From: baalke@zagami.jpl.nasa.gov   
      
   Nancy Neal/Don Savage   
   Headquarters, Washington                 December 9, 2003   
   (Phone: 202/358-1547)   
      
   Bill Steigerwald   
   Goddard Space Flight Center, Greenbelt, Md.   
   (Phone: 301/286-5017)   
      
   RELEASE: 03-397   
      
   EXTENSIVE DESTRUCTION POWERS SOLAR EXPLOSIONS   
      
        Large-scale destruction of magnetic fields in the sun's   
   atmosphere likely powers enormous solar explosions, according   
   to a new observation from NASA's Ramaty High Energy Solar   
   Spectroscopic Imager (RHESSI) spacecraft.   
      
   The explosions, called solar flares, are capable of releasing   
   as much energy as a billion one-megaton nuclear bombs. The   
   destruction of magnetic fields, called magnetic reconnection,   
   was a leading theory to explain how solar flares could   
   suddenly release so much energy, but there were other   
   possibilities as well. The new picture from RHESSI confirms   
   large-scale magnetic reconnection as the most likely scenario.   
      
   "Many observations gave hints that magnetic reconnection over   
   large areas was responsible for solar flares, but the new   
   pictures from RHESSI are the first that are really   
   convincing," said Linhui Sui of the Catholic University of   
   America, Washington. "The hunt for the energy source of flares   
   has been like a story where villagers suspect a dragon is on   
   the loose because something roars overhead in the middle of   
   the night, but only something resembling the tail of a dragon   
   is ever seen. With RHESSI, we've now seen both ends of the   
   dragon." Linhui is lead author of a paper on this research   
   published October 20 in Astrophysical Journal Letters.   
      
   Magnetic reconnection can happen in the solar atmosphere   
   because it is hot enough to separate electrons from atoms,   
   producing a gas of electrically charged particles called   
   plasma. Because plasma is electrically charged, magnetic   
   fields and plasma tend to flow together. When magnetic fields   
   and plasma are ejected from the sun, the ends of the magnetic   
   fields remain attached to the surface. As a result, the   
   magnetic fields are stretched and forced together until they   
   break under the stress, like a rubber band pulled too far, and   
   then reconnect -- snap -- to a new shape with less energy.   
      
   The thin region where they reconnect is called the   
   reconnection layer, and it is where oppositely directed   
   magnetic fields come close enough to merge. Magnetic   
   reconnection could power a solar flare by heating the sun's   
   atmosphere to tens of millions of degrees, and accelerating   
   electrically charged particles that comprise the plasma   
   (electrons and ions) to almost the speed of light.   
      
   At such high temperatures, solar plasma will shine in X-rays,   
   and RHESSI observed high-energy X-rays, emitted by plasma,   
   heated to tens of millions of degrees in a flare on April 15,   
   2002. The hot, X-ray emitting plasma initially appeared in the   
   RHESSI images as a blob atop an arch of relatively cooler   
   plasma protruding from the sun's surface. The blob-and-arch   
   structure is consistent with reconnection, because the X-ray   
   blob could be heated by reconnection, and the part of the   
   magnetic field that breaks and snaps back to the solar surface   
   will assume an arch shape.   
      
   These structures have been seen before and hinted at   
   reconnection, but the observations were not conclusive.   
   However, as RHESSI made images of the 20-minute long flare,   
   over the course of about four minutes during the most intense   
   part of the flare, the X-ray emitting blob exhibited two   
   characteristics consistent with large-scale magnetic   
   reconnection.   
      
   First, the blob split in two, with the top part ultimately   
   rising away from the solar surface at a speed of about 700,000   
   miles per hour, or around 1.1 million km/hr. This is expected   
   if extensive reconnection is occurring, because as the   
   magnetic fields stretch, the reconnection layer also   
   stretches, like taffy being pulled. Plasma heated by   
   reconnection squirts out of the top and bottom of the   
   reconnection layer, forming the two X-ray blobs in the RHESSI   
   pictures, when the top and bottom are sufficiently far apart   
   to be resolved as distinct areas.   
      
   Second, in both blobs, the area closest to the apparent   
   reconnection layer was hottest, and the area farthest away was   
   coolest, according to temperature measurements by RHESSI. This   
   is also expected if reconnection is occurring, because, as the   
   magnetic fields break and reconnect, other magnetic fields   
   nearby move in to the reconnection region and reconnect as   
   well, since the overall, large-scale field continues to   
   stretch. Thus, plasma is continuously heated and blasted out   
   from the reconnection layer. The plasma closest to the   
   reconnection area is the most recently expelled and therefore   
   the hottest. Plasma farther away was ejected earlier and had   
   time to cool.   
      
   "This temperature gradient in the hot plasma was the clincher   
   for me," said Dr. Gordon Holman, a Co-Investigator on RHESSI   
   and co-author of the paper, at NASA's Goddard Space Flight   
   Center, Greenbelt, Md. "If some other process was powering the   
   flare, the hot plasma would not appear like this." For images,   
   movies, and more information, refer to:   
      
   http://www.gsfc.nasa.gov/topstory/2003/1209rhessi.html   
      
      
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