Scientists funded by NASA have made big strides in learning how toforecast "all clear" periods, when severe space weather is unlikely.The forecasts are important because radiation from particles from thesun associated with large solar flares can be hazardous to unprotectedastronauts, airplane occupants and satellites.
"We have a much better insight into what causes the strongest, mostdangerous solar flares, and how to develop forecasts that can predictan 'all clear' for significant space weather, for longer periods," saidDr. Karel Schrijver of the Lockheed Martin Advanced Technology Center(ATC), Palo Alto, Calif. He is lead author of a paper about theresearch published in the Astrophysical Journal.
Solar flares are violent explosions in the atmosphere of the suncaused by the sudden release of magnetic energy. Like a rubber bandtwisted too tightly, stressed magnetic fields in the sun’s atmosphere(corona) can suddenly snap to a new shape. They can release as muchenergy as one, 10 billion megaton nuclear bomb.
Predicting space weather is a complicated problem. Solar forecastersfocus principally on the complexity of solar magnetic field patterns topredict solar storms. This method is not always reliable, because solarstorms require additional ingredients to occur. It has long been knownlarge electrical currents must be present to power flares.
Insight into the causes of the largest solar flares came in twosteps. "First, we discovered characteristic patterns of magnetic fieldevolution associated with strong electrical currents in the solaratmosphere," said ATC's Dr. Marc DeRosa, co-author of the paper. "It isthese strong electrical currents that drive solar flares."
Subsequently, the authors discovered the regions most likely toflare had new magnetic fields merge into them that were clearly out ofalignment with the existing field. This emerging field from the solarinterior appears to induce even more current as it interacts with theexisting field.
The team also found flares do not necessarily occur immediately uponthe emergence of a new magnetic field. Apparently the electricalcurrents must build up over several hours before the fireworks start.Predicting exactly when a flare will happen is like studyingavalanches. They occur only after enough snow built up. Once thethreshold is reached, the avalanche can happen anytime by processes notyet completely understood.
"We found the current-carrying regions flare two to three times moreoften than the regions without large currents," Schrijver said. "Also,the average flare magnitude is three times greater for the group ofactive regions with large current systems than for the other group."
The researchers made the discovery by comparing data about magneticfields on the sun’s surface to the sharpest extreme-ultraviolet imagesof the solar corona. The magnetic maps were from the Michelson DopplerImager (MDI) instrument on board Solar and Heliospheric Observatory(SOHO) spacecraft. SOHO is operated under a cooperative mission betweenthe European Space Agency and NASA.
The corona images were from the NASA Transition Region and CoronalExplorer spacecraft (TRACE). The team also used computer models of athree-dimensional solar magnetic field without electrical currentsbased on SOHO images. Differences between images and models indicatedthe presence of large electrical currents.
"This is a result that is more than the sum of two individualmissions," said Dr. Dick Fisher, Director of NASA's Sun-Solar SystemConnection Division. "It's not only interesting scientifically, but hasbroad implications for society."
For imagery about the research on the Web, visit:
http://www.nasa.gov/vision/universe/solarsystem/clear_weather.htmlFor information about NASA and agency programs on the Web, visit:
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