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A guide to last week's celestial light show

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The Inuit around Hudson Bay thought they were torches lit to guide the spirits of the deceased across the bridge between earth and the heavens. An old tale from Lapland attributes them to the mischievous arctic fox, who can light fires and spray snow with its brushlike tail. In their 1982 hit “I Ran,” Blade-coiffed new-wavers A Flock of Seagulls attribute their numinous appearance to a sort of trance state brought on by the mesmerizing looks of a girl “with auburn hair and tawny eyes.” Other European folk traditions have long held them to be omens of war, sickness and death.

But it was only in the past century that astronomers and physicists began to fine tune the notion that aurora borealis, or the Northern Lights, spring not from footlights for transient spirits or vulpine shenanigans, but from the magnetic interference of solar activity in the Earth’s atmosphere. The solar wind—a streaming gas of free electrons and protons formed by the violent disintegration of hydrogen at temperatures of several million degrees—escapes from the sun through holes in its magnetic field. Traveling at hundreds of kilometers per second, the plasma sweeps past Earth in the interplanetary magnetic field, and when some charged particles, particularly electrons, are drawn into our upper atmosphere by the Earth’s own magnetic action, they react with atmospheric gases to produce vivid curtains of color that cascade down along the magnetic field lines.

Last Friday’s fantastic aurora display, which peaked shortly before midnight when the cloud cover had mercifully lifted, was a singular pageant of gas and dancing particles, with sheets of yellow-green seemingly on every horizon and brilliant blossoms of reddish pink appearing to radiate from a spot high in the southwest. Auroral displays are always one-of-a-kind, but the reds and pinks that favored us over the weekend are quite uncommon, especially for such southerly latitudes. Each atmospheric gas glows with a distinctive color, the most common of which is the vivid yellow-green produced by low-altitude oxygen at about 60 miles up—several times higher than the highest-flying airplane. Ionized nitrogen produces blue light, while neutral nitrogen glows red; the comparatively rare all-red auroras are produced by high-altitude oxygen at about 200 miles up. Friday’s show was most likely a surf-and-turf of nitrogens and low-altitude oxygens—the most abundant gases in the atmosphere. If you were looking up, you saw sheets of shifting colors that corresponded with the spectrum lines of these gases in different states of ionization. That is to say, um, it was real purty.

According to Dr. David Friend, a professor of physics and astronomy at UM, we might be in for a bumper crop of auroral activity this year, thanks to the solar cycle which is currently peaking in intensity.

“It’s an 11-year cycle,” says Friend, reached at his campus office earlier this week. “Solar activity should stay high for another year or so, so we could continue to have auroras and radio interference.”

Kinda makes the Pink Floyd laser light show look like a booger, doesn’t it?

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