Everyone knows the picture of the sun. A bright orange ball with jets of fire spewing out thousands of miles into space with temps soaring above a million degrees. However, a new study from the National Center for Atmospheric Research (NCAR) brings into question coronal loops existence at all.
The report, published in The Astrophysical Journal, found that these may actually be optical illusions. While the researchers were able to pinpoint some of the coronal loops they were looking for, they also discovered that in many cases what appear to be loops in images taken of the Sun may in fact be wrinkles of bright plasma in the solar atmosphere. As sheets of bright plasma fold over themselves, the wrinkles look like bright thin lines, mimicking the look of distinct and self-contained strands of plasma.
“I have spent my entire career studying coronal loops,” said NCAR scientist Anna Malanushenko, who led the study. “I was excited that this simulation would give me the opportunity to study them in more detail. I never expected this. When I saw the results, my mind exploded. This is an entirely new paradigm of understanding the Sun’s atmosphere.”
Coronal loops are found around sunspots and across active regions of the Sun. These structures are associated with the closed magnetic field lines that connect attractive regions on the solar surface. Many coronal loops last for days or weeks, but most shift quite rapidly. The assumption that they exist is a normal one for scientists because it suits the most basic understanding of magnetism.
The findings, which have been coined the “coronal veil” hypothesis, could have substantial implications for solar research. These coronal loops have been used for decades as a way to garner info about density, temperature, and other physical characteristics of the solar atmosphere.
“This study reminds us as scientists that we must always question our assumptions and that sometimes our intuition can work against us,” Malanushenko said.
The research relied on a realistic 3D simulation of the solar corona produced by MURaM, a radiative magnetohydrodynamic model that was extended to replicate the solar corona in an effort led by NCAR several years ago. The model allowed the researchers to slice the corona in distinct sections in an effort to isolate individual coronal loops.
Since there is a significant magnetic field in the Sun, the existence of magnetic field lines that could trap a rope of plasma between them and create loops seems like an obvious explanation. And in fact, the new study confirms that such loops still likely exist.
However, the loops seen on the Sun have never really behaved exactly as they should, based on the knowledge of magnets. As an example, scientists would assume the solar magnetic field lines to expand as they move higher in the corona. Therefore, the plasma trapped between the field lines should also spread out between the boundaries, creating thicker, dimmer loops. But images of the Sun do not show this. Instead, they show the opposite. The loops further out still appear thin and bright.
The possibility that these loops are instead wrinkles in a coronal veil help explain this and other inconsistencies with scientists’ expectations of coronal loops. It also brings into question new mysteries such as what determines the shape and thickness of the folds and how many of the apparent loops in images of the Sun are actually real strands, and how many are optical illusions.
For the first time, the research group was also able to capture the entire life span of a solar flare, from the build-up of energy below the solar surface to the emergence of the flare at the surface, and finally to the fiery release of energy.
Malanushenko said that understanding the number of coronal loops which are actually optical illusions will require continued observations that probe the corona and new data analysis techniques.
“We know that designing such techniques would be extremely challenging, but this study demonstrates that the way we currently interpret the observations of the Sun may not be adequate for us to truly understand the physics of our star.”
