The usual concern of space weather is the
varying electromagnetic energy and atomic particle flux throughout
the interplanetary medium mainly from the sun
to the Earth.
However, larger particles traverse this region and influence
our space environment in a number of ways.
When these particles enter the Earth's atmosphere
frictional forces heat the surface of the particle
to high temperatures, creating light and atmospheric ionisation.
We call this phenomenon a meteor and
the particle that produces it a meteoroid.
Both the meteor and the meteoroid contribute to
space weather.
The meteor may be used to support communication
at low VHF frequencies. This idea was
explored in the 1950's and is now used
for low cost data transmission. The data
is transmitted in bursts following detection of a
pilot carrier when a meteor occurs with the
correct geometry. Such communication is suitable for remote
areas, and because the path is very sensitive
to the aspect geometry it allows for relatively
secure or private data transmission. Meteor burst communication
is also particularly suited to high latitudes and
to very disturbed propagation environments (eg following detonation
of nuclear explosives in the atmosphere).
Although meteors can enable a particular mode of
communication, they may also cause interference to HF
radio systems. In particular, Doppler HF radar can
suffer jamming during times of intense meteor activity.
The meteors reflect back the transmitted signal but
spread it over a wide area of 'Doppler space'. Such
interference makes it difficult to
sort out the desired echoes due to aircraft
or other targets.
The two effects discussed above, one constructive and
one destructive, are both produced by the meteor
ionisation. However, it is
possible that the meteoroid itself may produce direct
damage to a satellite. This was a large source of concern
in the early days of space exploration, but the
hazard turned out to be less
severe than initially expected.
However, this threat may increase dramatically during
certain years. The Leonid meteor shower, which occurs
around Nov 17 each year, increases a thousand
fold every 33 years. In 1966 the normal
hourly rate of around 5, shot up to
10,000! If this occurs again in the next
year or so (1998/9), we might expect at
least one large communications satellite to suffer serious
damage.
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