Fluctuation Dynamo in a Collisionless, Weakly Magnetized Plasma

Journal Article · Thu Aug 16 00:00:00 EDT 2018 · The Astrophysical Journal. Letters

DOI:https://doi.org/10.3847/2041-8213/aad638· OSTI ID:1466006

St-Onge, Denis A. ^[1]; Kunz, Matthew W. ^[1]

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1. Princeton Univ., Princeton, NJ (United States); Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)

Results from a numerical study of fluctuation dynamo in a collisionless, weakly magnetized plasma are presented. The key difference between this dynamo and its magnetohydrodynamic (MHD) counterpart is the adiabatic production of magnetic-field-aligned pressure anisotropy by the amplification of a weak seed field. This, in turn, drives kinetic instabilities on the ion-Larmor scale—namely, firehose and mirror—which sever the adiabatic link between the thermal and magnetic pressures, thereby allowing the dynamo to proceed. After an initial phase of rapid growth driven by these instabilities, the magnetic energy grows exponentially and exhibits a $${k}^{3/2}$$ spectrum that peaks near the resistive scale, similar to the large-magnetic-Prandtl-number ($$\mathrm{Pm}\gg 1$$) MHD dynamo. The magnetic field self-organizes into a folded-sheet topology, with direction reversals at the resistive scale and field lines curved at the parallel scale of the flow. The effective $$\mathrm{Pm}$$ is determined by whether the ion-Larmor scale is above or below the field-reversing scale: in the former case, particles undergo Bohm-like diffusion; in the latter case, particles scatter primarily off of firehose fluctuations residing at the ends of the magnetic folds, and the viscosity becomes anisotropic. The magnetic field ultimately saturates at dynamical strengths, with its spectral peak migrating toward larger scales. This feature, along with an anti-correlation of magnetic-field strength and field-line curvature and a gradual thinning of magnetic sheets into ribbons, resembles the saturated state of the large-$$\mathrm{Pm}$$ dynamo, the primary differences manifesting in firehose/mirror-unstable regions. Furthermore, these results have implications for magnetic-field growth in the weakly collisional intracluster medium of galaxy clusters.

• Research Organization: Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)
• Sponsoring Organization: This work was supported by U.S. DOE contract DE-AC02-09CH11466, and made extensive use of the Perseus cluster at the PICSciE-OITTIGRESS High Performance Computing Center and Visualization Laboratory at Princeton University.; USDOE
• Grant/Contract Number: AC02-09CH11466
• OSTI ID: 1466006
• Journal Information: The Astrophysical Journal. Letters, Journal Name: The Astrophysical Journal. Letters Journal Issue: 2 Vol. 863; ISSN 2041-8213
• Publisher: Institute of Physics (IOP)
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
dynamo
galaxies: clusters: intracluster medium
magnetic fields
plasmas
turbulence