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Unraveling the Mystery of the Sun’s Superhot Atmosphere

New Theory Might Explain Coronal Heating

UMass Lowell Image
The Sun’s outer atmosphere, called the corona, is a plasma (highly ionized gas) that is easily seen with the naked eye during a total eclipse. This view of the corona’s delicate structure was photographed by Edwin L. Aguirre during the July 11, 2010, total solar eclipse, which he observed from the South Pacific, on Tatakoto Atoll in French Polynesia’s Tuamotu Archipelago.

By Edwin L. Aguirre

The Sun, whose light and heat has made life possible here on Earth, has been studied in great detail since the invention of the telescope in the 17th century. Yet fundamental questions remain about the nature of this star nearest to our home planet.
One of them is the problem of the heating of the Sun’s tenuous outer atmosphere, known as the corona, which is composed of charged particles and extends millions of miles into interplanetary space. In the late 1930s and early '40s, scientists discovered that the corona is extremely hot — some 2 million degrees. And yet the Sun’s visible surface, called the photosphere, is a “mere” 6,000 degrees.
So how can the photosphere heat the surrounding corona to such incredibly high temperatures?
“This has been a longstanding problem in solar physics and astrophysics,” says Prof. Paul Song of the Physics and Applied Physics Department. “If the photosphere is indeed responsible for heating the corona, then it contradicts the second law of thermodynamics, which states that heat flows from higher-temperature regions to lower ones, and not vice versa.”
Song, who directs UMass Lowell’s Center for Atmospheric Research (CAR), says what makes the problem worse is that another observation shows that a large amount of energy actually radiates from the lower part of the solar atmosphere, a region known as the chromosphere.
“This means the energy from the Sun has to not only heat the corona but also provide energy for chromospheric radiation,” he says. “Quantitatively, the energy loss in radiation is a factor of 100 bigger than what is needed to heat the atmosphere to 2 million degrees!”
Song and CAR Distinguished Research Professor Vytenis Vasyliunas proposed a new theory that might help explain this apparent anomaly and lead to the final resolution of the coronal heating problem. Their findings were published last year in the “Journal of Geophysical Research.”
Like Atomic Bombs Going Off Everywhere
“Many people realize the Sun is very turbulent and the energy associated with that turbulence — similar to many atomic bombs going off everywhere on the Sun — can provide the required heat energy,” notes Song. “However, the question is how motional energy can be converted to thermal energy. Previous theories are able to come up with only about a few percent of the required thermal energy based on the observed turbulence energy.” 
In their paper, Song and Vasyliunas analyzed the perturbation (disturbance) of Alfvén waves as they propagate from the solar surface to the corona through the chromosphere, a rather difficult and complex mathematical-physical task. Alfvén waves are low-frequency traveling oscillation of the charged particles and the Sun’s magnetic field.
Song and Vasyliunas found that the waves’ perturbations are heavily damped, and that the motional energy of the perturbation can be converted to heat in regions where the magnetic field is weaker. The damping and heating is much less in regions of stronger magnetic field, they say. 
“Conventional wisdom has been to focus on regions of stronger magnetic field because these are where stronger perturbations are observed,” explains Song. “Now everything becomes clear: stronger perturbations are observed in stronger field regions because they are not damped, and vice versa. Our theory is thus able to convert the observed level of turbulence energy to the required thermal energy. We are continuing work on the details and other consequences of the heating mechanism to explain other observed solar phenomena.”
He says since coronal heating is essential to understanding the formation of the corona and solar winds, which are the cause of all space-weather phenomena, the new theory will substantially improve our current knowledge about solar storms and space weather.