Critical magnetic Reynolds number of the weakly-collisional turbulent dynamo in supersonic plasmas
Year: 2026
Type: preprint
Abstract: Weakly-collisional plasmas are found in several turbulent astrophysical environments, such as the Intra-galaxy-cluster medium (ICM) and the solar wind, where magnetic fields can be amplified by the turbulent dynamo. The onset of dynamo action is determined by the critical magnetic Reynolds number, ${\rm Rm}_{\rm crit}$, above which turbulent motions amplify magnetic energy. Although ${\rm Rm}_{\rm crit}$ has been studied extensively in collisional magnetohydrodynamic (MHD) turbulence, its value in supersonic weakly-collisional plasmas remains unknown. Here we determine ${\rm Rm}_{\rm crit}$ using three-dimensional hybrid particle-in-cell (HPIC) simulations of solenoidally driven turbulence with a sonic Mach number $\approx 2$. We vary the magnetic Reynolds number over ${\rm Rm}\approx31$-$1000$ and consider two initial plasma magnetisations, characterised by initial Larmor ratios $r_{\rm Larmor, 0}/L_{\rm box}=1$ and $100$. We find that the magnetic energy grows exponentially at sufficiently large ${\rm Rm}$ and decays when magnetic diffusion dominates at low ${\rm Rm}$. By fitting the magnetic-energy growth rate as a function of ${\rm Rm}$, we measure ${\rm Rm}_{\rm crit}=190^{+21}_{-17}$ and $198^{+16}_{-18}$ for the initially magnetised and unmagnetised plasma, respectively. These values are the same within their uncertainties, indicating that the initial plasma magnetisation does not significantly affect ${\rm Rm}_{\rm crit}$ in the supersonic regime. The measured values are higher than those previously obtained for the subsonic weakly-collisional dynamo, showing that increasing compressibility makes weakly-collisional turbulent dynamo action more difficult to excite. The measured values are also approximately 1.5 times higher than the ${\rm Rm}_{\rm crit}$ value reported for supersonic MHD turbulence.
Source: arXiv (Cornell University)
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: U.S. Department of Energy, University of Chicago, University of Rochester, National Computational Infrastructure, Innovation, Science and Economic Development Canada, Deutscher Akademischer Austauschdienst, University of Toronto, Alliance de recherche numérique du Canada, Gauss Centre for Supercomputing, Leibniz-Gemeinschaft, Compute Ontario
Awards: G3628240400, G6781880672, G8709026768
OpenAlex ID: W7223050863