The
Earth's Magnetic Field
The
Earth's
field lines are similar to those of a
simple
bar magnet.
The Earth's magnetic north is not at the 'true' north.
The Earth rotates around an imaginary line that joins the 'true north'
and 'true south' poles that we call the Earth's Axis or the 'Geographical
Meridian'.
Magnetic Declination
The line joining the magnetic north and magnetic south
is inclined at an angle to the true axis line.
This makes precise navigation
by the magnetic compass a complicated matter. The axis line for the
geographical true north/south is called the geographical meridian
and the axis line joining the magnetic north/south is called the magnetic
meridian.
The angle between them is called the magnetic declination.
Magnetic declination varies both from place to place and with the passage of time. As a traveller cruises the east coast of the United States, for example, the declination varies from 16o west in Maine, to 6o in Florida, to 0o in Louisiana, to 4o east (in Texas). The declination at London, UK is 1o 7' west (2014), and as the country is quite small that figure is fairly good for the whole of the country.
Magnetic declination is reducing, and it is predicted that in about 2050 it will be zero.
What causes the Earth's magnetic field?
The Earth has a substantial magnetic field that
is thought to be due to the movement of the charged particles in the
liquid core. . The origin of the Earth's magnetic field is not completely
understood by scientists, but is thought to be associated with electrical
currents produced by the spinning of the liquid metallic outer core
(made of iron and nickel) creating convection currents within this layer.
This mechanism is called the dynamo effect
The magnetic field of the Earth is not fixed. Rocks
formed from the molten state contain indicators of the magnetic field
at the time of their solidification. The study of such "magnetic fossils"
indicates that the Earth's magnetic field reverses itself every million
years or so (the north and south magnetic poles switch). Why this
should happen is a source of debate for scientists.
If a piece of magnetised iron is suspended freely (hung
from a fine thread or 'floated' on a pivot) it aligns itself with the
field lines from the Earth's core.
This played an important role in
the development of mankind's navigation skills. The stars could be used
to navigate by at night providing they were not hidden by clouds. Magnetic
compasses 'worked' regardless of the weather and therefore played an
important part in our exploration of planet.
The 'north' poles on these
little compasses sought out the magnetic north whether or not there
was cloud cover. They were called 'north poles'.
We know that the north
pole of a magnet seeks out the south of another so if we were to make
a model of the Earth with a magnet inserted inside it to simulate the
magnetic field we would have to position its south pole at the 'north' of the model!
The magnetic field protects the Earth
Magnetic fields exert forces on moving electrical charges.
So, the Earth's magnetic field can trap charged particles moving in
the atmosphere, making them spiral back and forth along the field lines.
The solar wind is a stream charged particles that are
emitted from our Sun. The Earth's magnetic field shields us from much
of the solar wind as it deflects many of the particles out into space
and traps some of them in the upper atmosphere.
It was discovered in
the late 1950s that the Earth is surrounded by two regions of particularly
high concentration of charged particles called the Van Allen radiation
belts.
It is believed that most of these particles come from our
Sun. These charged particles trapped in the Earth's magnetic field are
responsible for the aurora (Northern and Southern Lights) seen
in the sky around the poles.
Try these questions to test your understanding of this
passage: click here
To read news reports about the Earth's changing magnetic field click here.
LOJ (August 2001 - updated 2017)
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