THE SOUTHERN AURORAE : PART 1
The NATURE of the AURORA
TABLE 2 : Magnetic Deviation on 1st January 2015
|Alice Springs||04.76||-23.7||133.8E||-55.5||-33.66||6391||05 06|
|32.15||-54.5||158.9E||-78 83||-64.13||3347||15 26|
NOTES: The Table above gives the basic local magnetic data for southern cities or places within Australia and New Zealand. The explanation of the columns is as follows;
Magnetic Deviation is the variation of the compass from true south at the given location as of the 1st January, 2015. Positive values are towards the Eat, negative to the west.
Results are based on the Earth’s magnetic field models, and details about the methods and standards adopted are discussed in some detail within this site. Most locations of magnetic variations have to account for other anomalies. For most purposes, like trekking through the Australian bush, such knowledge may save you getting lost when using a magnetic compass for finding the cardinal directs. In most instances this has been replaced by satellite technology like GPS to locate where you are. At this site are several world maps showing magnetic variation. I.e. For Australia or the Rest of the World. Inclination, or the vertical needle dip, across Australia is Inclination at Epoch 2015.0.
More importantly during the appearance of aurorae, compass needles do not work very well. It is not unusual to see the direction erratically or wildly change by many degrees over a short time. This is not the only observed change. We often only think of compasses being affected in the horizontal plane to find 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, then the field lines intersecting with the surface of the Earth have a tendency of pointing the needle upwardly. When at quieter times of magnetic disturbance, the needle would be far more level. This last effect can be usefully employed to act as some aurorae detector. Here 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 some imminent auroral event.
The auroral process is actually far more complicated than merely the geomagnetic latitude. In summary, essentially the electrical beam following the field lines is pumped towards either of the magnetic poles of the Earth, which descends through the upper then lower atmosphere and then into the Earth surface. These magnetic field lines do not converge or intersects at the same location on the Earth but in fact at some distance away from the latitude and longitude of the observed geomagnetic poles. (This is simple due to the lines converging at the Earth’s core and NOT on the surface.)
All aurora therefore are found within 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 19° to 20° away from either the north or south magnetic pole. When considered in three-dimensional space. I.e. From the surface to 1000 kilometres 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 separate asymmetric ovals or torus-shaped belts.
The real size and shape of the auroral oval varies depending on several different factors. For example, during daylight hours, the oval may shrink to only about 15° in magnetic latitude from the poles. At night these ovals may gradually increase to 22.5°. Yet, 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, including one American satellite named POES. Any changes that occur are quickly recorded and made available across the Net. Current oval images can be found at Space Environment Center, and can be searched for recent images, movies and various plots.
Due to the varying height, the distance of the aurora from the observer can be considerably far away. This is mostly due to the curvature of the Earth, which an observer standing at sea level is about 5 kilometres. Assuming an object is higher, like a ship’s mast, an observe can see a bit further than this. We know that the base of an auroral display lies about 100 km. above the ground, meaning we can possibly detect a bright aurora when it is less than about 1300km. away. Higher aurorae, can be around 300km. in height, and such events could be seen around 2,300 km. away. From a low latitude location, like say Sydney, we know the central portion of the auroral oval averages in distance about 2,200 km. (and the geomagnetic pole is about 5,500 km. See Figure x.) This means when a rare very bright energetic aurora do appear, they become feasible to see from a lower latitude locations.
We know most low latitude aurora tend to most only red coloured event, which are known to be at the highest altitudes, between 300 km., and in some rarer circumstances, can rise to a maximum of 800 km.
However, see aurora on the horizon is almost impossible, so we must account for the maximum height the some aurora was seen. If say, the maximum altitude was 15° above the horizon, then the true distance that the aurora appears from us must be closer.
As the Kp index becomes about 7 or so, the auroral oval expand slightly by several hundred kilometres, making such an aurora possibly better viewed from low latitude.
Not all aurorae are necessarily seen due to just the auroral oval, such that, events can occur outside its boundaries. Under intense CME events, the magnetic field is so battered, that aurorae occur can become visible even as far as the equator, as happened in Singapore in 1903 or during the famous Carrington Event during 3rd September 1858.
Table 2 below, shows the height of the aurora and the possible distances they can be seen on the horizon.
In ancient mythology, Aurora was a Roman deity, but in the older Greek mythology she was known as Eos — the Dawn Goddess. 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 being 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 appearance were greatly admired and honoured. They were were certainly use to aurorae appearing regularly, so this became significant parts of their various elaborate folk-laws.
One of the first plausible reasonable explanation that was recorded — at least as best we know; appeared in 1250. Here this Norwegian writer explains;
“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 pertinent 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 familiar temperature scale based on the freezing and boiling points of water at sea level.)
Today, one of the most common misconceptions about the aurora were 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 phenomena. 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 properly described until about 1726, where the geophysical observer, Jean Jacques d’Ortous de Mairan (1678-1771) 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 (1731-1810), who found the height lay between 70 and 150 kilometres above the Earth. In modern determinations, the true height of the aurorae can vary between 60 to as high as 1,000 kilometres! Cavendish proved to be correct in his estimations. Heights are indeed typically in the range between 100 to 300 kilometres.
By 1872, a comet observer, 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 Eugen Goldstein (1850-1930), who incidentally worked on the original cathode ray tube suggested in 1879 that the auroral rays could be caused by solar storms — explaining the real connection between both the aurorae and observed magnetic storms. This concept was taken seriously by Norwegians, Kristian Birkeland (1867-1917) and Fredrik Carl Størmer (or Störmer) (1874-1957) in 1896, who produced new mathematical models on how these auroral mechanisms worked.
As we are naturally southern hemisphere orientated here, there is another historically reference that is also important to us. The first significant European explorer to journey into the far southern latitudes of the Southern Ocean, just happened to see and record the Aurora Australis, was Commander 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. Here an aurora was seen while returning to an agreed location in New Zealand, after Cook had lost sight of the companion ship, Adventure commanded by Captain Tobias Furneaux (1735-1781) at around −67°S within fields of icebergs. This bright aurora was seen from about latitude −54°S and longitude 61°E, some 750 km south of the French territory known as the Kerguelen Islands or Desolation Islands (−49° 21m S, 70° 13′E) — about 4,070 km from the western tip of Australia or 3,860 km from Africa. Cook describes in the ship’s log of 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), who arrived in Tasmania in 1844 as a convict. In 1870, he read a paper to the Royal Society of Tasmania, proving statistically there was direct correlations between sunspot numbers and the auroral frequency. He correctly theorised that the Earth was encompassed by a huge magnetic field, which was first espoused by Edmond Halley (1656-1742), but properly explained independently by English astronomer, Balford Stewart (1828-1887), just eight months before Abbott. Abbott had also stated that auroral appearances were likely related to the planetary 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 −45°, being roughly from Tasmania to southern New Zealand, will usually see aurorae more frequently than any other populated areas in the southern hemisphere. 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 mainly seen in its southern parts — namely Tasmania, southern Victoria, and sometimes South Australia and southern Western Australia. Aurorae, though less frequently, has been viewed from latitudes like Sydney, much of New South Wales, and even Brisbane in Queensland. Far more aurorae, however, are visible nearer the Antarctic continent. Macquarie Island, located at &;minus54.5°S, 158.9°E, sees more bright aurorae than any other locations in the southern hemisphere.
For southern places, like Invercargill in New Zealand, the number of aurorae averages five to ten per month. Probabilities in observing aurorae from eastern Australia is smaller roughly;
• Hobart 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 away from the geomagnetic poles that an aurorae can be seen. These powerful events are far more rare, 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 Earth’s magnetic poles around latitudes of about 60°N ; namely, Canada and Greenland in the northern hemisphere, Tasmania and New Zealand in the south hemisphere. At the actual magnetic poles, fewer aurorae occur than from the auroral ovals placed some 1,500 kilometres away.
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.
Last Update : 5th July 2015
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