The Aurora: New Light on an Old Subject

Dr. Sten F. Odenwald (Raytheon STX and NASA Goddard Space Flight Center)

For thousands of years, humans have admired the spectacle of the 'Northern Lights' also known as the Aurora Borealis. The multi-colored curtains of light that, from time to time, play across the skies like phantasmogoric serpents, have been seen by Scandanavian Vikings, Eskimos, and even on exceptional occasions, by inhabitants of the Mediterranean and Japan. No one has felt neutral about the experience, and it is often cast as a forewarning of doom, or some incomprehensible battle among celestial beings. Today, astronauts can see auroras from the vantage point of space where it appears as an oval-shaped glowing donut over 5000 kilometers in diameter, centered on the north magnetic pole.

During the last few decades, scientific investigation of this natural phenomenon have uncovered many new insights to how auroral displays are produced, and that many other planets such as Jupiter and Saturn also share such a phenomenon. The basic process seems to involve a flow of energetic charged particles into the atmosphere in the polar regions which discharges as much as a million megawatts of energy into the upper atmosphere at altitudes from 70 to 1000 kilometers. These particles collide with atoms of oxygen and nitrogen to produce the characteristic red, blue and green colors of auroral displays.

In the mid-19th century, Anders Jonas Angstrom noted that there was a similarity between auroral displays and certain kinds of electrical discharges that could be studied under laboratory conditions. This was the first recognition that some kind of electrical discharge was responsible for producing auroras. This was in distinction to earlier popular ideas that auroras were reflections of light from ice crystals high up in the atmosphere, or that they were related to terrestrial lightning. It wasn't until around 1925 that spectroscopic investigations finally identified one of the atoms causing the distinctive greenish light: Oxygen. This particular light is only produced at a single wavelength near 5577 Angstroms, about mid-way through the familiar visible spectrum. It is a feature caused by oxygen atoms at very low gas densities being excited by a specific amount of energy.

Around the turn of the century, physicists and astronomers had identified certain prominent atomic emission lines in such objects as distant, interstellar gas clouds and even the solar surface. Such elements as 'nebulium', 'coronium' and 'geocoronium'. Following decades of spectral analysis, these emission lines were finally tracked down, all except for one. The element 'helium' was discovered in the solar spectrum before it was finally found on earth, however the remaining mysterious lines turned out not to be from exotic new elements, but from ordinary iron atoms. The coronium lines were found in the coronal regions of the sun high above the solar surface. Originally it was thought that they were produced by an even lighter element than hydrogen which makes up the bulk of the solar material. Instead, the emission lines attributed to coronium were found to come from iron atoms that had been stripped of 13 of their electrons!

Auroras are now known to be electrical phenomena triggered by high speed electrons that enter the upper atmosphere in powerful currents, following the magnetic field of the earth into the polar regions. These electrons collide with atoms of oxygen and nitrogen to stimulate them to emit specific wavelengths of light. The process works very much like a neon sign, in which a current of electrons passes through a low density neon gas inside the tube to stimulate the atoms to emit light.

Auroras can never touch the ground, contrary to the many reports handed down by folklore. The emission of the light requires very low density gas conditions so that the atoms do not become 'collisionally excited' into other states. Too many collisions in a high-density environment will eliminate the specific electronic transition needed to produce the specific auroral lines. The density of the atmosphere near the lower range of the auroral limit near 70 kilometers is nearly the same as what is found inside a neon bulb. At the upper range of the auroral display at 1000 kilometers, the atmosphere is even more rarified.

In April, 1741 Hiorter discovered from studies of the earth's magnetism that ,whenever a prominent auroral display occurred, the magnetic field of the earth in the vicinity of the aurora would be disturbed. By 1770, J.C. Wilcke discovered that prominent auroral rays tended to align with the direction of the earth's magnetic field. A prominent solar flair on September 1, 1859 was observed by Richard Carrington and at the same time, several miles away at a local magnetic observatory outside of London, a major disturbance in the earth's magnetic field was recorded. These separate clues revealed that aurora are not just pretty lights in the sky, but are indicators of a process which often begins on the sun as a solar storm. These storms emit particles which sometimes collide with the earth and produce currents that flow into the magnetic polar regions. Aurora result from these flows of particles, and these flows also modify the earth's magnetic field to produce magnetic 'storms'.

Because aurora are indicators of severe magnetic activity, they are often correlated with many problems that can arise with electrical equipment. Aurora produce their own forms of radio radiation that can interfere with long distance communication. The rapidly changing magnetic fields near the ground can induce electrical currents in power lines that result in power black-outs. On March 13, 1989 a major solar storm produced a dazzling auroral display that was observed as far south as Florida and Japan. It also caused a power blackout for 9 hours that affected 6 million people in Quebec. Even natural gas pipelines are affected. As auroral electrical currents flow along these pipelines, they produce enhanced corrosion which can have catastophic consequences. Although the Alaskan pipeline was specifically designed with proper insulation to reduce this corrosion, the Siberian natural gas pipeline was built much earlier without this safeguard. In 1990, a portion of the pipeline ruptured and flooded a small valley with the vapors of the liquid natural gas. When two passenger trains entered the valley, the conductors smelled the gas and seconds later the entire valley exploded sending over 500 people to their deaths.

One possible way of reducing the risk for such catastrophes is to devise a way to successfully forecast when such major auroral 'storms' will happen. NASA satellites such as SOHO, ACE, TRACE and others in planning are parked about 1.5 million kilometers towards the sun so that this front guard can sense an approaching storm and provide up to an hour's notice of a major storm approaching from the sun. Other satellites monitor the solar surface to watch for flares which transmit their influences at nearly the speed of light and arrive at the earth within 10 minutes. Scientists have begun to elevate 'Space Weather Forecasting' to a high-precision art form even though there is an inevitable aspect of random chance to the way that the sun produces these storms. In the future, we may have better ways of protecting ourselves from the disruptive aspects of auroral displays so that we can, once again, return to admiring their beauty with a restored piece of mind. V