Event
Plasma Physics Seminar| Muni Zhou, MIT
Wednesday, November 18, 2020
3:30 p.m.
ZOOM
Taylor Prendergast
301 405 4951
tprender@umd.edu
Speaker: Muni Zhou
Speaker's Institution: MIT, Plasma Science and Fusion Center
Title: "From the Weibel instability to flux-tube coalescence-a pathway to seeding plasma dynamos"
Abstract: The amplification of seed magnetic fields by the turbulent dynamo is believed to be essential in forming large-scale cosmic magnetic fields with dynamical strengths. The Weibel instability is a crucial mechanism that can produce near-equipartition seed fields from unmagnetized plasmas, but only at the plasma kinetic scale. It remains unclear whether such microscopic seed fields, under the joint action of their own nonlinear evolution and the background turbulence, can contribute to the formation of macroscopic magnetic fields. In the first part of this talk, I will demonstrate how thermal pressure anisotropy can be generated by motions as simple as a shear flow and trigger the Weibel instability. The ensuing generation and nonlinear evolution of seed magnetic fields are studied within a fully kinetic framework. For the second part, I will focus on our study of a system composed of an ensemble of magnetic flux tubes, the dynamics of which can represent the long-term evolution of filamentary Weibel seed fields. The formation of large-scale magnetic fields from initial small-scale fields and the associated inverse energy transfer have been identified as a result of the coalescence of magnetic structures through magnetic reconnection. An analytical model for the time evolution of quantities such as the magnetic energy and the energy-containing scale is constructed within the magnetohydrodynamic description, and is confirmed by our direct numerical simulations. In the end, we apply our study to estimate the scale and strength of the initial seed field for the galactic dynamo problem by considering the interaction between inverse magnetic transfer and ambient turbulence.
