Magnetohydrodynamic turbulence regulates large-scale energy transfer in many astrophysical systems, including the solar atmosphere. For more than half a century, it has been widely accepted that the energy cascade in turbulent plasmas, such as the solar atmosphere, is controlled by MHD wave interactions.
In a new study, scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have uncovered a previously hidden heating process that helps explain how the “solar corona” can be vastly hotter than the solar surface it emits.
By using 200 million hours of computer time for the largest simulation of its kind ever, scientists were able to reveal the process. Their direct numerical simulation is the first to identify this heating mechanism in 3D space.
Chuanfei Dong, a physicist at PPPL and Princeton University, said: “Current telescope and spacecraft instruments may not have sufficient resolution to identify the small-scale process.”
The process known as magnetic reconnection, which forcibly splits and reunites magnetic fields into plasma — the soup of electrons and atomic nuclei that make up the solar atmosphere — is the secret ingredient. It was demonstrated by Dong’s modeling of how quickly the magnetic field lines reconnected, converting the large-scale chaotic energy into small-scale internal energy. Due to this efficient conversion of turbulent energy into thermal energy on a small scale, the corona is effectively heated.
Dong said, “Think of whipped cream in the coffee. The drops of cream quickly become wisps and slender curls. Likewise, magnetic fields form thin layers of electric current that disintegrate due to magnetic reconnection. This process facilitates the energy cascade from large-scale to small-scale, making the process in the turbulent solar corona more efficient than previously thought.”
“When the reconnection process is slow while the turbulent cascade is fast, reconnection cannot affect the transfer of energy across scales. But when the reconnection rate becomes fast enough to exceed the traditional cascade rate, the reconnection can more efficiently move the cascade down to small scale.”
“It does this by breaking and rejoining the magnetic field lines to generate 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 new finding couples the energy transfer rate to how fast the plasmoids grow, improving energy transfer from large to small scales and strongly heating the corona at these scales.”
The most recent discovery shows a regime with a solar corona-like magnetic Reynolds number that is unprecedentedly large. The sheer number characterizes the high energy transfer rate of the turbulent cascade. Reconnection-driven energy transfer is more effective as the magnetic Reynolds number gets higher.
The simulation is one of a kind and has consumed more than 200 million computer CPUs at NASA’s Advanced Supercomputing (NAS) facility.
PPPL physicist Amitava Bhattacharjee, a Princeton professor of astrophysical sciences who oversaw the research, said: “This numerical experiment has for the first time provided undisputed evidence of a theoretically predicted mechanism for a previously undiscovered range of turbulent energy cascade controlled by the growth of the plasmoids.”
“The impact of this finding in astrophysical systems at different scales can be explored with current and future spacecraft and telescopes. Unpacking the energy transfer process at different scales will be crucial to solving important cosmic mysteries.”
- Chuanfei Dong et al. Reconnection-driven energy cascade in magnetohydrodynamic turbulence. Scientific progress. DOI: 10.1126/sciadv.abn7627