From Magnetic Field Seed Generation to Dynamo in Collisionless Plasmas

Abstract

The origin of the cosmic magnetization present throughout the universe, greatly impacting the development of a diverse set of astrophysical systems, has long been a subject of research. Observations of cosmic magnetic fields, for example in the weakly collisional intracluster medium, show field magnitudes consistent with having been produced through the dynamo process, which causes small seed magnetic fields in plasmas to grow exponentially through the conversion to magnetic energy from kinetic energy in turbulent flows. Magnetohydrodynamics, the standard method of modeling dynamos, is not valid for weakly collisional systems since it assumes that a high collisionality dominates over competing small-scale processes, such as magnetic field seed-generating plasma instabilities. However, a fully kinetic treatment of the problem in electron-proton plasmas is prohibitively expensive numerically. In this thesis an advanced collisionless fluid plasma model, with a complexity between that of magnetohydrodynamics and a kinetic model, is used to study magnetic field growth in a plasma in the presence of driven turbulent flows. A significant decoupling of the ion and electron dynamics is observed for the employed mass-ratio, allowing access to previously unexplored physics. The simulations demonstrate both the dynamo process and magnetic field seed generation through the electron Weibel instability. The fluid and magnetic Reynolds numbers, key quantities in classical dynamo theory, are generalized for a collisionless system. This is done by studying the dependence of flow and magnetic energy damping rates on the strength of the pressure isotropization encoded in the ion and electron heat flux closures of the fluid model. A scan over the effective magnetic Reynolds number is performed for the dynamo simulations by adjusting the strength of the electron closure. Strong pressure isotropization from the closure leads to behavior of the magnetic field growth expected from classical dynamo theory, whereas a small isotropization results in seed generation from pressure anisotropy-driven instabilities dominating, as previously observed in kinetic simulations of electron-positron plasmas.