Aurora
"The northern lights have seen queer sights / but the queerest they ever did see / Was that night on the marge of Lake Lebarge / I cremated Sam McGee."
So the American poet Robert Service began his classic "The Cremation of Sam McGee," one of many poems he wrote about the arctic, a land which often witnesses the shimmering curtains of light called the aurora borealis.
As a widespread and highly visible phenomenon, the aurora has certainly been known to humans since long before written records developed. Often greenish but occasionally reddish-pink, aurorae occur chiefly in the northern latitudes and can over the entire sky. Probably aurorae instilled both awe and fear in the minds of these ancient peoples, who regarded celestial events as divine doings, often presaging unhappiness or ill fortune.
Written records of the occurrence of aurorae can be traced back to the ancient Chinese and Greek civilizations, but an understanding of what they were and what caused them had to wait much longer. With the advent of spectroscopy in the nineteenth century, it became possible to examine what elements were responsible for producing auroral light, and developments in the twentieth century of the theory of atomic structure and the nature of solar activity and the Earth's magnetic field allowed scientists to understand the cause of aurorae.
In the first half of the twentieth century, the astronomers Edward Milne and Eugene Parker developed our understanding of the solar wind, a stream of charged particles flowing outward from the Sun through the solar system in all directions. Rocket-borne experiments in the 1950s, led in part by James Van Allen, eventually led to a firm understanding of the origin of aurorae. When the solar wind reaches Earth, the charged particles are deflected by the Earth's magnetic field, streaming around our planet and into the space beyond. However, during periods of intense solar activity, including the outbursts of solar flares, the intensity of the solar wind may increase by a factor of ten or more. The magnetic field cannot deflect all the particles; a certain number of them "leak" through and stream down the magnetic field lines to where they intersect Earth's surface, near its north and south magnetic poles. These magnetic poles are not in the same place as the Earth's rotational poles, but they are located at high latitudes, explaining the tendency of aurorae to be seen very far north or south on the globe, but rarely in regions nearer the equator.
When the high-energy particles from the solar wind impact Earth's atmosphere, they ionize some of the atoms in the upper atmosphere. The recombination of the stripped e electron with an ionized atom of oxygen yields the greenish light so characteristic of aurorae, while the recombination of hydrogen leads to a redder glow. The light will persist as long as sufficient ionization exists to create a visible glow. In addition to the light show, however, the disrupted state of the atmosphere can interrupt radio transmissions, a source of concern to aviators among others.
In the best tradition of science, the ancient observers who recorded instances of aurorae were contributors to modern studies of global climate change. Solar activity could not be observed directly until the invention of the telescope in 1610 and the first observations of sunspots by Galileo. However, since aurorae are more frequent during periods of high solar activity, the frequency of aurora observations in ancient records can be used to trace (albeit somewhat crudely) solar activity levels back as far as the time of the Roman Empire. Since changes in the character of the solar activity cycle now appear to be linked in some way to long-term terrestrial climate change, these auroral observations have served as a useful tool for reconstructing the history of our Sun's behavior over the last two millenia.
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