title: Turbulent dynamo in a collisionless plasma.
abstract: Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas.
authors.affiliation: Université de Toulouse, Université Paul Sabatier-Observatoire Midi-Pyrénées, Institut de Recherche en Astrophysique et Planétologie, F-31400 Toulouse, France; CNRS, Institut de Recherche en Astrophysique et Planétologie, F-31400 Toulouse, France; francois.rincon@irap.omp.eu.
authors.forename: François
authors.initials: F
authors.lastname: Rincon
authors.affiliation: Physics Department, University of Pisa, 56127 Pisa, Italy;
authors.forename: Francesco
authors.initials: F
authors.lastname: Califano
authors.affiliation: The Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, United Kingdom; Merton College, Oxford OX1 4JD, United Kingdom;
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Schekochihin
authors.affiliation: Dipartimento di Fisica, Universitá della Calabria, I-87036 Rende (CS), Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Valentini
keywords: dynamo; intracluster medium; magnetic fields; turbulence