The Revolution Revolution: magnetic morphology driven spin-down.

Observations of young open clustersshow a bimodaldistribution of rotation periodsthat has been difficult to explain with existing stellarspin-down models. Detailed MHD stellar windsimulations have demonstrated that surface magnetic field morphology has a strong influence on wind-driven angular momentum loss. Observations suggest that faster rotating stars store a larger fraction of their magnetic flux in higher-order multipolar components of the magnetic field. In this work, we present a new model for stellarspin-down that, for the first time, accounts for the stellarsurface magnetic field configuration. We show how a magnetic complexity that evolves from complex toward simple configurations as a star spins down can explain the salient features of stellar rotationevolution, including the bimodaldistribution of both slow and fast rotators seen in young open clusters.