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Solar Deja Vu

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Solar Déjà Vu

SOHO records a dazzling series of rapid-fire coronal mass ejections on the Sun

coronal mass ejections recorded by SOHOOct 15, 1999: When Yogi Berra made his famous remark about "déjà vu all over again," he probably didn't have solar eruptions in mind. But this week solar physicists did a double take, and then a triple take, as the sun produced a rapid-fire series of coronal mass ejections. Even Yogi would have been impressed.

Right: These images captured by SOHO's LASCO C2 coronagraph on Tuesday, October 12 show two beautiful coronal mass ejections erupting from the east (left) limb of the sun. In a more detailed animation a smaller CME can be seen on the west (right) limb occurring about midway in time between the two larger ones.

Late on Tuesday, October 12, the space-based Solar and Heliospheric Observatory (SOHO) photographed two dramatic coronal mass ejections (CMEs) in a 8 hour period. Just 6 hours later, on October 13, the spacecraft's coronagraph captured another dazzling CME. Coronal mass ejections can carry up to 10 billion tons of plasma traveling at speeds as high as 2000 km/s. 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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Fortunately, none of the latest CMEs are headed in our direction. Earth-directed mass ejections produce what astronomers call "Halo events." As they loom larger and larger they appear to envelope the Sun itself. The recent batch of CMEs -- mostly seen in profile -- appears to be expanding tangled loops.

"Without SOHO we might not have seen these at all," says David Hathaway, a solar physicist at the NASA/Marshall Space Flight Center. "The coronagraphs on SOHO are simply indispensable for monitoring this kind of activity. None of these CMEs are headed our way, but they sure are pretty!"

SOHO (the Solar and Heliospheric Observatory) is a mission of international cooperation between NASA and the European Space Agency.

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"During solar maximum we often have more than one coronal mass ejection every day," continued Hathaway. "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 sun's 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."

SOHO CME animationLeft: Another dazzling CME captured by SOHO on Oct 13, 1999

"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."

Space weather forecasters at NOAA's Space Environment Center are currently monitoring active region 8731, a sunspot group exhibiting a complex "beta-gamma" magnetic field structure. A significant 'X-class' flare recorded at 9:01 UT on October 14 was associated with this region. (X-class flares are significant eruptions that register an x-ray flux in the 1 to 8 Angstrom band exceeding 10-4 Watts per square meter on NOAA's GOES 8 satellite.) The flare was accompanied by yet another CME shown in the animation below. Unlike the others seen earlier this week, this CME is Earth-directed. According to the Solar-Terrestrial Dispatch and NOAA's Space Environment Center the velocity of the ejected mass was approximately 700 kilometers per second near the Sun. The speed of the shock front will gradually decrease as the disturbance travels through interplanetary space. By the time it reaches Earth, the disturbance is expected to be weak compared to the background solar wind. It should produce few if any observable effects.

Below: This animation from SOHO's Extreme Ultraviolet Imaging Telescope shows the progress of a flare that erupted at 901 UT on October 14, 1999. An earth-directed coronal mass ejection can also be seen.

"Solar eruptions like these 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. Beta-gamma configurations are not as complicated as deltas, but they are still complex. 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.

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, including official alerts, warnings, and forecasts, please see NOAA's Space Environment Center web site at http://www.sec.noaa.gov/.

SOHO (the Solar and Heliospheric Observatory) is a mission of international cooperation between NASA and the European Space Agency. It is managed by the Goddard Space Flight Center for the NASA HQ office of Space Science.

Below: Solar x-ray emissions received by the GOES satellites indicate an X-class solar flare on October 14, 1999. Image Credit: NOAA Space Environment Center.

courtesy of the NOAA Space Environment Center

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