Researchers have uncovered a previously hidden heating process that helps explain how the atmosphere surrounding the Sun called the “solar corona” can be much hotter than the solar surface that emits it.
The discovery at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) could improve tackling a number of astrophysical puzzles such as star formation, the origin of large-scale magnetic fields in the universe and the ability to predict explosive space. weather events that can disrupt cell phone service and shut down power grids on Earth. Understanding the heating process also has implications for fusion research.
“Our direct numerical simulation is the first to provide a clear identification of this heating mechanism in three-dimensional space,” said Chuanfei Dong, a physicist at PPPL and Princeton University who revealed the process by spending 200 million hours of computing time on the largest simulation in the world. it’s kind “Current instruments on telescopes and spacecraft may not have high enough resolution to recognize the process occurring on a small scale,” said Dong, who details the discovery in the journal. Advances in Science.
The hidden ingredient is a process called magnetic reconnection that violently separates and reconnects magnetic fields in plasma, the soup of electrons and atomic nuclei that makes up the solar atmosphere. Dong’s simulation revealed how the rapid reconnection of magnetic field lines converts large-scale turbulent energy into small-scale internal energy. As a result, turbulent energy is effectively converted to thermal energy on small scales, thereby superheating the corona.
“Think about putting cream in the coffee,” Dong said. “The drops of cream soon become swirls and fine curls. Similarly, magnetic fields form thin sheets of electric current that break up due to magnetic reconnection. This process facilitates the large-scale to small-scale energy cascade, making the process more efficient in the turbulent solar corona than previously thought.”
When the reconnection process is slow while the turbulent cascade is fast, reconnection cannot affect the energy transport across the scale, he said. But when the rate of reconnection becomes fast enough to exceed the traditional cascade rate, reconnection can move the cascade to small scales more efficiently.
It does this by breaking and reconnecting magnetic field lines to create chains of small twisted lines called plasmoids. This changes the understanding of the turbulent energy cascade that has been widely accepted for more than half a century, the paper says. The new finding links the rate of energy transfer to how fast the plasmoids grow, enhancing energy transfer from large to small scales and strongly heating the corona at those scales.
The new discovery shows a regime with an unprecedented magnetic Reynolds number as in the solar corona. The large number characterizes the new high energy transfer rate of the turbulent cascade. “The higher the magnetic Reynolds number, the more efficient the reconnection-based energy transfer,” said Dong, who is moving to Boston University to take up a faculty position.
200 million hours
“Chuanfei performed the world’s largest turbulence simulation of its kind that has taken over 200 million computer CPUs [central processing units] at the NASA Advanced Supercomputing (NAS) facility,” said PPPL physicist Amitava Bhattacharjee, a Princeton professor of astrophysical sciences who oversaw the research. “This numerical experiment produced for the first time unequivocal evidence of a theoretically predicted mechanism for a previously undiscovered range of turbulent energy controlled by plasmoid growth.
“His work in the highly influential journal Advances in Science completes the computational program begun by his previously published 2D results Physical Review Letters. These papers are a codification of the impressive work Chuanfei has done as a member of the Princeton Center for Heliophysics,” a joint facility of Princeton and PPPL. “We are grateful for a PPPL LDRD [Laboratory Directed Research & Development] grant that facilitated this work and to the NASA High-End Computing (HEC) program for the generous allocation of computer time.’
The impact of this finding on astrophysical systems on a range of scales can be explored with current and future spacecraft and telescopes. Unpacking the energy transfer process at scales will be critical to solving key cosmic mysteries, the paper said.
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