“Such is the aurora in its grandeur. No colour, no brush can picture its magnificence, no words describe its sublime beauty... There we stand, lost in wonder which Nature writes in flaming cypher on a dark winter’s sky; here we stand, lost in wonder, obliged to confess - that we really know nothing.”
An aurora is an unusual atmospheric phenomena sometimes seen from time to time, that appears as bright illumination in the night’s sky. They are either a bright glow or organised as visual rays of light, which sometimes comprising differing colours, especially green and red. Aurorae when they are visible in the southern hemisphere are termed the Aurora Australis, while in the north sky are called the Aurora Borealis. Often this phenomena will be seen simultaneously in both hemispheres, and these are termed the Aurora Polaris. As for the general noun, modern usage calls the singular phenomena aurora, with the plural or genitive being aurorae and NEVER auroras. The adjective is termed auroral.
Any arrival of a bright and interesting auroral event can be very unexpected. Most appear when they are no more than a very faint, colourless unimposing glow, with perhaps small patches of light or just a single ray. This can quickly be followed by a rapid rise - perhaps only within minutes - of an illumination that may cover the entire sky wih light. Here it becomes possible to see a quite spectacular evolving multicoloured light showing many different forms or structures. These may removing all the stars for view - perhaps for a short time, perhaps for the entire night. Once this display has literally shocked you into disbelief, it may disappear as rapidly as it came into view.
Often you may have to wait considerable time before another bright aurora will show its wonderous display again. In places like the mainland of Australia you might have to wait several years, or even a decade or two, before you get a lucky chance to see another. If the event was seen in country skies and youlive in the city, you may never see another again. Others in more southern clines may aurorae on average about once per week as a regular occurance. On this latter point, I have even heard of the aurorae being described as an complete nuisance. For example, mothers complaining that they have to always cover windows with heavily made curtains just to stop some bright auroral event from waking their young children in the middle of the night!
Although all aurorae are an emotionally exciting experience, for some stargazers, its occurrence is scientifically reasonably well understood. Such astounding eerie light is caused by applying a simple electrical current that collides with normal neutral atoms like oxygen and nitrogen in the upper atmosphere. The ionising energy of this electric current is sufficent to cause these normally low energy ground state atoms to adsorb the energy and become electrically “excited”. Once “activated” the energy is held for a while, then after a short or long delay depending on the atom, reemit visible light at certain discrete wavelengths as single colours. For most aurorae this appears mainly green or red. This unusual illuminating process by this seemingly unusual cold ‘electronic chemical reacion’ is properly called fluoresence, and said to have causing atoms to fluoresce. Ie. This emitting light is under the same process that occurs in the everyday fluorescent light tube that you may have in your bathroom or perhaps outdoor for the backyard or shed.
Energy is transferred to these upper atmosphere atoms by very fast moving charged solar subatomic particles, like electrons, and to a lesser extent, protons. These are expelled by dramatic violent solar storms that are issued by flares or so-called coronal mass ejections (CMD’s. Such events hurl huge amounts of subatomic particles, movingat hundreds to thousands of kilometers per second, across the gulf of space and away from the Sun. By chance if these flares or CMD’s are facing towards the direction of the Earth, the planet becomes bombarded with these high velocity particles, to collide with the Earth’s protective magnosphere. Although the particles are travelling at tremendous velocities, it still take a day or two to reach us.
These particles are not destroyed by are entrapped and caught-up within the Earth’s own magnetic field. Rapidly they interact and are confined within this magnetic field and become powerful siralling streams that follow the many field lines. The field’s general curvature eventually forces them to intersect where they collide in the upper atmosphere, creating a fluorencent light-producing reaction - the aurorae.
One may think of these field lines being akin to some giant electric generator whose’s electrical circuitry holds truly awesome power that can generate tens of millions of amperes and anwhere between 40 000 and 55 000 volts. Here is the aurora’s particular natural wonder - an event producing enough power to support the electricity of a large industrialised nation. It is also something the technology cannot ever harness.
Down on the Earth surface, most of the time the electric current produced is very weak, but when some magnetic storm or disturbance happens - an auroral display is produced. The energies when it does can be sufficent fierce to disrupt short-wave radio transmissions, radio fadeouts, causing surges in the electrical power grid and sometimes even massive blackouts.
Other subtle effects are less noticeable. For example, the air may feel seemingly be full of static electricity, or making crackling or whooshing sounds. Much about the reason why this phenomenae occurs is not fully understood, but probably relates to the same things heard and felt near electrical motors and other such equipment.
The position of the Earth’s magnetic poles does not correspond to the rotational axis of the Earth, and hence, varies significantly in geographical latitude (and longitude). Positions for all places regarding the magnetic field of the Earth are commonly measured in terms of geomagnetic latitudes. This explains why location of the observer in seeing aurorae, as we have already seen, decreases the further north you go in the southern hemisphere, geomagnetically speaking. This has an immediate effect on where aurorae can be seen. For example, the Aurora Australis is seen in the Southern Ocean directly south of Australia, but are not seen in the South American countries - even on that continent’s far southern tip. In geomagnetic latitude, Sydney lies at a geomagnetic latitude of about 42oS, Melbourne is about 47oS while Hobart and New Zealand’s southern town of Invercargill is at geomagnetic latitude 51oS. As for South America’s southern tip the geomagnetic latitude is presently 20oS - thus not a good place for observing aurorae.
A map of the geomagnetic positions can be found at NOAA. You can also calculate the Magnetic Latitude and Longitude (1955) for your location using the following general formulae below.
Haymes, R.C. “Introduction to Space Science” pg. 216 (John Wiley & Sons)
The position of the geomagnetic poles are also where the travellers’ compass point towards. Although we say that the compass needle points north and south, this is not strictly true. There is a significant variation depending on where you live and that changes direction slowly through the years. For example in Sydney, Australia, true north was 11.9o further eastward (or 11.9o further west of true south) in 1976 - and was slowly gaining about +0.1o per year. Today (2005), true north deviates xx o and some x.x op per year.
More importantly during aurorae compass needles don’t work very well. It is not unusual to see the direction erratically change by many degrees over a short time. This is not the only change. We think of compasses only being affected in the horizontal plane in finding the cardinal points (north, south, east and west), but they are also significantly influenced in the vertical plane. If an aurora say was to occur overhead the field lines interescting the Earth’s surface have a tendence of pointing the needle more upwards. When at quieter times of magnetic disturbance, the needle would be far more level. This last effect can be usefully employed to act as a aurorae detector. Hene if a device is set-up where contact is made to complete an electronic circuit only when the compass needle rises sufficiently, this can be used to trip an alarm - alerting of an imminent auroral event.
The auroral process is actually more complicated than mere geomagnetic latitude. In summary, essentially the electrical beam following the feild lines is pumped towards either of the Earth’s magnetic poles, which descends through the upper then lower atmosphere and then into the Earth surface. These magnetic field lines don’t not converge or intersects at the same location on the Earth’s surface but in fact at some distance away from the latitude and longnitude of the observed geomagnetic poles. (This is simple due to the lines coverging at the Earth’s core and NOT at the Earth’s surface.)
All aurora therefore are found in a circular ring about 3 000 kilometres across on the Earth’s surface whose centre is place on either geomagnetic pole. During periods of normal activity this diameter averages about 19o to 20o away from either the north or south magnetic pole. When considered in three-dimensional space, Ie. From the surface to 1000 kilomtres up, the aurora will appear to move within the so-called auroral oval. So in reality, the electric current need to produce the aurora is not being pumped into one point but into two seperate asymmetric ovals or torus-shaped belts.
The real size and shape of the auroral oval varies depending on a number of different factors. For example during daylight hours, the oval may shrink to only about 15o in magnetic latitude from the poles. At night these ovals may gradually increase to 22.5o. However, this is only a general guideline. During periods of significant solar activity or when there are particles ejections from a solar flare passing the Earth, through coronal holes as CME (Coronal Mass Injections), then the auroral oval can dramatically expand in size and brighten. In some of these instances an aurorae display during violent geomagnetic storms can be seen from much higher magnetic latitudes than normally would be unexpected.
This auroral oval is continually photographed from several polar satellites, inluding one American staellite named POES. Any changes that occur are quickly recorded and are readily made available on the Internet. The current image can be found at Space Environment Center, and can be searched for recent images, movies and various plots.
In ancient mythology, Aurora was a Roman deity, but in the older Greek mythology she was known as Eos, - the goddess of the dawn. She is recognised as the one who announces the coming of the Sun in the first light of day. It has been only in recent times she has been associated with the nighttime phenomena - being more the goddess of the “False Dawn.”
Auroral displays have been known since ancient times. They were first recorded by Pliny, Seneca and Aristotle in the ancient literature. Most were feared by populations in Europe as evil portents signifying fire and great evil raining down on the world. Some saw them as terrifying events beng huge sky battles by dragons, gods or sky spirits. For those living closer to the poles, like the American Indians, Vikings or the Scandinavian peoples their apperance were greatly admired and honoured. They were were certainly use to aurorae appearing regularly, so this became a significant part of their various eloborate folk-laws.
One of the first plausible reasonable explanation that was recorded - at least as best we know - appeared in 1250. Here a Norwegian writer explained;
“Some people maintain that this light is a reflection of the fire which surrounds the seas of the north and the south; others say that it is the reflection of the sun when it is below the horizon... for my part I think that it is produced by the ice which radiates at night the light which it has absorbed by the day”.
Much can be found elsewhere on the history on understanding the aurorae, which remained in its infancy really until the beginning of the 20th century. Here I will skip this much of this part for the sake of brevity, but I will make just a few points;
One of the first scientific documents on the auroral phenomena was made by Auders Celsius (1701-1744) in 1733, who specifically travelled north to the Baltic Sea just to observe them. (Celsius was also the same scientist who introduced the familar temperture scale basedonthe freezing and boiling points of water at sea level.)
Today one of the most common misconception about the aurora was that it was caused by the influence of sunlight - either by being light somehow being reflected by the upper atmosphere or via some other similar physical phenomenae. Although is was later shown that some aurorae appeared blue or purplish due sunlight shining on the upper rays during twilight, it was not the principal process of the auroral light. A correct explanation did not begin to be described properly until about 1726, where a geophysical observer named Jean Jacques d’Ortous de Mairan determined that the aurora occurred around heights of 100 kilometres above the surface of the Earth. This was later calculated in 1784 by the English physicist Lord Henry Cavendish, who found the height lay between 70 and 150 kilometres above the Earth. In modern derminaions, the true height of the aurorae can vary between 60 to as high as 1000 kilometres! Cavendish proved to be correct in his estimations. Heights are indeed typically in the range between 100 to 300 kilometres.
By 1872, an observer of a comets, Giovanni Battista Donati (1826-1873) suggested that the cause of the aurora was closely linked to the Sun, which he concluded after some crude spectroscopy on the auroral light. Another by the name of Sagan Goldstein, who incidentially worked on the original cathode ray tube suggested in 1879 that the rays seen in the aurorae may be caused by solar storms - explaining the real connection between both the aurorae and the observed magnetic storms. By 1896, this concept was taken seriously by Birkeland and Carl Stõrmer, who produced mathematical models on how the auroral mechanisms worked.
As we are naturally southern orientated and biased, here another historically reference that is also important to us. The first European explorer to journey to the southern hemisphere to happen to see and record the Aurora Australis, was James Cook on 16th February 1773, from the deck of the 462 ton ship, the Resolution. This was on Cook’s second main southern expedition whose primary goal was to search for the then unknown southern continent of Antarctic. The aurora was seen while returning to an agreed location in New Zealand, after Cooke had lost sight of the companion ship, Adventure commmanded by Captain Furneaux. This occured at around -67oS in fields of icebergs. The aurorawas seen from about latitude -54oS and longitude 61oE - south of Kerguelen Island and about km the southern tip from the New Zealand. Cooke describes in the ship’s log this auroral appears as;
“In the night we had fair weather, and a clear serene sky; and between midnight and three o’clock in the morning, lights were seen in the heavens, similar to those in the northern hemisphere, known by the name of Aurora Borealis, or northern lights; but I had never heard of the Aurora Australis being seen before. The officer of the watch observed, that it sometimes broke out in spiral rays, and in a circular form; then its light was very strong, and its appearance beautiful. He could not perceive it had any particular direction; for it appeared, at various times, in different parts of the heavens, and diffused its light throughout the whole atmosphere...”
One of the first of the Australian amateur astronomers to observe and discuss the aurora was Francis Abbott (1799-1883) He arrived as a convict in Tasmania in 1844. In 1870 he read a paper to the Royal Society of Tasmania proving statistically that was a direct correlation between sunspot numbers and the auroral frequency. He correctly theorised that the Earth’s was encompassed in a huge magnetic field, which as previously espoused by Edmond Halley and by the English astronomer, Mr. Balford Stewart, eight months before. :He also stated that the apperance of the aurorae was likely related to the planet’s magnetic field lines.
In the southern hemisphere, the magnetic pole is placed just off the coast of Antarctica facing towards Australia. From latitudes south of about -45o- being from Tasmania to southern New Zealand will usually see aurorae more frequent than any other populated areas in the south. As the South Island is closer to the magnetic pole, therefore places like Queenstown and Invercargill will see many more aurorae. From Australia, the aurorae is see mainly in its southern parts - namely Tasmania, Southern Victoria and sometime of the South Australia. Aurorae, though less frequently, it has been viewed from latitudes like Sydney, much of New South Wales, and even Brisbane in Queensland. Far more aurorae, however, are visible from the Antarctic continent.
For southern places, from New Zealand’s Invercargill, the number averages five to ten ech month. The probability of aurorae in eastern Australia is smaller roughly being;
- Hobart was twice per month
- Melbourne once per month
- Sydney once every five years
- Brisbane once every ten to fifteen years
- Cairns once or twice per century
Generally the stronger the outburst of geomagnetic activity, the further from the geomagnetic poles an aurorae can be seen. These powerful events are far rarer, so the frequency of the aurora will decrease rapidly. Overall this frequency does changes significantly. It depends mainly on the proximity we are to solar maximum and when the sunspots are near maximum numbers - and this occurs near the earth’s magnetic poles around latitudes of about 60o; namely, Canada and Greenland in the north, Tasmania and New Zealand in the South. The actual magnetic poles show fewer aurorae than occurs about 1 500 kilometres away within the auroral ovals.
In theory, an aurora could be seen anywhere on Earth. It is possible to see aurorae nearer the equator - perhaps possibly as frequently as once or twice each century! Of course, much of this activity is also closely locked-in with the 11.3 year solar sunspot cycle. This explains why more aurorae and more probable nearer, or just after, solar maximum that at minimum. Furthermore, that most aurorae in high geomagnetic latitude will only be seen during solar maximum.