There is a profound mystery in our sun. While the sun's surface temperature measures around 10,000 degrees Fahrenheit, its outer atmosphere, known as the solar corona, measures more like 2 million degrees Fahrenheit, about 200 times hotter. This increase in temperature away from the sun is perplexing and has been an unsolved mystery since 1939, when the high temperature of the corona was first identified. In the ensuing decades, scientists have tried to determine the mechanism that could cause this unexpected heating, but so far, they have not succeeded.
Now, a team led by Sayak Bose, a researcher at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), has made a significant advancement in understanding the underlying heating mechanism. Their recent findings show that reflected plasma waves could drive the heating of coronal holes, which are low-density regions of the solar corona with open magnetic field lines extending into interplanetary space. These findings represent major progress toward solving one of the most mysterious quandaries about our closest star.
"Scientists knew that coronal holes have high temperatures, but the underlying mechanism responsible for the heating is not well understood," said Bose, the lead author of the paper reporting the results in The Astrophysical Journal. "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."
First predicted by Swedish physicist and Nobel Prize winner Hannes Alfvén, the waves that bear his name resemble the vibrations of plucked guitar strings, except that in this case, the plasma waves are caused by wiggling magnetic fields.
Bose and other members of the team used the 20-meter-long plasma column of the Large Plasma Device (LAPD) at the University of California-Los Angeles (UCLA) to excite Alfvén waves under conditions that mimic those occurring around coronal holes. The experiment demonstrated that when Alfvén waves encounter regions of varying plasma density and magnetic field intensity, as they do in the solar atmosphere around coronal holes, they can be reflected and travel backward toward their source. The collision of the outward-moving and reflected waves causes turbulence that, in turn, causes heating.
"Physicists have long hypothesized that Alfvén wave reflection could help explain the heating of coronal holes, but it has been impossible to either verify in the laboratory or directly measure," said Jason TenBarge, a visiting research scholar at PPPL, who also contributed to the research. "This work provides the first experimental verification that Alfvén wave reflection is not only possible, but also that the amount of reflected energy is sufficient to heat coronal holes."
Along with conducting the laboratory experiments, the team performed computer simulations of the experiments, which corroborated the reflection of Alfvén waves under conditions similar to coronal holes. "We routinely conduct multiple verifications to ensure the accuracy of our observed results," said Bose, "and conducting simulations was one of those steps. The physics of Alfvén wave reflection is very fascinating and complicated! It is amazing how profoundly basic physics laboratory experiments and simulations can significantly improve our understanding of natural systems like our sun."
Collaborators included scientists from Princeton University; the University of California-Los Angeles; and Columbia University. The research was funded by the DOE under contracts DE-AC0209CH11466, and DE-SC0021261, as well as the National Science Foundation (NSF) under grant number 2209471. The experiment was performed at the Basic Plasma Science Facility, which is a collaborative user facility that is part of the DOE Office of Science Fusion Energy Sciences program and is funded by DOE contract DE-FC02-07ER54918 and the NSF under contract NSF-PHY 1036140.
