This photo of the
Sun's edge, taken with the Solar Dynamics Observatory Atmospheric
Imaging Assembly, shows coronal loops in a variety of sizes. Although
the loops appear to have a constant width, like strands of rope, new
work suggests that this is an optical illusion. The loops are actually
tapered, wider at the top and narrower at the bottom. Credit: NASA/SDO. High Resolution Image (jpg) - Low Resolution Image (jpg)
Cambridge, MA - The
Sun's outer atmosphere, or corona, has posed an enduring mystery. Why
is it so hot? The Sun's visible surface is only 10,000 degrees
Fahrenheit, but as you move outward the temperature shoots up to
millions of degrees. It's like a campfire that feels hotter the farther
away you stand.
To understand how the corona is heated, some astronomers study coronal
loops. These structures are shaped like an upside-down U and show where
magnetic field lines are funneling solar gases or plasma.
Our best photos of the Sun suggest that these loops are a constant
width, like strands of rope. However, new work shows that this is an
optical illusion; the loops are actually tapered, wider at the top and
narrower at the ends. This finding has important implications for
coronal heating.
"You need less energy to heat the corona if the loops have a tapered
geometry, which is exactly what we found," says lead author Henry Winter
of the Harvard-Smithsonian Center for Astrophysics (CfA).
Winter presented his findings today in a press conference at a meeting
of the American Astronomical Society Solar Physics Division in Bozeman,
Mont.
Winter and his colleagues constructed a computer model of a tapered loop
using basic physics. Then they processed their model to show how it
would look when photographed by instruments like the High-resolution
Coronal Imager (Hi-C) or the Solar Dynamics Observatory's Atmospheric
Imaging Assembly (AIA).
They found that even the best available images wouldn't have the
resolution to show the loop's true structure. As a result, a tapered
loop would appear tubular even though it wasn't.
"In science we always compare theory to reality. But if your view of
reality is incorrect, your theory will be wrong too. What we thought we
saw could be just an effect of the instrument," explains Winter.
Historically, as we have gotten better and better photos of coronal
loops, they have revealed more and more structure. What first appeared
to be a single loop turned out to be made of many smaller strands. The
team's work shows that better instruments with higher resolution are
still needed to reveal the true shape and structure of the loops.
"Coronal loops are like Russian nesting dolls. We keep pulling them
apart but we haven't gotten to the smallest one yet," says Winter.
Winter's co-authors are Chester Curme (Boston University), Katharine
Reeves (CfA), and Petrus Martens (Montana State University).
Headquartered in Cambridge, Mass., the
Harvard-Smithsonian Center for Astrophysics (CfA) is a joint
collaboration between the Smithsonian Astrophysical Observatory and the
Harvard College Observatory. CfA scientists, organized into six research
divisions, study the origin, evolution and ultimate fate of the
universe.
For more information, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
