Dynamo processes in stars
The term ‘dynamo action’ appears in the literature describing two related but distinct processes. When the velocity v of a conducting fluid is driven by the non-magnetic forces in the direction opposite to that of the Lorentz force, the rate of working (j × B/c) · v is negative, implying an input of energy into the magnetic field; and in the high magnetic Reynolds number domain, the input on balance exceeds the Ohmic dissipation, so that an already existing field is amplified. This chapter discusses the ‘self-exciting dynamo’ problem, which demands that there be no sources of the magnetic field B other than the currents generated by fluid motions in the presence of B. It covers laminar kinematic dynamos; the Parker model; turbulent dynamos; kinematic models of the turbulent dynamo; non-linear dynamical feedback; fundamental problems; the role of magnetic helicity in the dynamo problem; numerical simulations; and dynamo action guided by a strong pre-existing field.
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