Constraints on dynamo action in plasmas

Abstract

Upper bounds are derived on the amount of magnetic energy that can be generated by dynamo action in collisional and collisionless plasmas with and without external forcing. A hierarchy of mathematical descriptions is considered for the plasma dynamics: ideal MHD, visco-resistive MHD, the double-adiabatic theory of Chew, Goldberger and Low (CGL), kinetic MHD, and other kinetic models. It is found that dynamo action is greatly constrained in models where the magnetic moment of any particle species is conserved. In the absence of external forcing, the magnetic energy then remains small at all times if it is small in the initial state. In other words, a small "seed" magnetic field cannot be amplified significantly, regardless of the nature of flow, as long as the collision frequency and gyroradius are small enough to be negligible. A similar conclusion also holds if the system is subject to external forcing as long as this forcing conserves the magnetic moment of at least one plasma species and does not greatly increase the total energy of the plasma (i.e., in practice, is subsonic). Dynamo action therefore always requires collisions or some small-scale kinetic mechanism for breaking the adiabatic invariance of the magnetic moment.

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Cited by in corpus (11)

• Dynamo theories
• Fluctuation Dynamo in a Collisionless, Weakly Magnetized Plasma
• Available energy and ground states of collisionless plasmas
• Fluctuation dynamo in a weakly collisional plasma
• Magneto-immutable turbulence in weakly collisional plasmas
• Dynamo in weakly collisional nonmagnetized plasmas impeded by Landau damping of magnetic fields
• Plasma physics of the intracluster medium
• A Measurement of the Effective Mean-Free-Path of Solar Wind Protons
• The effect of pressure-anisotropy-driven kinetic instabilities on magnetic field amplification in galaxy clusters
• Dynamical generation of macroscale magnetic fields and fast flows in a four-component astrophysical plasma
• Turbulent dynamo in the terrestrial magnetosheath