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

First discovered in 1939, this strange phenomenon has baffled scientists for decades. Despite numerous studies and hours of research, the mechanism behind this dramatic increase in temperature remains unclear.

But now researchers may have found a clue. A team led by Sayak Bos of the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) believes they have solved part of the Sun’s mystery. The study suggests that overheating in regions called coronal holes (areas of the corona with lower density and exposed magnetic field lines) could be caused by reflected plasma waves.

The mystery of the mysterious warmth of the sun

In a major step forward, the team found that reflected Alfvén waves can explain the mysterious warmth of the solar atmosphere.

“Scientists knew that coronal holes had high temperatures, but the underlying mechanism responsible for the heating was not fully understood,” Bowes said. “Our results show that plasma wave reflection can do this job. This is the first laboratory experiment to show that Alfvén waves are reflected in conditions consistent with coronal holes.”

Predicted by Nobel laureate Hannes Alfvén, Alfvén waves act like the vibrations of a plucked guitar string, except they are plasma waves driven by oscillating magnetic fields. The study provides the first experimental evidence that these waves can be reflected in the unique conditions around coronal holes.

Strange heating of coronal holes

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

The experiment showed that under certain conditions of plasma density and magnetic field strength, these waves can indeed be reflected from the boundaries and return to their source. This movement of the reflected wave caused turbulence in the plasma, which could heat the surrounding particles and offer a plausible explanation for the intense temperatures observed in the Sun’s outer atmosphere.

“Physicists have long suggested that Alfén wave reflection could explain the strange heating of coronal holes,” said PPPL Visiting Scientist Jason TenBarge. “This work provides the first experimental confirmation that reflection of Alfvén waves is not only possible, but also that the reflected energy is sufficient to heat coronal holes.”

Improving our understanding of the sun

To further validate their findings, the team performed detailed computer simulations that recreated the experimental setup and confirmed the results obtained in the laboratory. The simulations confirmed the team’s findings, showing that the reflection of Alfvén waves can occur under conditions similar to those of the solar corona, providing a robust model for the heating mechanism.

“We routinely perform numerous checks to ensure the accuracy of our observed results, and modeling was one of those steps,” Bowes said.

“The physics of the reflection of Alfvén waves is complex and extremely fascinating! It is incredible that rudimentary physical laboratory experiments and simulations can greatly improve our understanding of natural systems such as our Sun.”

Implications for space weather

Understanding the mysterious heat of the solar atmosphere has practical implications beyond academic interest. This phenomenon affects solar winds, which are flows of charged particles emitted by the Sun that can affect the Earth’s magnetic field. These winds affect satellites, GPS accuracy, and even power grids.

By shedding light on the role of Alfvén waves in heating the corona, this research could improve estimates of solar activity and help preserve important technologies on Earth.

Additionally, the laboratory validation of Alfvén wave reflection marks an important step forward in Solar research. This discovery allows scientists to improve their models of solar events such as 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 nearest star. The study was published in the journal Astrophysical Journal.