Helical dynamo growth and saturation at modest versus extreme magnetic Reynolds numbers

Journal Article · 18 January 2024 · Physical Review. E

DOI: https://doi.org/10.1103/PhysRevE.109.015206 · OSTI ID:2281957

Zhou, Hongzhe ; Blackman, Eric G.

Understanding magnetic field growth in astrophysical objects is a persistent challenge. In stars and galaxies, turbulent flows with net kinetic helicity are believed to be responsible for driving large-scale magnetic fields. However, numerical simulations have demonstrated that such helical dynamos in closed volumes saturate at lower magnetic field strengths when increasing the magnetic Reynolds number $\text{Rm}$ . This would imply that helical large-scale dynamos cannot be efficient in astrophysical bodies without the help of helicity outflows such as stellar winds. But do these implications actually apply for very large $\text{Rm}$ ? Here we tackle the long-standing question of how much helical large-scale dynamo growth occurs independent of $\text{Rm}$ in a closed volume. We analyze data from numerical simulations with a new method that tracks resistive versus nonresistive drivers of helical field growth. We identify a presaturation regime when the large-scale field grows at a rate independent of $\text{Rm}$ , but to an $\text{Rm}$ -dependent magnitude. The latter $\text{Rm}$ dependence is due to a dominant resistive contribution, but whose fractional contribution to the large-scale magnetic energy decreases with increasing $\text{Rm}$ . We argue that the resistive contribution would become negligible at large $\text{Rm}$ and an $\text{Rm}$ -independent dynamical contribution would dominate if the current helicity spectrum in the inertial range is steeper than $k^0$ . As such helicity spectra are plausible, this renews optimism for the relevance of closed dynamos. Our work pinpoints how modest $\text{Rm}$ simulations can cause misapprehension of the $\text{Rm}\to \infty$ behavior. Published by the American Physical Society 2024

• Research Organization: Univ. of Rochester, NY (United States); University of Rochester, NY (United States)
• Sponsoring Organization: USDOE; USDOE Office of Science (SC)
• Grant/Contract Number: SC0001063; SC0020103; SC0020432; SC0020434
• OSTI ID: 2281957
• Journal Information: Physical Review. E, Journal Name: Physical Review. E Journal Issue: 1 Vol. 109; ISSN PLEEE8; ISSN 2470-0045
• Publisher: American Physical SocietyCopyright Statement
• Country of Publication: United States
• Language: English