The Mystery of Coronal Heating   
   Published on: Nov 30, 2016   
       
   Imagine standing around a roaring campfire, roasting s'mores. You feel the   
   warmth of the flames as the marshmallows crackle. Now back away. You get   
   cooler, right?   
       
   That's not how it works on the sun. The visible surface of the sun has a   
   temperature of 10,000ø F.  Backing away from the inferno should cool things   
   down, but it doesn't.  Instead, the sun's upper atmosphere, or corona, sizzles   
   at millions of degrees - a temperature 200 to 500 times higher than that of   
   the roaring furnace below.   
       
   https://www.youtube.com/watch?v=RkUqX1TkiZo   
       
   For more than a half-century, astronomers have tried to figure out what causes   
   the corona to be so hot.  It is one of the most vexing problems in   
   astrophysics.   
       
   Solar physicist Bart De Pontieu of the Lockheed Martin Solar & Astrophysics   
   Laboratory says, "The problem of coronal heating was first discovered in the   
   1940s. The problem involves a variety of complex physical processes that are   
   difficult to directly measure or capture in theoretical models."   
       
   On June 27, 2013, with campfires blazing around the USA, NASA launched the   
   Interface Region Imaging Spectrograph (IRIS) - a space-based solar observatory   
   designed to get to the bottom of how the solar atmosphere is heated.   
       
   "IRIS studies the transition region between the sun's surface and the corona,"   
   explains De Pontieu, who is the science lead of the observatory. "It can track   
   the temperature and motions of hot gas at unprecedented spatial (0.33 arcsec),   
   temporal (2 s) and spectral (2 mi/s) resolution."   
       
   Most researchers agree that the corona is probably heated in several different   
   ways. For instance, plasma waves from the sun can rise into the corona and   
   crash, depositing their energy there. At the same time, "heat bombs" could be   
   going off. These explosions happen when magnetic fields in the corona   
   criss-cross and realign, exploding like a miniature solar flare.   
       
   One of the big questions of coronal heating has been: Is the corona heated   
   everywhere at once, or is heat delivered in discrete, bomb-like events?   
       
   De Pontieu says, "These two possibilities are very different, but the   
   distinction can be difficult to observe."   
       
   The problem is the corona is a great thermal conductor. If a heat bomb goes   
   off, the resulting heat rapidly spreads out over a large region. Blink, and it   
   looks much the same as uniform heating.   
       
   Fortunately, IRIS never blinks. A recent observation by the observatory's   
   spectrographs has found evidence for these discrete, explosive events.   
       
   Paola Testa of the Harvard-Smithonian Center for Astrophysics, lead author of   
   the paper reporting the results says, "Because IRIS can resolve the transition   
   region ten times better than previous instruments, we were able to see hot   
   material rushing up and down magnetic fields in the low corona. This is   
   compatible with models from the University of Oslo, in which magnetic   
   reconnection sets off heat bombs in the corona."   
       
   Testa emphasizes that other heating mechanisms may be at work, too. Even so,   
   these new observations could help tease out how much of the heating comes from   
   discrete heating events, helping researchers sort out a decades-old puzzle of   
   great complexity.   
       
   For more news about big mysteries, stay tuned to science.nasa.gov.   
       
       
   Regards,   
       
   Roger   
      
   --- DB 3.99 + W10 (1607)   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)