Aug 31, 1999

Solar Activity Heats Up





A coronal mass ejection this weekend could trigger auroral displays.


coronal mass ejection
August 31, 1999: This weekend the Sun issued a reminder to anyone who might have forgotten that solar maximum is right around the corner. On Saturday, August 28 at 18:05 UTC, a major solar flare erupted from a complex sunspot group crossing the sun's southern hemisphere. The x-ray flux from the explosion registered more than 10-4 Watts per square meter on NOAA's GOES 8 satellite, placing the flare in the most powerful "X" category. A somewhat weaker "M" class flare erupted from the same sunspot group on Friday, August 27.

Right: These images captured by SOHO's LASCO C2 coronagraph show the coronal mass ejection that accompanied Saturday's solar flare. Click on the image for an even .

Saturday's major solar flare was accompanied by a coronal mass ejection (CME), pictured above. The CME was not ejected directly toward Earth and most of the mass passed south of Earth's orbital plane. However, there is a chance that the outer edges of the disturbance will collide with our planet's magnetosphere around 12 UTC on August 31. There is no danger to satellites, power grids, or people, but there could be isolated episodes of intense auroral activity for 24 to 72 hours. Residents of Alaska and high northern latitudes are encouraged to watch for colorful auroral displays. The best times to observe are prior to and near local midnight, before the bright gibbous Moon rises in the east.


Left: This movie spanning 6 hours of actual time shows recent images of the Earth's auroral region taken from space by the Ultraviolet Imager (UVI) Instrument on board the POLAR spacecraft. New movies are generated every 6 hours. Hit reload for the latest animation or visit the UVI aurora home page for new pictures every 7 minutes.

The coronal mass ejection heading past Earth now is moving at about 600 km/s. That's not unusually fast as coronal mass ejections go. Previous CME's have been seen expanding away from the sun at speeds as high as 2000 km/s, and they can carry up to ten billion tons of plasma. When CME's collide directly with Earth they can excite geomagnetic storms, which have been linked to satellite communication failures. In extreme cases, such storms can induce electric currents in the earth and oceans that can interfere with or even damage electric power transmission equipment.


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"During solar maximum we often have more than one coronal mass ejection every day," says Dr. David Hathaway, a solar physicist at the Marshall Space Flight Center. "The basic cause of CME's is fairly well understood. Like solar flares, they occur whenever there's a rapid, large-scale change in the magnetic field. Solar flares and CME's often occur together, as they did this weekend, but not necessarily because the flare triggers the CME or vice versa. One can happen without the other and frequently during solar maximum we see CME's without an associated flare."

"Although we understand the basics of why CME's happen," he continued, "the details are still unclear. What makes the fields unstable? How rapid is the onset of the explosion? What's the detailed relationship between flares and CME's? All these questions are being actively researched, and we still can't predict CME events with any reasonable degree of accuracy."
white light image from Big Bear Solar Observatory
Left: A white light image of the sun from the Big Bear Solar Observatory, captured less than 2 hours before the August 28, 1999 X-class solar flare.

Space weather forecasters at NOAA's Space Environment Center have been monitoring active region 8674, which produced the latest crop of solar flares and the CME, for over a week. Since it first rotated into view, the sunspot group exhibited a complex magnetic field structure, including "delta field configurations," indicative of likely flare activity.

"Solar eruptions are clearly associated with sheared and twisted magnetic fields," continued Hathaway. "Whenever we see a 'delta configuration' -- that is, a sunspot where opposite magnetic poles are contained within the same penumbra -- it means something's probably about to go haywire. The trick to predicting the explosion lies in being able to look at the detailed geometry of the field around the sunspot group. Eventually, the experimental solar vector magnetograph facility here at Marshall and the vector magnetograph to be launched on the Solar-B mission may prove very useful for forecasting big eruptions.


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With solar maximum just around the corner -- Hathaway and colleagues predict it will occur in mid-2000 -- solar observers should have plenty of opportunity to study solar flares and CME's, and to hone their forecasting skills.

For more information about space weather and current solar activity, please see NOAA's Space Environment Center web site at

Below: Solar x-ray emissions received by the GOES satellites indicate 3 major solar flares between August 28 and August 31, 1999. Image Credit: NOAA Space Environment Center.


of the NOAA Space Environment Center


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Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: John M. Horack