Researchers have discovered a previously hidden heating process that helps explain why the atmosphere around the sun, known as the “corona,” is much hotter than the surface of the sun from which it is emitted.
The discovery, made at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), could help resolve a host of astrophysical puzzles, such as star formation, the origin of large-scale magnetic fields in the universe, and the ability to predict where eruptions might take place. A weather event that disrupts cell phone service and disrupts the planet’s power grid. Understanding the heating process also has implications for fusion research.
“For the first time, our direct numerical simulations provide a clear identification of this heating mechanism in 3D space,” said physicist Chuanfei Dong of PPPL and Princeton University, who dedicated 200 million hours of computer time to the world’s largest “current Telescopes and spacecraft instruments may not have high enough resolution to identify processes occurring at small scales,” said Dong, who details the breakthrough in the journal scientific progress.
The hidden ingredient is a process called magnetic reconnection, which separates and violently reconnects the magnetic fields in the plasma that form the soup of electrons and nuclei in the sun’s atmosphere. Dong’s simulations revealed how the rapid reconnection of magnetic field lines transforms large-scale turbulent energy into small-scale internal energy. As a result, turbulent energy is efficiently converted to thermal energy on small scales, superheating the corona.
“Think about adding cream to your coffee,” Dong said. “Cream droplets quickly become helical and elongated coils. Likewise, the magnetic field forms sheets of current that break apart due to magnetic reconnection. This process facilitates the cascade of energy from large scales to small scales, making the process more efficient in a turbulent corona than previously thought.”
Reconnection does not affect energy transfer across scales when the reconnection process is slow and turbulent cascades are fast, he said. But when the reconnection rate becomes fast enough to exceed the traditional cascade rate, the reconnection can move the cascade more efficiently to small scales.
It does this by disconnecting and reconnecting magnetic field lines to create chains of small twisted wires called plasmoids. According to the paper, this changes the understanding of turbulent energy cascades that have been widely accepted for more than half a century. This new finding links the rate of energy transfer to the growth rate of the plasmoid, enhancing the transfer of energy from large to small scales and strongly heating the corona at these scales.
The new discovery demonstrates a state with an unprecedentedly large magnetic Reynolds number, as in the corona. Large numbers characterize new high energy transfer rates for turbulent cascades. “The higher the magnetic Reynolds number, the more efficient the reconnection-driven energy transfer is,” said Dong, who is about to take a faculty position at Boston University.
200 million hours
“Transfy has performed the world’s largest turbulence simulation, using more than 200 million computer CPUs [central processing units] at the NASA Advanced Supercomputing (NAS) Facility,” said PPPL physicist Amitava Bhattacharjee, a professor of astrophysics at Princeton University who oversaw the study. The theoretically predicted mechanism of the undiscovered range consists of a turbulent energy cascade controlled by the growth of the plasmoid.
“His papers in high-impact journals scientific progress Having completed the computational program he started, his early 2D results were published in Physical Review Letters. These papers form the conclusion of Chuanfei’s impressive work as a member of the Princeton Center for Heliophysics, “Joint Princeton and PPPL Facility.” We thank PPPL LDRD [Laboratory Directed Research & Development] Grants facilitated this work, and thanks to the NASA High-End Computing (HEC) Program for generous computer time. “
The implications of this discovery for astrophysical systems at a range of scales can be explored using current and future spacecraft and telescopes. Unraveling the process of energy transfer across scales is critical to solving key cosmic mysteries, according to the paper.
Funding for the paper came from the DOE Office of Science (FES) and NASA, and computer resources were provided by NASA HEC with the National Energy Research Scientific Computing Center, DOE Office of Science user facilities, and the Computing and Information Systems Laboratory sponsored by NSF. Co-authors on the paper are researchers at PPPL, Princeton and Columbia Universities, and NASA Ames Research Center.
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