Abstract
Random waves in a uniformly rotating plasma in the presence of a locally uniform seed magnetic field and subject to weak kinematic viscosity and resistivity are considered. These “Lehnert” waves may have either positive or negative helicity, and it is supposed that waves of a single sign of helicity are preferentially excited by a symmetry-breaking mechanism. A mean electromotive force proportional to is derived, demonstrating the conflicting effects of the two diffusive processes. Attention is then focussed on the situation , relevant to conditions in the universe before and during galaxy formation. An -effect, axisymmetric about the rotation vector, is derived, decaying on a time-scale proportional to ; this amplifies a large-scale seed magnetic field to a level independent of , this field being subsequently steady and having the character of a “fossil field”. Subsequent evolution of this fossil field is briefly discussed.
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