This week, an intense solar storm produced an aurora borealis that extended well south of the Arctic Circle. Commonly known as the northern lights, the aurora is produced by the interaction of solar wind with the Earth's magnetic field. This wind, a perpetual stream of protons and electrons, is produced at the surface of the sun but its many sources (most closely associated with sunspots) are not spaced evenly across the globe; since the sun revolves every 28 days, the intensity of the solar wind varies through that period. Solar storms, known as flares, acutely increase the intensity and speed of the solar wind, which takes about 2 days to reach the Earth; the sighting of a solar flare thus indicates that the aurora will be especially intense and widespread two days later.
Giving the appearance of shimmering curtains or swirling veils of light, the aurora borealis is most often green in color but may be pink, yellow, orange, blue or, rarely, bright red; the altitude of its formation (generally 60-70 miles above the surface of the Earth) and the molecular composition of the atmosphere determine the color of each display. Though often called polar lights, the aurora borealis and its southern counterpart, the aurora australis, form in bands just north of the Arctic Circle and just south of the Antarctic Circle, respectively; during intense solar flares, these bands expand and the light displays are observed well into the Temperate Zones.
Aside from the occasional solar storms, the background solar wind waxes and wanes over an 11 year cycle, with aurora events most common and long-lasting during periods of increased sunspot activity. Throughout the cycle, the aurora is best viewed late at night and tends to be most intense near the spring and autumn equinoxes. This week's display brought a taste of the Arctic to many humans who have never entered its realm.