  Observations of young open clusters show a bimodal distribution of rotation
periods that has been difficult to explain with existing stellar spin-down
models. Detailed MHD stellar wind simulations 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 stellar spin-down
that, for the first time, accounts for the stellar surface 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 rotation evolution, including the bimodal distribution of
both slow and fast rotators seen in young open clusters.
