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The Ring like Shape of the Aurorae
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| Click on the picture above. You can watch "Aurora created by electrons
pouring in and forming a ring" on the CG. |
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Laboratory Aurora Display Device
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| Seen from space, the aurorae appear as rings encircling the poles.
The ringlike form of the aurora can be reproduced on a Laboratory Aurora
Display Device at the Ginga-no-Mori Observatory in Rikubetsu-cho, Hokkaido.
In a model of the Earth 40 cm in diameter, a light purple aurora can be
seen floating in a ring around the pole. |
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Why Does the Aurora Appear as a Ring?
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The particles in the solar wind are magnetized on the surface of the sun,
by the sun's own magnetic field. The interaction between the particles'
magnetic field and the Earth's magnetic field ( the magnetosphere)
generates an electric charge. This charge accelerates particles around the
surface of the magnetosphere, causing them to flow along the field lines of
the magnetosphere and strike the upper atmosphere around the north and
south poles. The luminescent discharge from this collision forms the
aurora. The ring shape of the aurora is explained by the way in which the
magnetic field lines emanate from the Earth. As we explained in step 1,
these lines of force are compressed by the solar wind, creating open and
closed sections. The electrons that fall in along the boundary between
these sections form rings in the upper atmosphere as they fall toward the
poles, giving rise to the aurora's halo-like appearance.
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the magnetic field lines of a bar magnet
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the magnetic field lines of a ring magnet
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Here we see a bar magnet and a ring magnet. Don't the ring magnet's magnetic
field lines look the most like the Earth's, which are divided into two
zones by the solar wind?
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Outline of Laboratory Aurora Display Device
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| Let's try an experiment to see whether the magnetic field lines of a ring
magnet really do form a ring like the aurora.
This is the Laboratory Aurora Display Device. At the top of a vacuum tank,
a plug is attached containing a tapered electrode and an air outlet. A
powerful magnet is placed at the bottom of the tank, capped with a model of
the conductive hemisphere. Voltage is applied to the vacuum tank, and the
air is evacuated. We'll start the experiment using a ring magnet, then
switch to a bar magnet, and compare the forms of the aurorae generated over
the hemisphere. |
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Aurora formed when a ring magnet is used
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Aurora formed when a bar magnet is used
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When the ring magnet is used, a ring-shaped halo is formed, just like the
real aurora. With the bar magnet, however, no such ring appears. This is
because the Earth's magnetic force lines are not all closed as they are in
the bar magnet, but divide into open and closed sections as in the ring
magnet.
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