The Mystery of Nanoflares
 
March 19, 2015:  When you attach the prefix "nano" to something, it usually
means "very small." Solar flares appear to be the exception.
 
Researchers are studying a type of explosion on the sun called a 'nanoflare.' 
A billion times less energetic than ordinary flares, nanoflares have a power
that belies their name.
 
"A typical 'nanoflare' has the same energy as 240 megatons of TNT," says
physicist David Smith of UC Santa Cruz. "That would be something like 10,000
atomic fission bombs."
 
https://www.youtube.com/watch?v=wjB9JtTU7SU&feature=youtu.be
 
A new ScienceCast video explores the mystery of the sun's tiniest flares. Play
it
 
The sun can go days, weeks or even months without producing an ordinary solar
flare.  Nanoflares, on the other hand, are crackling on the sun almost
non-stop.
 
"They appear as little brightenings of the solar surface at extreme
ultraviolet and X-ray wavelengths," continues Smith. "The first sightings go
back to Skylab in the 1970s."
 
The relentless crackle of nanoflares might solve a long-standing mystery in
solar physics:  What causes the sun's corona to be so hot?
 
Imagine standing in front of a roaring fire.  You feel the warmth of the
flames.  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 5500 C.  Moving away from the surface should provide some
relief.  Instead, the sun's upper atmosphere, known as the "solar corona,"
sizzles at a million degrees--a temperature almost 200 times higher than that
of the roaring furnace below.
 
For more than a half-century, astronomers have tried to figure out what causes
the corona to be so hot.  Every year or so, a press release appears purporting
to solve the mystery, only to be shot down by a competing theory a year or so
later.  It is one of the most vexing problems in astrophysics.
 
http://www.nustar.caltech.edu/image/nustar141222a
 
X-rays stream off the sun in this image showing observations from by NASA's
Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken
by NASA's Solar Dynamics Observatory (SDO).  [more] Smith thinks nanoflares
might be involved. For one thing, they appear to be active throughout the
solar cycle, which would explain why the corona remains hot during Solar
Minimum.  And while each individual nanoflare falls short of the energy
required to heat the sun's atmosphere, collectively they might have no trouble
doing to job.
 
To investigate this possibility, Smith turned to a telescope designed to study
something completely different.
 
Launched in 2012, NASA's NuSTAR X-ray telescope is on a mission to study black
holes and other extreme objects in the distant cosmos. Solar scientists first
thought of using NuSTAR to study the sun about seven years ago, after the
space telescope's design and construction was underway. Smith contacted the
principal investigator, Fiona Harrison of the California Institute of
Technology in Pasadena, to see what she thought.
 
"At first I thought the whole idea was crazy," says Harrison. "Why would we
have the most sensitive high energy X-ray telescope ever built, designed to
peer deep into the universe, look at something in our own back yard?"
 
Eventually, she was convinced.  As Smith explained, NuSTAR has just the right
combination of sensitivity and resolution to study the telltale X-ray flickers
of nanoflares. A test image they took in late 2014 removed any doubt.  NuSTAR
turned toward the sun and, working together with NASA's Solar Dynamics
Observatory, captured one of the most beautiful images in the history of solar
astronomy.
 
The next step, says Smith, is to wait for Solar Minimum.  The current solar
cycle will wind down in the years ahead, leaving the sun mostly free of
sunspots and other magnetic clutter that can obscure nanoflares.  NuSTAR will
be able to survey the stellar surface and gather data on these explosions like
no telescope has done before.
 
Will it solve the mystery of nanoflares and the solar corona?  "I don't know,"
says Smith, "but I cannot wait to try."
 
Credits:
Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:
Science@NASA
 
 
Regards,
 
Roger

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