From: baalke@zagami.jpl.nasa.gov   
      
   Microflares on Sun could play major role in heating corona   
   University of California Berkeley News Release   
   July 21, 2003   
      
   The Sun's big, bright, explosive flares are the attention grabbers,   
   but tiny, more numerous microflares may have nearly as much influence   
   on the solar atmosphere, according to new data from the University of   
   California, Berkeley's RHESSI satellite.   
      
   Solar flares, the largest explosions in the solar system, propel energetic   
   particles into space and are thought to be the main source of heat pumping the   
   Sun's outer atmosphere to a few million degrees Celsius - hotter than the   
   surface itself.   
      
   Now, solar observations by the RHESSI (Reuven Ramaty High-Energy Solar   
   Spectroscopic Imager) satellite show that microflares a million times smaller   
   are far more frequent and may together provide a major portion of the heat in   
   the corona.   
      
   "The big question for microflares is, are there enough of them? Do they occur   
   frequently enough and dump enough energy into the corona?" said Robert Lin,   
   professor of physics at UC Berkeley and principal investigator for RHESSI.   
   "RHESSI can see these tiny flares to lower energies than before, and our   
   observations are beginning to show that there is more energy released in these   
   tiny flares than people had originally thought."   
      
   Since solar flares play a major role in space weather, RHESSI's discoveries   
   about flares and microflares could eventually help predict the big storms that   
   interfere with radio communications on Earth.   
      
   Lin will present new data from RHESSI in a talk at 3:30 p.m. on Monday, July   
   21, at the meeting of the International Astronomical Union in Sydney,   
   Australia.   
      
   RHESSI, launched by NASA in February 2002 to study X-ray and gamma-ray   
   emissions from flares, has observed more than 10,000 microflares in the past   
   year and a half. These microflares are identified by the hard X-rays they emit,   
   which RHESSI is able to detect with 10 to 500 times the sensitivity of any   
   previous instruments flown in space.   
      
   These X-ray observations show that microflares are merely smaller versions of   
   their larger cousins, Lin said. Some astronomers have suggested that   
   microflares may be mainly thermal events, heating the Sun but not accelerating   
   particles like larger flares. If that were the case, they would produce more   
   low-energy soft X-rays than high-energy hard X-rays. But they do not.   
      
   "We've noticed that microflares are very similar to big flares. In big flares,   
   a lot of the energy, perhaps most of it, comes out in accelerated particles -   
   electrons, protons and heavy nuclei," Lin said. "We are finding the same to be   
   true of microflares."   
      
   Interestingly, a subset of microflares appears to be a different animal   
   entirely   
   and responsible for a type of radio burst from the Sun studied intensively by   
   pioneering Australian radio astronomer Paul Wild in the 1960s and 1970s.   
   These so-called Type III bursts are characterized by radio signals that   
   decrease in frequency, like the whistle from a departing train.   
      
   RHESSI has seen many Type III bursts, and they appear to be associated with   
   microflares that do very little heating of the solar atmosphere. Instead, the   
   stream of high-speed particles they produce seems to jet unchecked out of the   
   Sun at speeds up to one-third the speed of light, exciting radio oscillations   
   at   
   lower and lower frequencies as the particles pass through lower and lower   
   density plasma.   
      
   "This probably has to do with the magnetic field in the region around the   
   microflare, since particles are pretty much tied to the field lines and have to   
   run along them," Lin said. "We think that for normal microflares, the particle   
   acceleration occurs in a closed magnetic region so the electrons can't get   
   away;   
   they do more heating that way. In Type III bursts, the electrons are   
   accelerated   
   in an open magnetic field, and they have an easy way to escape, so they do less   
   heating in the corona."   
      
   Aside from RHESSI's numerous observations of microflares, the satellite's   
   X-ray and gamma-ray instruments have also captured several large flares.   
   These have allowed the RHESSI team to investigate the relationship between   
   flares and coronal mass ejections (CME), which are another type of large   
   stellar explosion that sends shock waves into space. One conclusion, Lin said,   
   is that the fastest coronal mass ejections - those moving at 1 to 5 million   
   miles   
   per hour (1.6 to 8 million kilometers per hour) - are linked directly to solar   
   flares.   
      
   "With RHESSI, we can image the location of a flare's initial release of energy   
   and accelerated particles," Lin said. "When we look at extremely big and fast   
   coronal mass ejections and extrapolate back to the Sun, we find that at the   
   very   
   point where the coronal mass ejection is initiated, that is exactly where the   
   flare energy release happened. The flare starts everything off."   
      
   These largest of the mass ejections are the ones that have the greatest effect   
   on Earth, exciting geomagnetic storms that can cause power outages and   
   damage communications satellites. The shock wave from coronal mass   
   ejections also produces energetic particles that pose a hazard to satellites   
   and   
   astronauts.   
      
   "If we understood the process, we could begin predicting when coronal mass   
   ejections should happen," Lin said. "We're still a long way from that, but it   
   makes it extremely interesting to discover the relationship between flares and   
   coronal mass ejections."   
      
   It is still unclear whether other types of coronal mass ejections are related   
   to   
   solar flares, he said.   
      
   Both flares and coronal mass ejections are produced by the roiling magnetic   
   fields in the surface of the star. As the surface churns, magnetic field lines   
   get twisted like rubber bands. When the tension becomes too great, they break,   
   snapping and flinging charged particles outward in a solar flare.   
      
   Flares can trigger coronal mass ejections, which are massive rising bubbles of   
   plasma entangled with the magnetic field. But some mass ejections seem   
   unrelated to flares, Lin said. One possible explanation is that these come from   
   magnetic fields that kink as they twist, so the magnetic field intensity   
   doesn't   
   get compressed enough to explode into a flare.   
      
   "In this case, the magnetic fields slowly kink and eventually start to rise,   
   dragging plasma with it them," he said. "They're not associated with a flare   
   because they don't break suddenly.   
      
   "The very fast, powerful CMEs are probably the breaking kind."   
      
   RHESSI will continue its observations of solar flares for at least another two   
   years, and probably longer.   
      
   The RHESSI scientific payload is a collaborative effort among UC Berkeley,   
      
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