Science Gazette

Solar mystery solved by scientists

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The air becomes colder as we travel away from a heat source. Surprisingly, the same cannot be true for the Sun, but researchers at the University of Otago may have just discovered why.

According to Dr. Jonathan Squire of the Department of Physics, the surface of the Sun begins at 6000 degrees Celsius, but over a few hundred kilometers, it abruptly warms up to more than a million degrees, forming its atmosphere, or corona.

“This is so heated that the gas leaves the Sun’s gravity as’solar wind,’ sails into space, and collides with Earth and other planets.

“We know from observations and theory that the abrupt temperature increase is caused by magnetic fields that emanate from the Sun’s surface. However, the specific mechanism by which they operate to heat the gas is unknown; this is known as the Coronal Heating Problem.

“Astrophysicists have various alternative hypotheses on how magnetic-field energy may be transformed into heat to explain the heating,” he adds. “However, most have problems explaining some part of data.”

Dr. Squire and co-author Dr. Romain Meyrand collaborated with scientists from Princeton University and the University of Oxford and discovered that two earlier ideas may be blended into one to answer a critical portion of the ‘issue.’ The results of the researchers have recently been published in Nature Astronomy.

The common ideas are based on turbulence-induced heating and heating generated by a kind of magnetic wave known as ion cyclotron waves.

“Both have some problems — turbulence struggles to explain why Hydrogen, Helium, and Oxygen in the gas become as hot as they do, while electrons remain surprisingly cold; while the magnetic waves theory could explain this feature, there doesn’t seem to be enough of the waves coming off the Sun’s surface to heat up the gas,” Dr. Meyrand says.

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The researchers utilized six-dimensional supercomputer simulations of coronal gas to demonstrate how these two hypotheses are essentially part of the same process, connected together by a strange phenomenon known as the ‘helicity barrier.’

Dr. Meyrand conducted a previous Otago investigation that uncovered this remarkable pattern.

“If we consider plasma heating as a little like water pouring down a hill, with electrons heated right at the bottom, the helicity barrier functions like a dam, halting the flow and redirecting its energy into ion cyclotron waves. In this sense, the helicity barrier connects the two theories while also resolving their respective concerns “He elaborates.

In this recent study, the researchers disturbed the magnetic field lines in simulations and discovered that the turbulence formed waves, which triggered the heating.

“As a result, the structures and eddies that emerge resemble cutting-edge observations from NASA’s Parker Solar Probe mission, which just became the first human-made object to sail into the corona.

“This gives us confidence that we are properly capturing crucial physics in the corona, which, when combined with the theoretical discoveries concerning the heating processes, is a promising road to solving the coronal heating issue,” adds Dr. Meyrand.

Dr. Squire argues why learning more about the Sun’s atmosphere and subsequent solar wind is critical because of the enormous effects they have on Earth.

The effects of solar wind interacting with the Earth’s magnetic field are known as’space weather,’ and can generate anything from Aurora to satellite-destroying radiation and geomagnetic currents that disrupt the electrical system.

“All of this is supplied, basically, by the corona and its heating by magnetic fields, thus the solar-corona dynamics are not only significant for our overall knowledge of the solar system, but they may also have major effects on Earth.

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“Perhaps, with a greater knowledge of its underlying physics, we will be able to construct better models to anticipate space weather in the future, permitting the application of protective techniques that might save billions of dollars in damage.”

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