Satgen495  Magnetism Pt2  by GM4IHJ    (BID SGEN495)    1998-09-19

The magnetic field of the earth gives us excellent protection from
most charged particle bombardment coming in from space. Without it we
would probably not survive for long. Bombardment by the Solar wind can
however, sometimes, penetrate the magnetic field for some considerable
distance - PROVIDED - that the polarity of the incoming solar wind
magnetic polarity is opposite to that of the earth. By contrast , same
polarity solar wind is repelled. Once inside the outer sections of the
magnetic field , opposite polarity solar wind can take the easy route
down between the magnetic field lines to the earths polar regions ,
causing aurora. Noting that charged particle are impeded when they try
to cross field lines , but they can move freely between/parallel to
field lines.

Unfortunately Geosats orbiting 36000 kms sunwards of the earth are not
even safe if the solar wind polarity is the same as the earths. The
force of the solar wind can compress the earths magnetic field
inwards, pushing it in past the sunside geosats, which then have to
withstand its full fury. A situation which can cause geosat failure in
some circumstances.

When the solar wind polarity is opposite to that of the earth , and it
penetrates the magnetic field protecting the earth, it has not as yet
been known to cause any low earth orbit satellites sudden failure.
But the production of aurora over sub polar regions can temporarily
affect satellite performance. Whereby satellite on the same side of
the aurora as your station, can get multiple signals, one clear tone
direct, the others at varying doppler shifts reflected or scattered by
the aurora back to your station. When the satellite is the other side
of the aurora from your station , severe signal scintillation and
doppler broadening of bandwidth may occur if the signal path to you
transits the auroral zone. Auroral disturbance rarely rises up to
satellite height ( above 500kms or more ), but the auroral curtain at
heights between 80 and 200 kms often intercepts the satellite signal
path. Even geostationary satellite signals at VHF and low UHF can be
reflected / refracted / scattered by aurora back to your station, but
the doppler shift on the auroral signals usually places it well clear
of the clean direct signal.

On its own behalf , without the aid of the solar wind , the earths
magnetic field is central to another form of VHF propagation -FAI
Field Aligned Irregularities. FAIs are collections of charged
particles, sometimes, but not always connected with the beginings or
ends of Sporadic E events. The particles are constrained temporarily
in thin sheets between the earths magnetic field lines. Eg Over the
equator the field lines are horizontal , so the FAI regions are
horizontal acting as trans equatorial message propagators/conductors.
But away from the equator the magnetic field lines are say (over
Scotland) dipping down into the earth at about 57 degrees , so FAI
patches over Scotland slope at 57 degrees - to point almost directly
towards the true magnetic pole's location in the earths core, where
the earths self sustaining magnetic dynamo, is located.