GUIDE TO THE AURORA

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.

Hobart Aurora
Green aurora over Hobart, Tasmania in August 2005
Image from Dallas and Beth Stott

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.

STS59 Aurora
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]

THE NAMING OF THE AURORA

The name aurora is a latin word meaning dawn, from the Roman goddess Aurora. It is believed that either Galileo in 1619 or the French scientist and philospher Gassendi in 1622 were the first to "christen" the northern phenomenon with the name Aurora Borealis, the latter word simply meaning north. Thus we have the "northern dawn" or northern lights. This is of course a misnomer, as aurorae have absolutely nothing to do with the dawn. The aurora australis was undoubtedly seen by New Zealand Maoris, particularly in the south island, and also occasionally by Australian Aborigines, particularly those living in Tasmania, as well as inhabitants of Patagonia. However, the first European records of the southern aurora are ascribed to the expeditions of Captain James Cook in the late 1700's when he investigated the regions to the south of Australia. This was then given the complementary name Aurora Australis or southern dawn.

Note that the plural of aurora may be either aurorae or auroras.

WHY ARE AURORAE IMPORTANT?

Interest in aurorae is manyfold. It ranges from a casual interest in observing the beauty of the spectacle to a problem of system interference that has strategic security implications. Some of these are listed below.

The Magnificent Aurora
The emotion that the aurora brings forth in many people is perhaps best expressed by a quote from Donald G Carpenter (USAF Academy) in "Environmental Space Sciences":
The awe-inspiring shimmering aurora is a splashing of radiant colors on the TV screen of our atmosphere. It is part of a cosmic display conceived by God, produced by the Sun, directed by Earth's magnetic field, starring plasma fluxes and released photons, and with makeup by atomic de-ionizations and de-excitation. It is one of the classic beauties of nature, and fortunately its "run" will not be limited to a single season.

Historical Aurora
The reaction of various peoples in history to auroral displays has depended largely on where they lived. Those people who live near the auroral zones and see frequent aurorae tend to regard it as a benign or even beneficial event. Some Canadian eskimos believed that the aurora was the dance of the animal spirits. In Scandanavian folklore an aurora was thought to enhance the fertility of the Earth, with a promise of an abundance of seeds and a rich harvest, or a large fish catch from the ocean. Chinese and Japanese myths associated it with fertility, and even today Japanese honeymooners travel to Alaska in the belief that a child conceived under the aurora will prosper and be blessed with good luck. Some Alaskan motels are happy to oblige with bedrooms with glass ceilings. On the other hand, in societies where the aurora is only seen infrequently, it has often been seen as a bad omen, particularly as aurorae seen at lower latitudes (closer to the equator) are most often present as red glows. A spectacular aurora seen over middle Europe in 1570 brought forth the following admonition:
Wherefore, dear Christians, take such terrible portents to heart and digilently pray to God, that He will soften His punishments and bring us back into His favor, so that we may await with calm the future of our souls and salvation. Amen.
The drawing below was made of the same aurora, from what is now the west of the Czech Republic.
1570 auroral drawing "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.

Solar Wind
Solar wind is continuously blown out from the very high temperature corona (outer atmosphere) of the Sun.
[Image: Nikkei]
CME
A coronal mass ejection (CME) is a huge cloud of plasma released by the active Sun
[Image: NASA GSFC]

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.

Magnetosphere

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.

CME LASCO 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 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.

Auroral Crowns
The auroral crowns on opposite poles of the Earth. [Dynamics Explorer Image - NASA]
Auroral Oval
The auroral oval around the north pole can be clearly seen. [Dynamics Explorer Image - NASA]

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.

Aurora around Magnetic Pole
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]
Auroral Oval Differences
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).

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.

Aurora from the ISS
The aurora as imaged from the International Space Station [NASA image].
Aurora from the Space Shuttle
The aurora as seen from the Space Shuttle. Note how the colour varies with altitude [NASA image].

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.

Auroral Spectral Colours 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.

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.

Color TypeDescription
AGreen aurora with red tops
BGreen aurora with red bottom
CPure green aurora
DPure red aurora

Aurorae occur in a variety of forms. Ground observers use single letters for shape and structure:

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

These letters may be combined to describe a specific auroral form:

HA homogeneous arc
RB rayed bands
PA pulsating arcs
FC glowing corona

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):

IBCkRDescription
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.

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.

SIAuroral Form
1Glow or Patch
2Arc, Veil or Band
3Rayed Arc, Veil or Band
4Ray Bundles
5Active, Moving or Flaming Forms
6Corona
7All Sky Storm

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:

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 +

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.

Auroral photography parameters

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.

Theta Aurora A theta aurora as seen
by the Dynamics Explorer
satellite.

[NASA-Univ of Iowa]
How this aurora forms, and the circumstances that lead to its formation are still matters of debate and investigation.

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.

Coast Hugging Aurora An aurora "hugging" the
coast of Greenland.

[POLAR satellite image
NASA - Univ of Iowa]

The strange phenomenon of auroral sounds is discussed in the following section.

AURORAL SOUNDS

For over 200 years occasional reports have been made by both scientific and lay people about hearing sounds associated with some aurora, usually those that are bright and rapidly varying. The sounds are described as hissing, whooshing, swishing, and sometimes crackling noises. And the amazing part of the reports is that these sounds seem to relate directly to visible changes in the auroral display.

Although there appears no doubt that auroral sounds are real, they present science with a conundrum, because even if sound could travel through the near vacuum of the upper atmosphere where aurorae are produced, it would take a minimum of 300 seconds to travel over the distance between the aurora and the observer. And thus the sight and sound should be totally uncorrelated.

The only possible explanations must involve electromagnetic phenomena, which travel at the speed of light, together with a transduction mechanism which converts the electromagnetic quantity to sound near the observer. Some researchers have proposed that the explanation lies with the electric field associated with auroral particle acceleration, whereas others maintain that electromagnetic radiation (very low frequency radio waves) are emitted by the aurora, and that this signal is rectified/converted into sound energy near the ground. Possible conversion detectors have been suggested as dry pine needles or even frizzy hair on the observer.

It appears that no-one has recorded this sound or a VLF signal directly associated with it, although VLF signals have been recorded that are related to auroral activity, and also from bright meteors, which might be related to the auroral sound phenomenon. A Finnish group from Sodanklya Geophysical Observatory and the Finnish Meteorological Institute have made attempts in this area of research, and their results indicate a possible connection between VLF signals and auroral sounds, but more work needs to be done in this area.

OTHER PLANETS

The Earth is not the only place to have aurorae. The Hubble Space Telescope has now photographed aurorae on Jupiter, Saturn, Uranus and Neptune. To host aurorae a planet needs an atmosphere and a reasonable magnetic field. The images below show the auroral ovals on the two largest planets in the solar system.

Jovian aurora The northern auroral oval on Jupiter.

NASA - Hubble Space Telescope

Saturn's aurorae 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.

AURORAL RESOURCES

Books

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