  Angular momentum evolution in low-mass stars is determined by initial
conditions during star formation, stellar structure evolution, and the
behaviour of stellar magnetic fields. Here we show that the empirical picture
of angular momentum evolution arises naturally if rotation is related to
magnetic field strength instead of to magnetic flux, and formulate a corrected
braking law based on this. Angular momentum evolution then becomes a strong
function of stellar radius, explaining the main trends observed in open
clusters and field stars at a few Gyr: the steep transition in rotation at the
boundary to full convection arises primarily from the large change in radius
across this boundary, and does not require changes in dynamo mode or field
topology. Additionally, the data suggest transient core-envelope decoupling
among solar-type stars, and field saturation at longer periods in very low-mass
stars. For solar-type stars, our model is also in good agreement with the
empirical Skumanich law. Finally, in further support of the theory, we show
that the predicted age at which low-mass stars spin down from the saturated to
unsaturated field regimes in our model corresponds remarkably well to the
observed lifetime of magnetic activity in these stars.
