WHAT IS AN AURORA?
An aurora is the name given to the light that is produced in the upper
atmosphere when electrons and protons precipitate from the Earth's
magnetosphere down into the lower regions of the upper atmosphere.
This precipitation typically takes place along a ring which encircles
the polar regions. Aurorae around the north pole are termed Aurora
Borealis or Northern Lights, and around the south pole are termed
Aurora Australis or Southern Lights.
Particles in the magnetosphere typically originate from the sun via the
solar wind. Some of these particles precipitate into the lower
atmosphere continuously, and the aurorae are thus normally present at
all times, although they may not always be visible (due to limited
intensity and the obscuring effect of daylight). At times of injection of
large numbers of particles from the solar wind (following solar
activity) the aurora become brighter and the ring region in which they
occur (termed the auroral oval) expands and moves closer to the
equator.
When these particles strike molecules of air at heights
from 70 to 600 km they produce various colours
of light that may be seen from the ground and
from space. The process is similar to what
happens inside a TV tube when electrons are
accelerated toward the screen. When they hit the
phosphor coating, coloured light is emitted.
Aurorae take many different forms which follow
the patterns and variations of the Earth's magnetic
field.
Green aurora over Hobart, Tasmania in August 2005
Image from Dallas and Beth Stott
This image taken from the Space
Shuttle shows the Aurora Australis glowing
above the Earth's surface below the star
constellation of Orion. [NASA image / STS-59]
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"A shocking prodigy which was seen from Kuttenberg
in the kingdom of Bohemia and independently in other towns and
places round about on the 12th of January, for four hours in the
night. As it stood within the clouds of the sky in this year 1570."
Crawford Library, Royal Observatory, Edinburgh |
Disturbance to High Frequency Communications
Any high frequency or shortwave signal that has to propagate via a
great circle over the polar regions is likely to be subject to disturbance
by the auroral plasma. This typically produces what is known as
auroral flutter. This is a rapid (several times per second)
fading of the signal that breaks it up and makes signal readibility very
difficult.
HF communications is often of greater importance in the polar regions because geosynchronous satellites are either very close to or below the horizon, rendering communication via such satellites very difficult or impossible. It is important to know the location of the auroral oval to be able to predict HF disturbances, and possibly suggest alternate signal routings. Aircraft flying from South Africa to Australia along a great circle path will often pass through the southern auroral region. As HF is often their only means of communication, it is important for them to know if and when they may experience communication outages, so that lack of communications is not interpreted as an aircraft problem.
Satellite Communications
The same effect that causes disturbances to HF signals is also
responsible for producing scintillations of UHF satellite signals that
trace a transionospheric path. Essentially a similar fading called
scintillation is produced on the signal if it passes through an active
auroral oval. The effect becomes more pronounced as the path through
the auroral region is increased, that is, as the elevation of the satellite
from the ground station becomes less. Knowledge of the diurnal and
geomagnetic variation of the oval can allow communications to be
planned for a time when scintillation effects are expected to be
minimised. This problem is of concern to Australian Antarctic bases.
Radar Interference
A radar looking towards the poles can be affected in two ways by an
active aurora. Emissions from the aurora act as a noise source to
reduce the sensitivity of the radar and its detection probability for
smaller objects. The aurora can also act at times as a radar reflector
to VHF signals. This can introduce false targets onto the radar display.
Both of these effects could be devasting to a radar operator who may
be expecting intercontinental ballistic missiles to be arriving along a
polar great circle route. Again, knowledge of auroral behaviour is
vital to prevent potentially catastrophic consequences. This problem
is of importance in the northern hemisphere where the most direct route
from Russia to North America is via the polar regions. However, in
the southern hemisphere scientists have established two radars, one in
Tasmamia and one in New Zealand to probe the southern auroral zone.
This project is named Tiger and is coordinated from Latrobe University,
ORIGIN
It all starts at the Sun. A solar wind streams out from the
Sun continuously, but occasionally huge clouds of
plasma, called Coronal Mass Ejections (CME) are flung
out in violent solar outbursts.
The Earth's magnetic field deflects most of the particles
away from the Earth. However, at the end of the field
lines, which are blown out behind the Earth by the solar
wind, particles can enter, and are accelerated back toward
the polar regions of the Earth. There they form the
auroral ovals, centred around the Earth's magnetic poles,
south and north.
The solar wind produces a continuous aurora, but this is usually
faint, and of course not always visible from the ground due to daylight
and weather. The aurora is usually brighter on the nightside of the
Earth where most of the particles are injected. If a CME ejected
from the Sun is travelling in the direction of the Earth, it will inject
large quantities of particles (mostly electrons and protons) into the
Earth's magnetosphere, producing aurorae much brighter than normal.
THE AURORA FROM SPACE
From the ground we can appreciate the beauty of the aurora. But
we really need to view it from space to get the big picture. When
we do so we see that the aurorae sit as crowns on the polar regions
of the Earth.
If you look closely at the right image above you can also see a
phenomenon that very few people are aware of - the equatorial
aurorae. These can be see as arcs emanating from the daylight
sector of the Earth. Don't however, expect to see them from the
ground with your eyes.
The image below left clearly shows that the auroral oval is centred on
the Earth's magnetic poles and not the geographic poles. The right
image shows the difference in the auroral oval between quiet and
active geomagnetic conditions.
All the above images in this section were taken from a very high
altitude by the Dynamics Explorer satellites with instrumentation from
and analysis by the University of Iowa under the
leadership of Lou Frank.
This altitude allows us to see the whole Earth, and the complete
auroral ovals.
However, there are some spacecraft, such as the Space Shuttle and
the International Space Station that are low enough so that they
occasionally fly through the aurora. Some of the photos taken
from the ISS have an ethereal or ghostly look, and it almost appears
as if one could reach out and touch the aurora.
AURORAL COLOURS
When particles precipitate down
from the magnetosphere into the auroral zone they encounter
ever increasing density of air molecules. Eventually they will strike
one of these molecules, transferring energy to it, and leaving it
in an "excited" state. It only remains in this state for a short while.
The excess energy is disposed of by emitting a particle or photon of
light. The wavelength, and hence the colour of the emitted light is
determined both by the type of air molecule, and the amount of energy
it is given in the collision.
Up to an altitude of about 100 km, the Earth's atmosphere is
homogeneous, consisting of about 20% oxygen molecules (two
atoms of oxygen joined together) and 80% nitrogen molecules
(also two atoms of nitrogen joined together). Above 100 km
oxygen molecules start to 'dissociate' and so we tend to find
increasing concentration of individual oxygen atoms. Even
higher, above about 500 km we start to find significant numbers
of hydrogen atoms (these come from the Sun rather than the Earth's
lower atmosphere).
Although most aurora are formed in the altitude range between
about 100 and 300 km, sometimes the precipitating particles
have energies high enough to
penetrate down to 70 km, and in some conditions very high
altitude aurorae can form up to around 600 km.
Near the auroral zones green is the most common colour seen in
auroral displays. Green aurora tend to occur at altitudes from
100 – 250 km by oxygen atoms emitting light at 557.7 nanometres.
Red aurora are less common and form around 200 – 500 km from
oxygen atoms emitting light at 630 nm. These are often the aurora
seen at mid-latitudes following large solar outbursts.
When particles are energetic enough to penetrate down to altitudes
below 100 km, blue aurora may be seen, and this is produced by
nitrogen molecules emitting light at 423.6 and 427.8 nm. Converse
to this in altitude, a rare high altitude form of red aurora around
600 km is due to hydrogen atoms emitting light at the hydrogen-alpha
wavelength of 656.3 nm.
Occasionally a yellow colour may be seen in bright aurora. This is
due to a combination of red and green auroral emission.
Most faint aurorae show no color to the eye at all, appearing only as
white forms. This is not because the aurora is not coloured, but
because of how the eye perceives low intensity light. In the retina,
the light sensitive part of the eye, on the back wall of the eyeball,
there are two types of light sensors: rods and cones. The cones
are concentrated near the central field of vision and are used to
give high resolution seeing under moderate to high light levels. The
rods are spread around the peripheral field of view.
These are more sensitive to light than are the cones, and are the
sensors used at night and in low light levels. Only the cones are
sensitive to color. And thus in low light levels, we are unable to
appreciate any color that may be present. Thus we see faint stars
and faint aurora as being devoid of colour. A camera however,
will reveal any colour that is present.
CLASSIFYING THE AURORA
People have categorised aurora in many different ways, according
to colour, shape, structure, brightness and energy. However, before
discussing any of these classification schemes we should point out a
major division between what are called discrete aurora and
diffuse aurora.
Discrete aurora are those that have reasonably
well defined boundaries, and are the ones that are most readily seen.
Diffuse aurora on the other hand are much fainter and spread out over
a wide area. Diffuse aurora can be divided into two sub-categories
of pulsating aurora and hydrogen arc aurora. The former undergo
fluctuations in brightness with periods of from 1/10 of a second to
20 seconds. The latter is a uniform broadband of light that results
from a steady precipitation of protons and electrons from the outer
Van Allen radiation belt.
When most people talk about the aurora they are
referring to the discrete aurora.
Aurorae may be classified according to their colour. However, because
color is related to brightness when viewing the aurora (see the last
paragraph of the last section), this type of classification must be
used with caution. This list also only includes the most common
auroral colours.
Aurorae occur in a variety of forms. Ground observers
use single letters for shape and structure:
These letters may be combined to describe a specific
auroral form:
An atlas of auroral forms is a great help in serious auroral observing.
Auroral brightness is specified by an International
Brightness Coefficient (IBC) or with a light meter (photometer) that
measures in units called kiloRayleighs (kR):
The extent of the aurora may also be specified,varying
from a small glow or patch to an all sky storm. The aurora section
of the Royal Astronomical Society of New Zealand uses a seven
point Storm Intensity (SI) code to indicate the extent of the aurora.
A satellite can also be used to measure the integrated brightness or
activity of the aurora by measuring the electron power input to the
polar regions. The brightness of the aurora is proportional to the
precipitating electron flux, which may vary from 1012
electrons per square metre for a very faint aurora to over 1017
electrons per square metre for a very bright aurora. The power
associated with this flux is measured in gigawatts (109W)
according to the table below:
VIEWING & PHOTOGRAPHING THE AURORA
Aurora are not often seen in Australia, but
observers in Tasmania will see them more
frequently than those along the southern coast of
the mainland. Scientists in Australian Antarctic
bases may see an aurora every dark night, weather
permitting.
The occurrence of moderate intensity aurora tend to follow the
sunspot cycle which has a period of about 11 years. Thus when
there a lot of sunspots on the Sun we expect to see more aurora
further away from the polar zones. However, the really big
aurora tend to occur only a very few times during a single sunspot
cycle, and they may occur at virtually any time within the cycle.
People living in the south of the south island of New Zealand
see frequent auroral displays. However, those of us living in
mainland Australia are not so lucky. Auroral alerts can be received
by email and even by SMS on a mobile phone (see the Auroral
Alerts section below), and for anyone who is interested in viewing
and imaging the aurora australis this is a very worthwhile service.
For those interested in photographing aurorae the graph below
gives a guide to camera settings. Although it was devised for
film cameras, many digital cameras will now allow an ISO
"film speed" setting to be entered into the camera. Because long
exposures are required for all but the brightest aurorae, it will
be necessary to mount the camera on a tripod. And of course, with
digital cameras, the instant display feature allows another exposure
to be taken immediately if the parameters were not set right in the
first place.
CURRENT ISSUES
Three current issues in auroral studies relate to the origin of the theta
aurora, the reality of coast hugging aurorae, and the nature of sounds
sometimes heard from the aurora.
The Theta Aurora
Most aurora as seen from space take the form of an oval centred on
either the north or the south magnetic pole. Occasionally, however,
a line appears across a diameter of the oval, forming the greek letter
theta, after which this type of aurora is named.
Coast Hugging Aurorae
Up until recently scientists assumed that only the Earth's magnetic
field and the upper atmosphere had any effect on auroral formation.
However, examination of many satellite auroral images seems to
indicate that there is an excess (with respect to expected statistical
occurrence) of aurora that seem to line up with coastlines on the Earth's
surface below the aurora. Why the Earth's surface topography should
influence auroral behaviour is very unclear at this time.
The strange phenomenon of auroral sounds is discussed in the following
section.
Solar wind is continuously blown out from the very
high temperature corona (outer atmosphere) of the Sun.
[Image: Nikkei]
A coronal mass ejection (CME) is a huge cloud of
plasma released by the active Sun
[Image: NASA GSFC]
This image was taken with a white light
coronagraph aboard the SOHO spacecraft stationed at the
Lagrange point between the Sun and the Earth. It shows a
large coronal mass ejection leaving the Sun. The Sun itself is
behind the light blue occulting disc, but its size is indicated by
the white circle. Considering that the diameter of the Sun is about
1.5 million km, the size of the CME is approximately 10 million
km across, and it carries about 5 billion tons of matter away
from the Sun. It is this material, that if it were to impact the Earth,
would produce a bright auroral display.
[Image from LASCO on SOHO, a NASA/ESA spacecraft]
The auroral crowns on opposite poles of the Earth.
[Dynamics Explorer Image - NASA]
The auroral oval around the north pole can be clearly seen.
[Dynamics Explorer Image - NASA]
The auroral ovals are always centred on either the
north or the south magnetic poles, not the geographic poles
about which the Earth rotates. [All images here are from the
Dynamics Explorer satellite - NASA]
The image on the left is taken during quiet geomagnetic
conditions when few particles are precipitating. The auroral
oval can barely be seen. The left
image was taken during a major geomagnetic storm resulting
from the impact of a huge solar CME on the Earth. The
auroral oval is extremely bright and has expanded in size
toward the equator, giving southern Australia spectacular
auroral views (1989 March 13/14).
The aurora as imaged from the International Space Station
[NASA image].
The aurora as seen from the Space Shuttle. Note how the
colour varies with altitude [NASA image].
The most common auroral colours,
the elements that produce them, and
their wavelengths (nanometres).
Only nitrogen is in its molecular
form, all the other elements are in
atomic form.
Color Type Description A Green aurora with red tops B Green aurora with red bottom C Pure green aurora D Pure red aurora
Shape Structure A – arc D – diffuse B – band F – glowing C – corona H – homogeneous D – drapery P – pulsating G – glow R – rayed R – ray(s) S – surface
HA – homogeneous arc RB – rayed bands PA – pulsating arcs FC – glowing corona
IBC kR Description I 1 Faint, brightness of milky way.
No colour apparent. II 10 Brightness of thin moonlit
cirrus cloud. III 100 Brightness of moonlit cumulus
cloud. IV 1000 Bright as the full moon.
Casts shadows. Very rare.
SI Auroral Form 1 Glow or Patch 2 Arc, Veil or Band 3 Rayed Arc, Veil or Band
4 Ray Bundles 5 Active, Moving or Flaming Forms 6 Corona 7 All Sky Storm
Activity Index Power (GW) 1 0 – 2.5 2 2.5 – 4 3 4 – 6 4 6 – 10 5 10 – 16 6 16 – 24 7 24 – 39 8 39 – 61 9 61 – 96 10 96 +
A theta aurora as seen
by the Dynamics Explorer
satellite.
[NASA-Univ of Iowa]
An aurora "hugging" the
coast of Greenland.
[POLAR satellite image
NASA - Univ of Iowa]
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The northern auroral oval on Jupiter. NASA - Hubble Space Telescope |
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Both north and south auroral ovals on Saturn can
be seen in this image. NASA - Hubble Space Telescope |
AUSTRALIAN AURORAL ALERTS
Auroral alerts are issued by email and SMS message from
IPS Radio and Space Services. For details see
IPS Auroral Alerts. These alerts are based on intensity of
geomagnetic activity reported by an Australian network of
magnetometers, instruments that measure variations in the Earth's
magnetic field. Particles from the Sun not only precipitate into the
atmosphere creating aurora, but they also increase the particle
population in the Earth's magnetosphere. These increases cause
a net decrease in the geomagnetic field, and they also cause large
oscillations in the field. The larger the influx of particles the
greater is the field decrease and the amplitude
of the oscillations. These oscillations
are a result of the magnetospheric particle population building up
and then the dumping of these particles down toward the Earth
(a little like flushing a cistern when it becomes full).
It is just this activity that produces both aurorae and geomagnetic
activity (or storms). The correlation is not one hundred
percent, particularly when the magnetic field sensors are located some
distance from the auroral zone, but usually gives a reasonable indication
of when auroral activity is occurring. The geomagnetic activity is
specified by a magnetic index called K which ranges from 0 (quiet
geomagnetic conditions) to 9+ (a very large geomagnetic storm in
progress).
IPS auroral alerts are first initiated when the K index reaches 7, and
are re-iterated for values of K=8 and then K=9. A value of 7
usually indicates that people in Tasmania will see an aurora, whereas
a value of 9 indicates that an aurora may be visible from Sydney or
Perth.
The IPS web site also runs a model showing where the auroral oval
is expected to be at any given time, and thus how close it may be to
various parts of New Zealand and Australia.