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Geomagnetic Storm

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see captionApril 7, 2000 -- As stargazers ventured outside on the evening of April 6 for a view of Mars, Jupiter, Saturn and the Moon clustered in the western sky, many observers were also treated to a rare and unexpected display of aurora borealis.

"[We are] seeing stunning Aurora Borealis here in the Netherlands!" wrote Peter Lipton from Amsterdam. "You don't see the Northern Lights very much down here [in] Amsterdam, and it is the first time I've ever seen a decent display .... the complete northern sky was lit up red and white. This lasted for 10 minutes and then faded and moved on westward."

Right: Observers reported scenes like this one in northern Europe, Canada, Alaska and in the continental United States as far south as North Carolina last night. This picture of aurora borealis in Alaska was captured by Jan Curtis during a magnetic storm on February 24 using Fuji's 800 NHGII print film, 35 mm lens at f/2.0 (exposure between 5-12 seconds). [more pictures from Jan Curtis's Northern Lights Pictures web site]

"Who said nothing ever happens when planets line up?," wrote another aurora-watcher. "This is great!"

In fact, the dazzling display of Northern Lights had nothing to do with Thursday night's picturesque alignment of 3 planets with the slender crescent Moon. It all started earlier in the day, at 1600 UT on April 6, when NASA's ACE spacecraft detected an interplanetary shock wave headed toward Earth. About an hour after the wave passed ACE, it reached our planet. The magnetosphere -- a region of space around Earth controlled by our planet's magnetic field -- forms a shield that normally protects Earth from solar wind storms. This gust of solar wind was so strong that it compressed the magnetosphere and triggered the ongoing geomagnetic storm.

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Above: These data from NASA's Advanced Composition Explorer (ACE) spacecraft show the speed of the solar wind measured 1.5 million km from Earth at the L1 Sun-Earth libration point. The discontinuity, where the wind velocity jumps from 375 km/s to nearly 600km/s, marks the passage of an interplanetary shock wave caused by a solar coronal mass ejection on April 4, 2000. Events detected by ACE usually reach Earth about an hour later.

The material in the shock front left the Sun two days ago. The Solar and Heliospheric Observatory captured pictures of a full halo coronal mass ejection (CME) at 1541 UT on April 4, 2000. Coronal mass ejections can carry up to 10 billion tons of plasma traveling at speeds as high as 2000 km/s. "Halo events," like the one on April 4, are CMEs moving almost directly toward Earth. As they loom larger and larger they appear to envelope the Sun, forming a halo around our star.

see captionRight: This sequence shows a coronal mass ejection billowing away from the Sun after the onset of a solar flare on April 4, 2000. The solid colored circle in the middle is the occulting disk of the Solar and Heliospheric Observatory's C3 coronagraph. It blocks out the intense light of the Sun so that the tenuous corona is visible. The C3 coronagraph is able to observe the Sun's corona between 3.5 to 30 solar radii. [click for a more complete animation]

As fast-moving material from a CME flows away from the Sun, it piles up against slower-moving gas that had been ejected earlier. This produces a sharp, dense shock front like the one ACE detected yesterday. The shock wave from the eruption on April 4 traveled two days through interplanetary space before reaching Earth. When it arrived it triggered aurora so bright that they could be seen from a brightly lit parking lot in Yonkers, New York.

"At 8:15 p.m., I was out in the brightly-lit parking lot of News 12 Westchester with cameraman Barry Gerber taking a shot of the Moon and planets for our 10:00 Night Edition show, when I took a glance overhead and saw a distinct glow about 15 to 20 degrees in diameter, shining with a beet-red-type hue," recounts Joe Rao, a well-known sky watcher and meteorologist for Channel 12 News in Westchester, NY. "I suspected that this might be an aurora, but because of the surrounding lights I could not really be sure. I called my wife at our home in Levittown, Long Island and asked her to go into our backyard and tell me if she saw anything unusual. About 30 seconds later she returned excitedly saying that there was a 'crimson red glow high in the southern sky and nearly overhead.'"

In the United States aurorae were reported as far south as North Carolina.

"This certainly appears to be the largest storm of the year," said Dr. Jim Spann of the NASA/Marshall Space Flight Center. Spann is a co-investigator on the ultraviolet imager carried by NASA's Polar satellite, which monitors auroral activity from Earth orbit. Measurements of geomagnetic activity like the Earth experienced last night can teach scientists a great deal about how our planet's magnetosphere reacts to shock waves from the Sun. Spann is busy sifting through data from Polar now for a closer look at what happened during the storm.

More to Come?

"We're not sure if the storm will be as strong tonight," says Gary Heckman, a space weather expert at the NOAA Space Environment Center. "[It looks like] it's about over but typically these storms make a comeback for us in the Western Hemisphere when darkness rotates back around again. So there is a chance of more sightings tonight [April 7] but probably not as likely as last night. I plan to look between 10 pm and 2 am just to see."

"This storm reached G4 on the Space Weather Scale," he continued. "The great storm of March 1989 would have been a robust G5 for two nights running. So maybe the activity we expect with the solar maximum period is now heating up."

If you plan to be outside tonight, the best time to watch for Northern Lights is around midnight local time (local time is the time wherever you live). The Moon is only three days past new on April 7, so skies will be dark outside urban areas. Even faint auroral emissions could be visible.

For more information about the ongoing geomagnetic storm, visit the NOAA Space Environment Center web site or SpaceWeather.com. SOHO is a cooperative project between the European Space Agency (ESA) and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA.

Web Links

NOAA Space Environment Center -official forecaster of space weather events

SpaceWeather.com -daily updates and news about solar flares, coronal mass ejections and geomagnetic activity

NOAA Space Weather Scales -- find out what a "G4 storm" really means.

SOHO home page -real-time images, screen savers, and more

Thursday's Classroom -- lesson plans and educational activities about space weather.