A. The aurora borealis, or northern lights, is among the natural world's most spectacular displays: shimmering curtains of green, red, and violet light that ripple across the polar night sky. For centuries the phenomenon inspired myth and legend. Some cultures saw the dancing lights as the spirits of departed ancestors, others as omens of coming war or disaster. Only in the modern era did scientists come to understand that the aurora is not supernatural at all, but the visible result of a continuous interaction between the Sun and the Earth's upper atmosphere. A matching display, the aurora australis, appears around the southern magnetic pole.

B. The lights are ultimately powered by the Sun, which constantly streams charged particles into space in what is called the solar wind. When these particles reach the Earth, most are deflected harmlessly by the planet's magnetic field, but some are funnelled downward toward the magnetic poles. There they collide with atoms of oxygen and nitrogen high in the atmosphere, often around 100 kilometres above the ground, transferring energy that the atoms then release as light. The colour of the aurora depends on which gas is struck and at what altitude: oxygen typically produces the familiar green and, more rarely, a deep red, while nitrogen can add faint tinges of blue and purple to the display.

C. Because the particles are drawn toward the magnetic poles, auroras are seen most reliably within a ring-shaped zone at high latitudes, taking in places such as northern Canada, Iceland, and Scandinavia. Clear, dark skies far from the glow of city lights give the best viewing, and the displays are naturally most visible during the long nights of autumn and winter. Auroral activity waxes and wanes with the Sun's roughly eleven-year cycle of storms. During periods of intense solar activity, the lights can occasionally be seen far south of their usual range, astonishing observers in unexpectedly temperate regions.

D. Beyond their beauty, auroras are a visible sign of what scientists call space weather, and studying them has genuine practical importance. The same solar storms that brighten the sky can also disrupt human technology, interfering with radio communications, satellite navigation, and even electrical power grids on the ground. Researchers now monitor the Sun continuously in order to forecast such storms, much as meteorologists forecast ordinary weather. For most travellers, however, the aurora remains chiefly a source of wonder, drawing thousands of people each winter to the far north in the hope of seeing the night sky come alive.

E. Not given in any of the above paragraphs.