Atmospheric Phenomena 511 Auroras

The aurora is a phenomenon widely experienced by people of high latitudes, although occasionally seen much closer to the equator. In the Northern Hemisphere, we call it the Aurora Borealis (or Aurora Polaris), "Northern Lights," and in the Southern Hemisphere, the Aurora Australis. The pale light, with its suggestions of blue, green, and sometimes red color, displays a variety of forms from pulsating parallel stripes to shimmering curtains (cf. Figure 5.1 for a particularly striking example).

The name derives from the Roman goddess of dawn, Aurora, corresponding to Eos in Greece. Her father, according to Hesiod,2 was the titan Hyperion, and among her children were the winds Zephyrus, Notus, and Boreas; Hesperus, the evening star; and the stars. In Homer's epic poems (e.g., the Iliad, Ch. 1) dawn is rosy-fingered, and is driven out of the east every morning in a horse-drawn chariot.

The Aurora could hardly have been missed by ancient observers, although they could not have had any idea of its true nature. It was not clearly recognized as a unique phenomenon in China (for good reasons, as we indicate later) and so had many names, such as "coloured emanations,"

2 Hesiod probably flourished in the 8th century b.c. and was reputedly from Boeotia, Greece.

Figure 5.1. The Aurora Borealis, as captured by Prof. J.S. Murphree, at Fort Yukon, Alaska: The exposure was 1/2s with 400 ASA Ektachrome slide film. Courtesy, J.S. Murphree.

"strange lights," "red vapour," "north polar light," and "cracks in the heavens" (Needham/Ronan 1981, p. 233). The "Historical Record," Shih Chi, refers to them as "vapours." Pang et al. (1988) suggest that the phrase "the sky rained blood," found in both ancient European as well as Chinese texts, is a description of a "Type A Red Aurora," which has a deep "blood-red" color (see Stothers 1979). Of the five types of color patterning, three involve red colors (A, B, and D), but only one is entirely red—type D. See Davis (1992) for descriptions and photographs of each type. The phenomenon is often shrouded in folklore and myth; so a detailed description of its physical properties is appropriate.

The aurora is caused by the interaction of energetic, charged particles with the Earth's upper atmosphere. The bulk of the charged particles, which are mostly electrons and protons, originate in the Sun and are expelled in a "solar wind'; they are normally deflected by the Earth's magnetic field, sometimes into separate toroidal regions of electrons and protons several thousand kilometers above the Earth's surface. These radiation belt regions are together known as the Van Allen belts, after their discoverer, James A. Van Allen (1914- ). After a large influx from the Sun, the particles may be "dumped" into the Earth's upper atmosphere along a broad annular region (the auroral oval) close to the geomagnetic poles.3 Thus, it is mainly a high-latitude phenomenon. Although satellite photographs reveal that aurorae are nearly always present at some level of brightness (cf. Figure 5.2), increases in auroral activity occur with

3 The positions of the magnetic poles vary somewhat irregularly with time over several hundred kilometers. The Northern geomagnetic pole is currently in western Greenland;the Southern, near Tasmania. The auroral oval is located at about 23° from this point and has a width of less than ~1000km. A further discussion of the effects of the changing geomagnetic field and its uses in archeological dating is given in §4.4.

Figure 5.2. A Viking satellite view of the auroral oval, visible on the dark side of Earth, but present on the sunlit side as well. Courtesy, Space Science Group of the University of Calgary, from research sponsored by the Canadian Space Agency.

"gusts" in the solar wind, which is the rain of charged particles that is constantly evaporated from the outer atmosphere of the Sun. Solar flares can produce dramatic effects when the energetic ions emitted during these eruptions reach the Earth because they trigger substorms, resulting in large-scale dumping into the upper atmosphere. Collisions cause ionization and excitation of atmospheric atoms and ions, and these interactions lead to light emission. The color comes from the species of ions or atoms that are doing the emitting; the numbers of the different species4 vary with height (as well as time) in the atmosphere—and so does their contribution to the light. The ions are constrained by lines of magnetic force, but can in turn alter them if sufficiently energetic. This accounts for their many curious patterns: rippling curtains, pulsating globs, traveling pulses, and many others. The aurora can vary in height from just below 100 to ~1000Km, with maximum emission typically coming from

4 The most important contributors of emission lines are the atomic and molecular forms of oxygen and nitrogen; lesser contributions come from hydrogen, sodium, calcium, and other constituents of the upper atmosphere.

about 110Km. Thus, the area of its visibility on the Earth's surface can vary greatly.

The occurrence of aurorae is regulated by the 11-year sunspot cycle (see §5.3.1). The numbers of sunspots vary with time, and as they increase, so does the frequency of spectacular aurorae. Consequently, there is both a seasonal and a long-term cyclicity in the appearance of the aurorae, but they are seen even at solar minimum, when sunspots are infrequent or even absent. In addition, the long winter nights of the high-latitude sites (where the aurorae are more frequent) make the phenomenon more visible in the winter. The Royal Navy's overland Arctic Land Expedition of 1820/1821 in northern Canada had among its duties the study of the Aurora Borealis. They recorded frequent auroral occurences (the journal of John Richardson, Richardson Houston, ed. 1984), despite the circumstance that the solar cycle was then near a minimum (Parker 1978, p. 20).

The North American Inuit, from Greenland to the Bering Strait, traditionally regard the aurora, which they describe as aqsarniit ("football players"), as the spirits of the dead who are playing football with the head of a walrus (MacDonald 1998, pp. 146-156). Such a description captures adequately both the relatively quiet and the rapid movements of the auroral streamers, the motions of which seem remarkably unpredictable although constrained by magnetic field lines. See §13.6 for a broader discussion of Inuit astronomy.

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