= Planetary dynamo
A planetary dynamo maintains an intrinsic <magnetic field> by motion of an electrically conducting fluid. The <resistive induction equation> balances stretching and transport against <magnetic diffusion>. A sufficiently large <magnetic Reynolds number> is necessary but does not ensure that the flow regenerates every required field component. Thermal or compositional <convection> supplies mechanical power; rotation organizes the flow through the <Coriolis force>, and the <Lorentz force density> eventually modifies the flow and limits growth. A fluid core can fail to operate a dynamo if it is stably stratified or inadequately driven.
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