Pressure-anisotropy-mediated Helical Dynamo in Turbulent Collisionless Plasmas

Abstract

The origin of coherent, near-equipartitioned magnetic fields in the universe remains poorly understood, especially in collisionless scenarios. By means of theoretical modeling that includes indispensable kinetic effects and fully kinetic particle-in-cell simulation that contains sufficient scale separation, we self-consistently show that the collisionless large-scale dynamo is quite efficient and the system-scale coherent fields can be generated from kinetic-scale seeds under a turbulent helical drive. We find that, by triggering kinetic mirror and firehose instabilities, the pressure anisotropy—a kinetic effect that is unresolved in conventional magnetohydrodynamics—is the key physics that produces net magnetic helicity that can be inversely transferred to large scales. The magnetic helicity generation rate can be formulated as $\propto {u}_{{\rm{rms}}}^{2}/{L}_{{\rm{in}}}$ , where u _rms is the turbulent velocity and L _in is the driving scale. Our results profoundly refine the picture of cosmic dynamos and potentially resolve the critical issue of dynamo quench.

Keywords

• Plasma astrophysics
• Cosmic magnetic fields theory
• Magnetic fields
• Plasma physics