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)