'Extreme' aurora predicted tonight, weather permitting

Anchorage Daily NewsSeptember 9, 2011 

If the skies are clear, tonight may offer particularly good aurora viewing in Anchorage and over most of Alaska.

The Geophysical Institute at the University of Alaska Fairbanks is forecasting a level 5 intensity on the planetary magnetic index, or Kp scale.

"We first noticed a coronal mass ejection yesterday," said Hans Nielsen, associate director of the Institute. "Earlier today, a very high level of magnetic activity started. I think it was up at 7 at one point."

The Kp scale goes from 0 to 9, with 0 meaning no activity and 9 meaning the most extreme activity. However, Nielsen said, the high number doesn't necessarily mean good viewing in Alaska because the energy moves lower on the globe.

"If you have a 9 you don't get anything in Alaska," he said. "It's all 'way down in the lower 48."

The magnetic storm reaching earth on Friday is expected to last into the night. "Weather permitting, highly active auroral displays will be visible overhead from Barrow to Bethel, Dillingham and Ketchikan, and visible low on the horizon from King Salmon," read the Institute's aurora forecast site.

The site explains that the short term forecast is a calculation based on measurements of the solar wind made at a satellite that gets the readings about one hour before they reach earth.

The best time to observe aurora is near local midnight, says the site. "More precisely, the time to shoot for is an hour or two prior to local geomagnetic midnight, and the forecast maps found here are calculated for that time. If you are a serious aurora watcher, plan to spend the night from about 9 P.M. to 3 A.M. watching for auroral action."

Which won't be happening in Fairbanks, Nielsen said. "It's overcast here."

Weather reports call for obscured skies over much of Alaska tonight. But there may be patches of relatively clear viewing between clouds rolling in from Siberia. The forecast for much of Southcentral Alaska, including Anchorage, is for partly cloudy skies this evening becoming cloudy.

The Geophysical Institute is forecasting moderate aurora activity on Saturday and active northern lights on Sunday.

Reach Mike Dunham at mdunham@adn.com or 257-4332.

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