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Aurora

The Aurora Australis is a luminous glow in the night sky of the Southern Hemisphere, most commonly in the polar region over Antarctica. It has a great variability in brightness, from a feeble glow to a brilliant display of many colours. And it can transform from quiet arcs stretching from horizon to horizon to a sky filled with spectacular, dancing rays and bands. It is analogous to the Aurora Borealis in the Northern Hemisphere.

The existence of these "southern lights" was first suggested in Europe in 1733 by de Mairan, in the first textbook published that was devoted to the aurora. They were confirmed by Captain James Cook, who observed the phenomenon during his voyage of discovery to Australia in Endeavor in 1770. During his subsequent expedition to the Antarctic in Resolution (1772-75), he and his crew had several sightings. He then coined the term Aurora Australis, and compared its colours and motions to those of the Aurora Borealis that they had viewed over Sweden. Subsequent voyages of Antarctic exploration pushed farther south and reports of auroras became common in ship's logs—such as in 1831 from John Biscoe, captain of the ship Tula, when surrounded by icebergs at the Antarctic Circle:

"At the same time, nearly the whole night the Aurora Australis showed the most brilliant appearance, at times rolling itself over our heads in a beautiful column, then as suddenly forming itself as the unrolled fringe of a curtain, and again suddenly shooting to the form of a serpent and at times appearing not many yards above us; it was decidedly transacted in our own atmosphere, and was without exception the grandest phenomenon of nature of its kind I have ever witnessed"

The Aurora Australis occurs well to the south of inhabited regions and only very major ones expand equatorward sufficiently to be seen over such continents as Australia (and there only a very few times a year). One of the earliest reported from Australia was by Francis Abbott in Hobart, Tasmania:

"On August 29, 1859 there appeared a most brilliant Aurora Australis extending from the W. through the S. to the Eastern part of the horizon in one continued arc of about 190 degrees and shooting up to the zenith.—The phenomenon had for thirty minutes a most magnificent appearance, the bands (i.e. rays) being in complete repose formed a truncated cone of glory, the apex of which, if projected, would have terminated in the zenith.—

A second display of the Aurora appeared on the night of 2nd September following, equally brilliant and extensive, but less transitory, and with this difference, that from Sunset the whole of the southern sky was of deep ruby colour—

From 12 to 1 o'clock on the morning of the 3rd the Aurora broke out into flickering streamers and corruscations, as brilliant, and with as much diversity of colour as on the 29th of August, forming in the zenith a well defined corona, which shortly after became diffused and then dispersed.—"

These two auroras were also seen in North America, Jamaica, Rome, and Athens and are on the list of historical great auroras. The latter one was also viewed and sketched from Melbourne.

Many countries sent expeditions to the Antarctic in the early part of the twentieth century. One was the British National Antarctic Expedition of 1901-04. On this, Robert Falcon Scott and his party were forced to establish winter quarters on Ross Island, near McMurdo Sound. Many sightings of auroras were made from there. Douglas Mawson, a member of Ernest Shackleton's British Antarctic Expedition of 1907-09, recorded very detailed observations of the aurora. He continued these scientific studies during his 1911-14 Australasian Antarctic Expedition.

The geographical distribution of the aurora in the Southern Hemisphere was first reported by Boller in 1898, based on some 780 nights during which auroras were observed. There were several later efforts to map the southern auroral zone but it was not until the International Geophysical Year (IGY) during 1957-58 that more observational data allowed Feldstein to produce much improved maps of both auroral zones. His southern map showed that this auroral zone roughly encircled Antarctica, well poleward of the southern continents. And that the percentage of nights that one would expect to see overhead auroras in major capital cities there was very low (Hobart, 3, Sydney or Auckland 1, Capetown 0.5, Buenos Aires .01). It is not surprising that observations and studies of the Aurora Australis in early times were scanty, compared to the much more extensive studies of the Aurora Borealis, which in contrast occurred over many populated areas.

The Aurora Australis generated several legends and beliefs among early natives of Australia and New Zealand, even though it occurred usually well to the south of them and perhaps was not an integral part of their culture. The auroras they saw were usually red, so they quite naturally associated them with glowing fires. The Maori of New Zealand believed that some of their ancestors had journeyed far south by canoe and become trapped in ice. Their descendants in that inhospitable zone sometimes lit huge bonfires signaling their kinsmen in hope of being rescued. They thought aurora was a reflection of these great fires in the night sky as Tahu-nui-a-Rangi (Great Glowing of the Sky).

Certain aboriginal people in Northern Australia viewed the aurora as the feast fires of the Oola-pikka folk—ghostly beings who sometimes spoke to the people through these auroral flames. Only the elders dared look at the lights and interpret their messages. Those in the south explained auroras as the campfires of spirits, flickering over Kangaroo Island off the south coast of Australia. Among the Kurnai people of southeast Australia the aurora needed no interpretation: it was an unequivocal and terrifying warning from the Mungan Ngour, the "Great Man" and a sign of his wrath. The Kurnai would run about trying to fend it off, and shouting at it to go away.

It had been long recognized that the southern and northern lights had some similarity and that the magnetic field of the Earth exerted some influence over them. There was some evidence that auroral displays occurred simultaneously in both hemispheres. A literature study about 1865 revealed that of 34 displays seen in the Southern Hemisphere, there were 23 coincident with auroras reported in North America. Photographs taken in 1968 from coordinated aircraft flying over Alaska and south of New Zealand showed auroras that were near identical and, in fact, mirror-images of each other. More detailed studies with spacecraft imagers have since established that the two auroras are conjugate (occurring on both ends of the same magnetic field line). Comparisons of such joint images show some small differences in detail (intensities and form), explained as due to irregularities in the geomagnetic field. So, the southern and northern lights can be considered integral parts of a global phenomenon. Comments on the aurora that follow can therefore apply equally to either the southern or the northern lights.

As for general characteristics, the aurora varies in brightness from a faint glow at quiet times to a brightness approaching that of the full moon during active periods. It is a permanent optical feature of the upper atmosphere, appearing in each hemisphere as an oval about 100 km or more above the earth. Its position varies with geomagnetic activity. During moderate activity the aurora is located about 23 degrees from each magnetic pole on the night side of the Earth and about 15 degrees on the dayside. During magnetically quiet times the oval shrinks poleward by as much as 5 degrees, significantly reducing the size of the polar cap, that region poleward of the aurora. The South Magnetic Pole is near Vostok Station, Antarctica, and the southern "auroral oval", the zone of most frequent auroral occurrence, roughly encircles the Antarctic Continent. It is only during very major disturbances that the southern lights move sufficiently equatorward to be seen over continents such as Australia.

While spectacular auroral displays have been recorded through history as far back as 500 BC, it has been through concerted international efforts, such as the IGY, and succeeding in situ balloon, rocket, and satellite investigations that most understanding of the phenomenon has emerged. The aurora is caused by particles, mainly electrons, bombarding the upper atmosphere gases. Its visible spectrum consists mainly of emissions from excited oxygen atoms (green and red emissions) and nitrogen molecules (violet and pink). The variation in colour seen in different displays is due to differing depths of penetration into the atmosphere by the bombarding electrons.

The emission spectrum extends over a wide frequency range extending from x-rays to radio emissions. Some major emissions are in the extreme ultraviolet region and are absorbed by the atmosphere, but can be detected from above by spacecraft imagers. Orbiting satellites such as the Dynamics Explorer, Viking, Polar, and Image have been used routinely since 1981 to photograph the aurora globally, even in the presence of full sunlight. They have verified that the aurora is a permanent optical feature, consisting of two full haloes encircling the earth. Viewed around this "24-hour oval" there are typically quiet arcs in the evening sector, dynamic brighter auroras in the midnight sector, diffuse auroral remnants in the morning sector and faint, red aurora through the noon sector.

The IGY prompted the establishment of many Antarctic bases in the 1950s, many of which have had active research programs, including auroral studies. At the Australian Mawson station and the Japanese Syowa station, aurora can be seen on practically every clear night. They are about 25 degrees from the South Magnetic Pole. Amundsen-Scott Station at the South Geographic Pole, established in 1956, is just poleward of the center of the auroral zone, but an ideal location for studies of auroras near midday - aided by several months of 24-hour darkness each year.

The period 1996-2003 went from solar minimum (~1996) to solar maximum (~ 2002) and the observations and photographs of Robert Schwarz, a scientist then at Amundsen-Scott Station, illustrate how auroral activity and poleward expansion increase with solar activity during the 11-year sunspot cycle. They are in agreement with the more extensive evidence from the Northern Hemisphere.

Auroral activity is controlled to a major degree by solar activity and the solar wind, that continuous stream of electrons and protons emanating from the sun. Major auroras are due to Coronal Mass Ejections (CMEs) from the sun, while auroral substorms are usually triggered by changes in the solar wind. Auroral substorms occur periodically and typically last for about 3 hours. The first sign is a sudden brightening of the quiet auroral arc in the midnight sector. This brightening spreads westward along the auroral oval, then the aurora expands poleward (termed the expansive phase). During this time the aurora is most active and colourful, with draperies, transient rays and rapidly moving arcs. The aurora then fades and recedes to lower latitudes and is replaced by fainter patches, often pulsating. This recovery phase lasts for up to 2 hours.

Great auroras expanding down to low latitudes and lasting up to 2 days occur very occasionally, and have been marvelled at through the ages. They are marked by their near global extent, their long duration and by their brightness and vivid colour. As they expand to lower latitudes they usually exhibit a deep red colour. Global power inputs via particle precipitation have been estimated as high as 1000 gigawatts during the peak of such auroras. They tend to occur around or following the peak of the 11-year cycle of solar sunspot activity (but can occur any time). While there may be something special causing these very unusual auroras, so far it appears they are just "bigger" than the usual substorms and not greatly different.

One of the great auroras of the past century occurred on March 13-14, 1989, with sightings reported from Argentina, Britain, and over much of North America. Satellites recorded images of greatly expanded auroras over both southern and northern latitudes. That aurora lasted more than 30 hours and caused a major power disruption along the American eastern seaboard. Another aurora approaching the same magnitude was on April 6-7, 2000. It was seen in South Africa, north of Capetown, a bright red glow lasting for three hours. It was also viewed over much of Europe and throughout North America.
One last example was the great Aurora Australis seen on March 30-31, 2001. As reported in detail by the Royal Astronomical Society of New Zealand, it was visible over much of New Zealand and Southern Australia through the whole night and was often red in colour. There were Aurora Borealis sightings at the same time over much of North America and across Norway.

It is noteworthy that for the latter two auroras there was a full array of spacecraft in orbit and it was possible to trace their full evolution from beginning to end. SOHO observed the solar CMEs, which caused them. The progress of the clouds and the solar wind streaming earthward were monitored by the ACE and WIND satellites, and the auroras resulting some 2-3 days later (the sun-earth transit time) were recorded by several satellites.

It is difficult to get auroral observations from the ground in both hemispheres simultaneously, due to the asymmetry of seasons. The best opportunities are around the equinoxes, when there are similar day and night conditions in both auroral zones. Fortunately, auroral activity is somewhat greater through the spring and autumn seasons due to increased magnetic activity, which adds to the chances of joint observations. Major displays of Aurora Australis were reported by viewers in New Zealand and Australia on May 4, 1998, October 24 and November 4, 2001, October 23, 2003, and November 7, 2004.

Poleward of the auroral zones there are often auroras, referred to as "polar auroras". They were first noted by early Antarctic (and Arctic) explorers. They are of similar origin to lower latitude auroras but are somewhat different in appearance and character. They occur under quiet magnetic conditions when auroral oval activity is minimal. They consist of very narrow arcs, usually faint and colourless, and they are always aligned along the Sun-Earth line. They usually are seen to split off the poleward edge of the auroral oval and drift across the polar cap, or to linger for hours depending on the state of the solar wind.

There is now a general understanding of auroral phenomena, but some questions still remain and these are now being addressed by several countries in on-going research on the Aurora Australis in Antarctica, at over 20 stations located around the continent. This work has been recently extended with the addition of several remote automated geophysical observatories on the continent, and with an array of auroral radars that cover much of the southern auroral zone. So, while historically much of the knowledge about auroras came from observations of the Aurora Borealis, the Aurora Australis has captured its proper share of attention during the past half-century with the establishment and operation of the Antarctic research bases.

Donald J. McEwen

See also: Antarctic: Definitions and Boundaries; Auroral Substorm; Australasian Antarctic Expedition (1911-14); Biscoe, John; British Antarctic (Nimrod) Expedition (1907-09); British National Antarctic (Discovery) Expedition (1901-04); Cook, James; International Geophysical Year; Magnetic Field; Mawson, Douglas; Ross Island; Scott, Robert Falcon; Shackleton, Ernest; Solar Wind; South Pole; Vostok Station.

References and Further Reading

Bond, F.R. Background to the Aurora Australis, Kingston, Tasmania: Commonwealth of Australia (ANARE Report 135), 1990.

Bond, F.R., and Fred Jacka. Distribution of auroras in the Southern Hemisphere, II: nightly probability of overhead aurora. Australian Journal of Physics 15 (2): 261-272, 1962.

Bond, F.R., and I.L. Thomas. The southern auroral oval. Australian Journal of Physics 24: 97-202, 1971.

Brekke, Asgeir, and Alv Egeland, The northern lights: their heritage and science. Translated by James Anderson. Oslo: Grondahl og Drewers Forlag AS, 1994.

Eather, Robert. Majestic lights: the Aurora in science, history and the arts. Washington, DC: American Geophysical Union, 1980.

Liversidge, Douglas. The last continent. London: Jarrold Publishers, 1958.

Savage, Candace. Aurora: the mysterious northern lights. Vancouver: Greystone Books, 1994.

Schwarz, Robert. Polarlichter. Nordendstr. 58, 8080 Munchen, Germany, 2004.

Stormer, C. The polar aurora. Oxford: The Clarendon Press, 1955

Vallance Jones, A. Historical review of great auroras. Canadian Journal of Physics 70: 479-487, 1992.

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