Aurora Borealis/Australis

March 23, 2009, 9:01 pm
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Introduction

For thousands of years humans have been able to look up at the northern sky and see strange, but colorful, glows of light. In ancient Rome, Julius Caeser dispatched troops to put out non-existing fires in northern cities thinking that the crimson glows were towns engulfed in flames. By the 1800's, spectroscopic studies revealed that auroral light was actually caused by oxygen and nitrogen atoms made to emit light in only a few specific wavelengths.

The most spectacular manifestation of the connection between the Sun and the Earth is the Aurora Borealis (Northern Lights) and the Aurora Australis (Southern Lights). For millennia, people have watched them and worried about what ill portents they were heralding. It wasn't until the mid-1800s that scientific studies began to uncover many of their mysteries.

Photo of an aurora taken from the ground in December,2004 on Kam Lake in Yellowknife, NT (Courtesy Andrew Eaton

Photo of an aurora taken from the ground in December,2004 on Kam Lake in Yellowknife, Northwest Territories, Canada. (Courtesy Andrew Eaton)

Early History

A popular scientific idea in the 1600’s was that aurora were vapors exhaled from earth that exploded upon reaching high altitude. By this time, Gilbert had also demonstrated that Earth was a magnet, which is why compasses worked so well to navigate. In the 1700’s, Edmund Halley offered the idea that aurora were the invisible material emitted from earth’s magnetic poles, but ‘…in certain degrees of intensity or velocity, becomes self-luminous and betrays itself to the eye in the aurora’. [Lovering, 1860). ( Phil Trans Vol. 395. P. 94) The magnetic nature of aurora was pretty well established by the end of the 1700’s thanks to experiments beginning with Celcius and Hoiter in 1740, and continuing into the early 1800’s with Alexander von Humbolt’s investigations. These simple experiments showed that, instead of Earth’s magnetism being constant in time so that a compass held a fixed bearing, this field sometimes erupted in ‘magnetic storms’. These magnetic disturbances to compass needles were especially pronounced whenever there were bright aurora nearby.

By the later half of the 1700’s, Dr. Priestly also concluded that aurora are an electrical phenomenon as well. Even Ben Franklin had something similar to say about it in 1779 in a paper he wrote ‘ (Edinborough Phil Journal Vol. XLII p. 561). For a time, it was thought that the electrification of the air itself caused aurora. Aurora were considered to be electric discharges in the upper atmosphere caused, perhaps, by electrical ‘fluids’ that flowed from pole to pole. De la Rive also had his reason to support an electrical origin for aurora in the charged atmosphere, but preferred to see aurora as an exhalation from the Earth which drifted into the atmosphere and discharged to produce the light.

We also have the idea by Captain John Ross (Appendix to his second voyage published in 1833) that aurora are the reflected light from arctic snow and ice. However, by 1839 many attempts had been made to detect polarized light from aurora but failed to find any such evidence that aurora are reflected light, even though auroral light reflected from the ocean was polarized as expected. (Lovering 1860.

Some scientists also took a more radical view of the origins of aurora. In 1733, Mairan developed the idea that, since to his mind the Zodiacal light was the outer atmosphere of the sun, whenever the earth passed into it and it became entangled with earth’s upper atmosphere, it would be seen as an aurora.

Better Data

As the 19th century unfolded, we have several different and competing ideas about what aurora are, and what causes them? It was a complex stew of ideas and ‘theories’ in which each one had discovered a small piece of the bigger puzzle. One can easily forgive the non-scientists for thinking that no progress had actually been made in understanding aurora. What was to happen during the first 50 years of this new century was than old ideas would be disguarded and new ones created. The biggest contrast between the 1700’s and the 1800’s was in the huge increase in high-quality data, and spectacularly new scientific instruments to measure magnetic fields, electrical currents and even dissect the light from the aurora itself.

Prof. D. Olmstead at Yale in 1856 published an article in which he combined several basic aspects to aurora, and came to a basic and profound conclusion. The first basic aspect that was irrefutable was that they aren’t just observed in one location on earth – they are global in extent. It was also known to be true at this time that the forms of an aurora during a single ‘exhibition’ go through a specific sequence of changes, and that these phases occur at the same local times on all meridians. They are not choreographed by some absolute time…. The only possible conclusion from this was that the matter in aurora must be foreign to earth like Mairan had proposed, but more importantly, it had to have something to do with the position of the sun – the great ‘clock’ that regulated local time. (American Journal of Science VII p. 127-293)

Even Euler got into the act and proposed that the zodiacal light AND aurora are caused by exhalations from the sun…. What all these ideas of causation really lacked was a way to point the finger in a specific direction. As with many phenomenon in nature, sometimes only statistics can lead the way.

An English artillaryman, Edward Sabine, was fascinated by aurora as were many amateur and professional scientists of his day. Sabine's credentials in this particular persuit were better than most, and even rivaled Humbolt's own resume. Sabine had accompanied both John Ross and later Edward Perry in their quest for the Northwest Passage. He had spent many hours on the ship's deck observing the polar lights and wondering about their causes. The combination of Samual Schwabe's discovery a few years earlier of the solar sunspot cycle, and the massive data provided by Humbolt's magnetic observatories led Sabine in 1852 to the discovery that magnetic storms ebb and flow with the sunspot cycle.

Schwabe was well out of his league in providing a coherant physical model for the correlation. He was not a scientist with an impeccable pedigree from a prestigeous observatory or university. So this result lay dormant for years. But the correlation between aurora/magnetic storms and the sunspot cycle was the clear evidence that was later used to support Olmstead and Mairan’s idea that the cause of aurora had something to do, not with purely terrestrial sources, but with the sun, and in particular, the ebb and flow of sunspots across its surface.

So, by 1859 there was at least a theory in print that proposed aurora were caused by solar interactions with Earth. What wasn’t known is what the nature of this interaction could possibly be. It was also, certainly, true that just because there were sunspots on the sun, aurora were not always seen. Scientists during the 1800's eventually learned that aurora often accompanied magnetic 'storms' and an unsettled magnetosphere; they were produced by flows of charged particles entering the atmosphere; they came and went with the sunspot cycle; and their colors were the product of excited oxygen and nitrogen atoms hundreds of miles above the surface of the Earth.

By the turn of the 20th century, scientists actually created artificial aurora in their laboratories, and once television and the fluorescent lamp were invented, it was pretty clear just how aurora worked. What scientists still didn't understand was what was triggering them. Some thought it was from direct currents of particles from the Sun itself. Others felt it was more complicated than that.

Satellites Add to Data

Thanks to intensive study by research satellites during the Space Age, aurora have been substantially de-mystified, even as their ethereal beauty has remained to dazzle us and inspire awe. When the magnetism of the solar wind is the same as the south-type polarity in the daytime side of the Earth, an invisible valve opens in the magnetotail region, allowing particles and energy to penetrate deep into the magnetosphere.

In the delicately balanced magnetic tail of the Earth, magnetic fields can become crossed just as in solar flares. The energy stored in the magnetic field can be liberated as currents of charged particles.

These accelerated particles flow into the equatorial regions of near-Earth space and become trapped in the ring current. Positively charged particles drift westward while negatively charged particles drift eastward. In a process scientists don't fully understand, some of these particles also flow along the magnetic field into the polar regions. There, these million-ampere currents cause the spectacular displays we see as aurora, and the currents that cause them also heat the atmosphere and ionosphere.

Satellite view of the auroral oval taken by the NASA IMAGE satellite.

Satellite view of the auroral oval taken by the NASA IMAGE satellite.

During the 1900's, the source of this excitation was traced to currents of electrons and protons flowing down the geomagnetic field lines into the Polar Regions where they collided with the atmospheric atoms. The collisions took place at altitudes of 100 to 1000 kilometers, high above the ionosphere.

The explanations of how and why aurora are produced has had a complicated and contentious history. The popular misconception is that aurora are produced by solar wind particles directly entering the polar regions. In actuality, there are a number of different causes for aurora, each with its own characteristic qualities. For example, Aurora are not produced by solar flares, although radio communications black outs are caused by flares on the dayside of the Earth, as these high-energy particles disturb the ionosphere. Other solar events, such as coronal mass ejections, do contribute to the conditions that cause some of the strongest aurora to light up the skies. At other times, the simple change in magnetic polarity of the solar wind from north-directed to south-directed is enough to trigger aurora.

Because of the existence of the cusp in the magnetosphere on the dayside, solar wind particles can under some conditions diffuse or flow down this entryway into the Polar Regions. This causes dayside aurora, which are normally invisible to the eye, but detectable with the proper instruments. This is, virtually, the only case where solar wind particles can directly cause aurora, and it is not the cause of the spectacular nighttime polar aurora. To understand how nighttime aurora are produced, it is helpful to imagine yourself living inside a television picture tube. Beams of electrons, guided by magnetic forces, paint serpentine pictures on the atmosphere, which we see as the aurora. The origin of these beams is in the distant geotail region, not in solar wind plasma. Solar flares are also very ineffective in directly producing aurora, though they do cause disturbances in the ionosphere on the dayside which disrupt shortwave communication.

As a severe disturbance in the solar wind flows by the Earth, it causes several important electrical and magnetic changes. When the solar wind turns southward, its lines of force encounter the also-southward lines of the Earth's north geographic Polar Regions on the dayside. (Note, because your compass needle points north, it is actually being attracted by the south-type pole of the geomagnetic field.) The solar wind field lines then connect up with the Earth's field in a complex event that transfers particles and energy into the Earth's magnetosphere. While this is happening near the Earth, in the distant geotail, other changes are causing the geomagnetic field to stretch like taffy, and snap into new magnetic shapes. This causes billions of watts of energy to be transferred into the particles already trapped in the magnetosphere out in these distant regions. These particles, boosted in energy by thousands of volts, then flow down the magnetic field lines into the nighttime polar regions. The changes in the shapes of the aurora during an evening are not random, but are tightly choreographed by specific events going on in the geotail as it releases its stored magnetic energy.

Preview Image

"Aurora Australis" - This view of the Aurora Australis, or Southern Lights, which was photographed by an astronaut aboard Space Shuttle Discovery (STS-39) in 1991, shows a spiked band of red and green aurora above the Earth's Limb.

 

Citation

Odenwald, Sten (Contributing Author); Bernard Haisch (Topic Editor). 2008. "Aurora Borealis/Australis." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 13, 2007].
<http://www.cosmosportal.org/articles/view/135461/>

 

Glossary

Citation

(2009). Aurora Borealis/Australis. Retrieved from http://www.cosmosportal.org/view/article/135461

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