updated 9/16/2005 8:54:55 PM ET 2005-09-17T00:54:55

A huge sunspot has been blasting Earth with magnetic clouds for weeks, producing some of the most vibrant and visible summertime auroras in years, according to NASA scientists.

Scientists said the magnetic flare-ups from Sunspot 798 may last through the weekend.

"It is a fairly large geomagnetic storm that we've had over the past 24 hours, and it should continue a little while longer," said aurora researcher Dirk Lummerzheim, at the Geophysical Institute at the University of Alaska Fairbanks.

Skywide northern lights have awed Alaskans since last week and produced red displays as far south as Arizona. However, current forecast maps predict the auroras will not be visible south of southern Canada.

A North Pole photographer who signed e-mail messages as "Santa" posted a dazzling picture of a display on Thursday on the Geophysical Institute's online aurora forum.

The shimmering green light came courtesy of Sunspot 798, which sent a gusting magnetic cloud hurtling toward Earth at more than a million miles per hour.

Sunspots are planet-sized splotches formed by the sun's roiling magnetic field. The sunspots become unstable and explode, producing flares and propelling charged particles and radiation into space.

Sunspot activity can produce a geomagnetic storm that makes regular daily auroral activity much more visible than usual.

Solar scientists say the sun is supposed to be in the quiet phase of its 11-year cycle, with sunspot activity close to minimum.

But the year has so far produced four severe geomagnetic storms and 15 extreme flares.

"The sunspots of 2005, while fewer, have done more than their share of exploding," said solar physicist David Hathaway, of the National Space Science and Technology Center in Huntsville, Ala.

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