Operation and Saturation of the Turbulent Dynamo in a Collisionless Magnetized Plasma

Turbulent amplification of cosmic magnetic fields is constrained in collisionless plasmas by phase mixing of flow variation along the magnetic field and by conservation of the first adiabatic invariant. Using hybrid-kinetic simulations, we show that ion-Larmor-scale instabilities (firehose, mirror), triggered by the adiabatic production of magnetic-field-aligned pressure anisotropy as the magnetic field is stretched, efficiently pitch-angle scatter particles and obviate these constraints. As a result, many features of the turbulent dynamo in a collisionless magnetized plasma are similar to those found in its Pm » 1 MHD counterpart, including an exponential kinematic phase, during which a folded-field topology and Kazantsev spectrum are established, and saturation, in which the magnetic and kinetic energies come into approximate equipartition (albeit not scale by scale). However, there are key differences in the field statistics and dynamo efficiency due to the interplay between the forcing scale, the shrinking ion-Larmor scale, and the evolving scales on which the magnetic field bends and folds. Comparisons with MHD and Braginskii-MHD simulations of turbulent dynamo are made