This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell is a science writer at the institute.

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A black spot on the sun will probably give Alaskans some great auroras for about five days beginning March 21, according the Charles Deehr, one of the Geophysical Institute scientists who forecasts auroras. The dark splotch on the x-ray image of the sun is known as a coronal hole, an area of the sun devoid of solar flares, sunspots, and anything else scientists can identify from our perch millions of miles away.

“I can’t say how far south the aurora is going to be, but it’s going to be active,” Deehr said of the period from March 21 to March 26, 2000.

Until Skylab astronauts photographed the sun with an x-ray camera in 1974, scientists knew little about coronal holes, according to Syun-Ichi Akasofu, director of the International Arctic Research Center. In 1905, British astronomer E.W. Maunder noticed that magnetic disturbances on Earth sometimes occurred about every 27 days. He also knew that the sun spins in a complete rotation every 27 days, so he thought that perhaps some region on the sun was causing auroras to peak when that region of the sun was facing Earth. Unlike sporadic solar flares—explosions on the sun that cause auroras—the magnetic activity Maunder noticed was predictable. In the 1930s, German scientist Julius Bartels confirmed Maunder’s finding. Bartels named the black spot the M-region, which some scientists took to mean “Mysterious region” or “Magnetically active region.”

The launch of the SOHO and Yohkoh satellites allowed scientists to fingerprint coronal holes and follow their movement. The satellites hover 150,000 kilometers (93,000 miles) in space, a point where the Earth and sun both tug at the satellite with the same gravitational force. As Earth rotates around the sun, the satellite remains stationed between the two and sends x-ray and ultraviolet images of the sun back to Earth.

In his office at the Geophysical Institute, Deehr recently pulled up a series of pictures of the sun taken by the satellite. The images showed a large dark blot that migrated across the surface of the sun until it faced Earth in late February. At that time, when the coronal hole faced Earth, auroras were outstanding. Auroras were also great about 27 days earlier, on Dec. 31, 1999.

“We started off the millennium with zapping good aurora,” Deehr said. “Too bad it was 50 below in Fairbanks and nobody noticed.”

Coronal holes are the Old Faithfuls of the sun’s surface. Though solar flares produce good auroras, scientists’ only chance to predict their effects come in the three days between their occurrence and their arrival at Earth’s ionosphere. Coronal holes can exist for one year or more, and holes such as the one now affecting Earth’s magnetism tend to come back with every rotation of the sun. If the electrical circuit between Earth and sun is oriented in a favorable way for aurora production—as it has been during the life of this coronal hole—an aurora forecaster’s job becomes easier, and a bit more fun.

Coronal holes create memorable aurora displays by spitting out a fast stream of electrons that react with gas molecules high in the ionosphere, from 60 to 200 miles above Earth. When the coronal hole faces Earth, as it will March 21-26, our planet is basically looking down the barrel of a gun that fires aurora. Try to get out and watch the show.

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