Genesis, dynamics, and dissipation of turbulent magnetic fields
Astronomical observations indicate that coherent, dynamically important magnetic fields are ubiquitous in the Universe. However, neither the origin problem -- what are the physical mechanisms that generate the initial ``seed'' magnetic fields —- nor the dynamo problem—how magnetic fields are amplified and sustained by turbulent plasma motions -- are well understood. In addition, the role played by these fields in determining the material and thermodynamic properties of cosmic plasmas, which strongly impact various astrophysical phenomena, is still unknown. This talk will follow the ``life cycle'' of cosmic magnetic fields and addresses problems including the magnetogenesis, the formation of large-scale magnetic fields, the magnetized turbulent cascade through kinetic scales, and the plasma heating that ultimately ensues.
I will first demonstrate in a fully kinetic framework the generation of seed magnetic fields through the Weibel instability under a generic large-scale shear flow. The resulting spontaneous plasma magnetization confirms kinetic plasma processes as a plausible cause of magnetogenesis and that cosmic plasmas are thereby ubiquitously magnetized. This study sets the stage for studying whether such microscopic seed fields with filamentary morphology, under the joint action of their own nonlinear evolution and background turbulence, can contribute to the formation of macroscopic magnetic fields. We address this question by studying the dynamics of a large ensemble of interacting magnetic flux tubes, which resembles a wide range of astrophysical systems in addition to the Weibel seed fields. The emergence of large-scale magnetic structures from small-scale turbulence is identified as a consequence of magnetic reconnection of magnetic flux tubes, leading to the inverse transfer of magnetic energy. In the last part of the talk, I will discuss the ensuing strongly magnetized plasma turbulence in the kinetic range (below the ion gyroscale). The self-organization and dynamics of the magnetic fields is found to give rise to intermittency, which determines the turbulent spectrum, and efficient phase mixing around current sheets, which leads to electron heating. These results advance a first-principles understanding of the origin and dynamics of cosmic magnetic fields and their implications for astrophysical phenomena.