An Introduction to the Aurora
The aurora is a beautiful, shimmering display
of light from the upper atmosphere that is commonly seen during the
night in polar regions of the Earth. The northern aurora is known as
the aurora borealis and its southern counterpart is known as the aurora
What causes the aurora? The sun emits a
continuous stream of charged particles called the solar wind. This wind
interacts with the magnetic field of the Earth and produces large
electrical currents. These currents flow along the magnetic field lines
down into the upper atmosphere surrounding the north and south magnetic
poles (see diagram). These currents cause the atmospheric gases to glow
like the gas in a fluorescent tube. A very large quantity of energy is
deposited in the upper atmosphere during an auroral display. For
example, about 1,000,000 kW, the equivalent to the power capacity of a
large power plant, is needed to power a medium-strong auroral light of
about 10 x 1,000 km. It should also be noted that only 1% of the energy
of the precipitating particles actually goes into the production of
visible light !
Where do aurora occur? Auroras occur
around the Earth's north and south geomagnetic poles in regions known
as auroral ovals. The auroral ovals are constantly in motion, expanding
towards the equator or contracting towards the pole, and constantly
changing in brightness. Occasionally the aurora may be seen as far
south as the south of England. However, this would be a rare event -
with maybe 1 or 2 displays every 10 years. This plot shows the typical
location of the auroral oval during conditions of moderate activity.
to see what the weather is like in space today.
The gases in the aurora glow with
characteristic colours. The emissions are the results of processes
where atoms or molecules are excited by collisions with precipitating
particles, returning to the ground state by emitting photons of a
characteristic frequency or set of frequencies. This photograph shows a
view of a green-rayed auroral arc. Green is the most common colour in
the aurora and is caused by atomic oxygen at altitudes of between about
100 and 200 km.
This photograph shows an auroral form
that is predominantly green and violet. This colour is due to molecular
This photograph shows several auroral
arcs that appear to sweep across the constellation of Orion, standing
majestically above the horizon.
This photograph shows a view of the aurora
with a red and a green component. The red colour, seen predominantly in
the left half of this frame, is due to atomic oxygen at altitudes of
about 250 km.
This view of the aurora australis was taken by
the Space Shuttle Discovery from low Earth orbit at an altitude of
about 250 km. The variation of the colour of the glow of the aurora
with height may be clearly seen with the red aurora occurring at
greater heights than the green aurora. (NASA photo).
This image shows a view of the entire auroral
oval taken by the Dynamics Explorer 1 satellite on 8 th November 1981
from high above the north polar region. This image was taken at an
altitude of approximately 20,000 km. Here the glowing oval of auroral
emission is about 4,500 km in diameter. Click here
for the latest visible images from the Visible Imaging System also on
Polar. (image courtesy of Louis A. Frank)
Does the aurora occur on other planets?
The answer is yes! The Earth is not the only planet in the solar
system on which auroras may be seen. Auroras may also be seen on the
gas giants, Jupiter, Saturn, Uranus and Neptune. These planets all have
strong magnetic fields and vast atmospheres.These images taken by the
Hubble Space Telescope, show the aurora at Jupiter. These auroras
happen for the same reason as on Earth. The gases in the upper
atmosphere are excited by charged particles pouring in near the north
and south magnetic poles. Most of the particles come from the solar
wind but some flow from Jupiter's nearest moon, Io, and are trapped by
the magnetic field. Click here
for the official press release. (NASA photo)
Good starting points for further information on the aurora are
available at www.geo.mtu.edu and www.exploratorium.edu and links therein.
8th November 2006
Yulia V. Bogdanova