Study Reveals That Solar Coronal Loops May Be Optical Illusions
By: April Carson
According to a new study, which challenges conventional ideas about what we understand and don't know about the Sun, certain coronal loops—ropey strands of plasma that scientists have long thought existed in the Sun's atmosphere—may actually be optical illusions.
The National Center for Atmospheric Research (NCAR) recently published a study in The Astrophysical Journal that used a cutting-edge, realistic 3D simulation of the solar corona to examine how cosmic rays affect the upper atmosphere. The scientists were able to cut the corona into distinct parts with a sophisticated NCAR simulation completed several years ago in order to isolate individual coronal loops.
They learned that a significant portion of the loops weren't real loops at all.
While the research team was able to locate some of the coronal loops they were searching for, they also discovered that in many situations what appear to be loops on photographs of the Sun may actually be wrinkles in the solar atmosphere's brilliant plasma. When sheets of brilliant plasma fold over each other, forming folds, small bright lines resembling distinct and self-contained strands of plasma are produced.
The "coronal veil" hypothesis, which the study's researchers are calling the "coronal veil," has significant implications for our understanding of the Sun because decades-old supposed coronal loops have been used to infer information about density, temperature, and other physical features of the solar atmosphere.
"It's so beautiful to be able to study something that I've been researching for my entire career," said NCAR scientist Anna Malanushenko, who conducted the research. "I was thrilled that this model would allow me to scrutinize coronal loops in greater depth. This has come as a total surprise to me." When I saw the findings, my mind went blank. This is an entirely new approach of seeing the Sun's atmosphere."
The study was funded by NASA and had experts from NCAR's High Altitude Observatory, Lockheed Martin Solar and Astrophysics Laboratory, the Southwest Research Institute, and NASA Goddard as collaborators. The National Science Foundation funds NCAR.
In extreme ultraviolet light, images of the Sun reveal what appear to be coronal loops. Because it is in line with our most fundamental knowledge of magnetism, scientists have presumed them to exist since long ago.
At some point, everyone has observed how iron filings are sprinkled around a bar magnet. The filings align themselves along magnetic field lines that loop from one pole of the bar magnet to the other, which become stronger and more dense as they get farther away from the magnet.
The apparent coronal loops in pictures of the Sun are strikingly similar, and since there is a large magnetic field on the Sun, the notion of magnetic field lines trapping a rope of plasma between them and forming loops appears logical. And, in fact, the new research confirms that such loops are probable.
However, based on our current understanding of magnets, the coronal loops on the Sun have never functioned in a way that was predicted. For example, as you progress higher into the corona, scientists would expect the magnetic field lines to spread apart just like in an iron filings experiment.
If this occurred, the plasma between the field lines would spread out across the boundaries, resulting in thicker, less brilliant loops. However, photographs of the Sun do not exhibit this effect. Rather than thin and bright, the loops radiating outward continue to appear slender and brilliant.
Our expectations of the loops are based on computer simulations of coronae, which emerged after Bierstadt's careful study and high-resolution photography from space. These wrinkles in a coronal veil help explain this and other discrepancies with our preconceptions; but it also raises new questions. What determines the form and thickness of these folds, for example? And how many of the apparent loops visible on pictures of the Sun are real strands, and how many are optical illusions?
"This research serves as a reminder to scientists that we must always question our preconceptions, and that our intuition may be deceptive at times," said Malanushenko.
The revolutionary design provides a new perspective on the Sun
An extremely detailed simulation of the solar corona produced by MURaM, a radiative magnetohydrodynamic model that was expanded to model the solar corona in an NCAR-led project, made it possible to realize that coronal loops may be illusory.
The first simulation was revolutionary because it could represent what was going on in many parts of the Sun, from the upper convective zone—about 10,000 kilometers below the surface of the Sun—through to nearly 40,000 kilometers into the solar corona. The Sun's various regions are separated by several types of physical characteristics, including density and pressure differences. Because scientists had not found a way to mathematically combine these areas in a single simulation, they had not previously figured out how to represent them coherently.
The new simulation was also able to capture the whole life cycle of a solar flare for the first time, from the buildup of energy beneath the sun's surface to the emergence of flare at the surface, and then to the enormous release of power.
The scientists used the model to produce three-dimensional data sets that included the magnetic field and plasma structure, which may be utilized to simulate "synthetic" observations. Because the solar corona is optically thin—that is, it's simple to see through it—structures in the corona overlap one another in photographs of the Sun.
This makes determining whether a "loop" that overlaps other loops is ahead or behind difficult.
It's difficult to determine whether the loop has a little cross section, like a garden hose, or if it looks more like a long ribbon viewed face on. It's also conceivable that what appears to be a thin strand is an optical illusion caused by a fold in a sheet of brilliant plasma.
The solar atmosphere may also be studied by using MURaM's data cubes, which give scientists the opportunity to break down and analyze the overlapping features separately, something that can't be done with current observatories and instruments.
While the MURaM simulation is one of the most realistic representations of the solar corona, it's still a model. Probing the corona and employing new data analysis techniques will be required to understand how many coronal loops are mere optical illusions.
"Although previously, there was no need for a mechanism to understand the Sun's behavior. However, this research shows that our current method of interpreting the Sun's data may not be adequate to truly comprehend solar physics," said Malanushenko.
The study was published in the journal Science.
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About the Blogger:
April Carson is the daughter of Billy Carson. She received her bachelor's degree in Social Sciences from Jacksonville University, where she was also on the Women's Basketball team. She now has a successful clothing company that specializes in organic baby clothes and other items. Take a look at their most popular fall fashions on bossbabymav.com
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