Mysterious heat of the sun's atmosphere is finally explained

The sun’s heat is one of the most fascinating mysteries of the solar system. While the surface of our star burns at an intense 10,000 degrees Fahrenheit, its outer atmosphere – known as the solar corona – reaches a staggering 2 million degrees Fahrenheit. That’s about 200 times hotter than the surface.

This strange phenomenon, first discovered in 1939, has puzzled scientists for decades. Despite many studies and countless hours of research, the mechanism behind this drastic temperature increase has remained elusive.

But now, researchers may have found a clue. A team led by Sayak Bose at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) believes they have unlocked part of the sun’s mystery.

The research suggests that the extreme heating in regions called coronal holes – areas of the corona with lower density and open magnetic field lines – may be driven by reflected plasma waves.

The puzzle of the sun’s mysterious heat

In a significant step forward, the team discovered that reflected Alfvén waves could explain the mysterious heat of the sun’s atmosphere.

“Scientists knew that coronal holes have high temperatures, but the underlying mechanism responsible for the heating is not well understood,” said Bose.

“Our findings reveal that plasma wave reflection can do the job. This is the first laboratory experiment demonstrating that Alfvén waves reflect under conditions relevant to coronal holes.”

Alfvén waves, predicted by Nobel laureate Hannes Alfvén, act similarly to the vibrations of a plucked guitar string, except they’re plasma waves driven by oscillating magnetic fields.

The research provides the first experimental evidence that these waves can reflect back under the unique conditions around coronal holes.

Strange heating of coronal holes

Using the Large Plasma Device (LAPD) at UCLA, Bose and his team generated Alfvén waves within a 20-meter plasma column, carefully simulating the conditions present around solar coronal holes.

The experiment revealed that, under specific conditions of plasma density and magnetic field strength, these waves could indeed reflect off the boundaries and travel back toward their origin.

This reflected wave movement generated turbulence within the plasma, which, in turn, could heat the surrounding particles, offering a plausible explanation for the intense temperatures observed in the sun’s outer atmosphere.

“Physicists have long hypothesized that Alfvén wave reflection could explain the strange heating of coronal holes,” said Jason TenBarge, a visiting research scholar at PPPL.

“This work offers the first experimental verification that Alfvén wave reflection is not only possible, but also that the reflected energy is sufficient enough to heat the coronal holes.”

Improving our understanding of the sun

To further validate their findings, the team conducted detailed computer simulations that replicated the experimental setup and confirmed the results observed in the lab.

The simulations reinforced the team’s conclusions by demonstrating that Alfvén wave reflections could occur under conditions akin to those in the solar corona, providing a robust model for the heating mechanism.

“We regularly conduct multiple checks to ensure the accuracy of our observed results, and simulation was one such step,” said Bose.

“The physics of Alfvén wave reflection is intricate and utterly fascinating! It’s incredible how rudimentary physics lab experiments and simulations can significantly improve our understanding of natural systems like our sun.”

Implications for space weather

Understanding the mysterious heat of the sun’s atmosphere has practical implications beyond academic interest.

This phenomenon affects solar winds, which are streams of charged particles emitted from the sun that can influence Earth’s magnetic field. These winds impact satellite operations, GPS accuracy, and even power grids.

By shedding light on the role of Alfvén waves in coronal heating, this research could improve predictions of solar activity and help protect critical technology here on Earth.

Moreover, the confirmation of Alfvén wave reflection in the lab marks a significant step forward in solar research. This breakthrough allows scientists to refine their models of solar phenomena like solar flares and coronal mass ejections.

Thanks to this experimental leap, researchers now have a powerful new tool to better understand the complex dynamics of our closest star.

The study is published in the journal The Astrophysical Journal.

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