Researchers have made a major breakthrough in revealing how the sun’s atmosphere heats up, discovering that reflected plasma waves might be responsible for the heating in coronal holes.
There is a great enigma surrounding our sun. The temperature of the sun’s surface is approximately 10,000 degrees Fahrenheit, yet its outer atmosphere, known as the solar corona, reaches an astonishing temperature of around 2 million degrees Fahrenheit—roughly 200 times hotter. This perplexing rise in temperature away from the sun has puzzled scientists since 1939, when the high temperature of the corona was first observed. Over the years, researchers have sought to uncover the mechanisms behind this unexpected heating, but have yet to find a definitive explanation.
Recently, a team led by Sayak Bose from the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has achieved a significant milestone in understanding this heating mechanism. Their new research indicates that reflected plasma waves may play a role in the heating of coronal holes, which are areas in the solar corona with low density and open magnetic field lines that extend into outer space. This discovery marks a substantial advancement toward unraveling one of the greatest mysteries of our nearest star.
“While it was known that coronal holes have elevated temperatures, the exact mechanism behind this heating was still unclear,” explained Bose, the lead author of the study published in The Astrophysical Journal. “Our results show that plasma wave reflection can account for this effect. This study represents the first laboratory experiment showing that Alfvén waves reflect under conditions that resemble those near coronal holes.”
The phenomenon was first hypothesized by Swedish physicist and Nobel Prize laureate Hannes Alfvén, whose namesake waves mimic the vibrations of plucked guitar strings; however, in this case, they are generated by oscillating magnetic fields.
Bose and the research team utilized a 20-meter-long plasma column from the Large Plasma Device (LAPD) at the University of California-Los Angeles (UCLA) to generate Alfvén waves that replicate conditions found around coronal holes. The experiment illustrated that when Alfvén waves encounter areas with different plasma densities and magnetic field strengths—like those surrounding coronal holes—they can reflect and move back toward their origin. The interaction between these outward-moving and reflected waves creates turbulence, which in turn generates heat.
“Physicists have long proposed that Alfvén wave reflection could explain the temperature increase in coronal holes, but it has previously been impossible to verify this in the lab or through direct measurements,” stated Jason TenBarge, a visiting research scholar at PPPL and co-researcher. “This study provides the first experimental evidence that Alfvén wave reflection is both feasible and that the amount of reflected energy is enough to heat coronal holes.”
In addition to the laboratory experiments, the team also conducted computer simulations that supported the findings regarding the reflection of Alfvén waves under conditions akin to coronal holes. “We routinely perform numerous checks to validate the accuracy of our results,” noted Bose. “Simulations were one such check. The study of Alfvén wave reflection is incredibly intriguing and complex! It is remarkable how fundamental laboratory experiments and simulations can enhance our comprehension of natural phenomena like our sun.”
The research team included scientists from Princeton University, UCLA, and Columbia University. Funding was provided by the DOE through contracts DE-AC0209CH11466 and DE-SC0021261, as well as a grant from the National Science Foundation (NSF) under number 2209471. The experiments took place at the Basic Plasma Science Facility, a collaborative user center operated under the DOE Office of Science Fusion Energy Sciences program, funded by DOE contract DE-FC02-07ER54918 and NSF contract NSF-PHY 1036140.
