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Northern lights, mostly called aurora, are the fluttering
lights that can be observed in the sky over the northern-most
parts of the world. A similar light is located over the southern-most
parts of the world; it is called southern lights. Together
the northern- and southern lights are called polar lights.
The Latin word for northern lights, Aurora Borealis,
was first used in the early seventeenth century. Aurora was
the goddess of dawn, i.e. red light of dawn, in ancient Roman
mythology and Borealis means northerly. A translation would
be the northerly light of dawn. The Latin word for southern
lights is Aurora Australis, the southerly light of
dawn. Also the Lappish word for auroras, guovsahas, is related
to the light of dawn.
Beliefs in ancient times
Before people knew what they know today they tried, in
their own way, to explain why there were auroras. People knew
that there were auroras but they did not know why. At that
time people were very superstitious so their explanations
sometimes became very imaginative.
Three old Nordic explanations are mentioned in the book "Kongespeilet"
from the thirteenth century. At that time people thought the
Earth was flat and surrounded by oceans. One explanation was
that the oceans were surrounded by fire and that auroras were
the light from those fires, reflected in the sky. One other
possibility was that the sun threw its beams high in the sky
although the sun it self was located beneath the edge of the
earth plate. A third possibility was that glaciers could absorb
so much power that they began to shine.
One of the ancient Swedish names for aurora is sillblixt
(herring flash). The name comes from people who thought that
the aurora was a reflection of large herring shoals in the
ocean. This is preserved in documents from Närke and
different parts of Norway.
The natives, for example Indians and Lapplanders, that lives
in the aurora zones today think that the aurora is something
to be respected. This opinion is still active in our century.
A lot of elderly people living in the north of Sweden can
remember as children being told to act nice and silent when
there were auroras in the sky. To misbehave at that time was
very serious.
For the Lapplanders, as for other people in northern Europe,
Asia and America the aurora was a place for the dead. Above
all it was people who had died a violent or too early death
who came to live in the aurora. It could be people who were
murdered, killed in war, took their own life, died in child
birth or unborn children.
The Lapplanders thought that auroras and the weather were
connected. When the aurora was flaming high in the sky the
weather should be warm. By magic influence of the aurora they
thought it was possible to also influence the weather. This
could be done in many different ways. In Kvikkjokk they called
out a chant which started "gokseth (aurora) lipi, lipi".
Lipi is short for lihphuit that means flutter. From Vilhelmina
it is told that you could make the aurora flutter by waving
a white sheet.
All people did not think that a fluttering aurora ment warm
weather, some thought that it was getting cold others that
there was a storm comming. Most people did belive that a fluttering
aurora ment changes in the weather though.
Why are there auroras?
The Earth is surrounded by a thin gas cover, the atmosphere,
and in the space above fast charged particles, plasma, are
moving. Auroras arise when some of those particles enter the
Earth's atmosphere and collide with atoms and molecules. When
the particles collide the energy used to give them their velocity
changes into a light, the aurora.
The particles that make auroras come from the ionosphere
but have got an extremely high velocity due to the energy
from the solar wind.The particles are caught by the Earth's
magnetic field and are steered towards the poles. When a particle
reaches the atmosphere it collides with one of the many present
atoms.
When the particle collides with an atom the atom takes over
some of the energy thet has given the particle its velocity.
The particle keeps on moving but with less velocity, since
it has lost some energy to the atom. The particle soon collides
with a different atom.
The atom that has taken over some energy from the particle
has now got too much energy and lets go of it. The surplus
energy becomes light. The next atom that collides with the
particle also takes over some of its kinetic energy, resulting
in the particle losing even more velocity. The new atom also
lets go of the energy. As the particle moves down through
the atmosphere the atoms become more and more crowded, resulting
in more collisions for the particle. Each time the particle
collides it moves a bit slower and more light is emitted.
When the particle has collided a number of times it has lost
so much of its kinetic energy that it stops moving. This occurs
when the particle is approximately 100 kilometres from the
Earth's surface. When a lot of particles collide with atoms,
releasing light, an aurora occurs.
For auroras to arise on a planet five things are required.
First of all the planet has to have an atmosphere. The atmosphere
is the screen upon which the aurora is shown. If there was
no atmosphere the particles from space would find no atoms
to collide with and no light would be visible.
Second, there must be charged particles, plasma, that can
collide with the atmosphere. If there were no particles the
situation would be the same as above.
Third, something that can steer the plasma particles down
to the atmosphere is needed. That is a magnetic field. If
the magnetic field was not present most of the particles would
miss the earth and keep moving through space. Fourth, an energy
source that can give the plasma particles all the energy they
need to create auroras is required. On Earth that energy source
is the Sun. If the particles were not provided with all the
necessary energy auroras would not occur.
Lastly, something to carry the energy from the Sun to the
particles is needed. This is the solar wind.
Tips for studying auroras
- It has to be dark and fairly calm weather.
- It is very important to look often. The most intense part
of an aurora often lasts only between 10- 30 minutes.
- It can sometimes be hard to see the difference between
a faint aurora and a cloud. If it is an aurora you can see
stars through it. Auroras are often greenish and change
shape in a different way than a cloud.
- In Scandinavia the most active auroras are often seen
before midnight. The aurora can be photographed by an ordinary
camera, but a fast film is recommended (for example 400
ASA).
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