts
, but they are poorly protected from cosmic radiation circa 50
rad/year. By contrast mighty Jupiter has a massive magnetic field and
deadly radiation belts. Belts which are made worse by the output from
Io , Jupiters highly volcanic inner moon. Io's constant output of
sulphur forms a giant radiation torus around its orbit . Entering the
radiation zones around Jupiter/Io is not recommemnded.