Dynamical magnetic relaxation: A nonlinear magnetically driven dynamo

A non-linear, time-dependent, magnetically driven dynamo theory which shows how magnetically dominated configurations can relax to become helical on the largest scale available is presented. Coupled time-dependent differential equations for large scale magnetic helicity, small scale magnetic helicity, velocity, and the electromotive force are solved. The magnetic helicity on small scales relaxes to drive significant large scale helical field growth on dynamical (Alfv\'en crossing) time scales, independent of the magnitude of finite microphysical transport coefficients, after which the growing kinetic helicity slows the growth to a viscously limited pace. This magnetically driven dynamo complements the nonlinear kinetic helicity driven dynamo; for the latter, the growing magnetic helicity fluctuations suppress, rather than drive, large scale magnetic helicity growth. A unified set of equations accommodates both types of dynamos.