Abstract: Weakened magnetic braking (WMB) was originally proposed in 2016 to explain anomalously rapid rotation in old field stars observed by the Kepler mission. The proximate cause was suggested to be a transition in magnetic morphology from larger to smaller spatial scales. In a series of papers over the past five years, we have collected spectropolarimetric measurements to constrain the large-scale magnetic fields for a sample of stars spanning this transition, including a range of spectral types from late F to early K. As we gradually expanded the sample, we attempted to trace the onset of WMB back to its root causes: from its influence on stellar rotation periods, to the hypothesized shift in magnetic morphology, and ultimately to the evolution of the global stellar dynamo. Recently, we reanalyzed the entire sample with a focus on uniformity for the observational inputs, leading to a paradigm shift in our understanding of this phenomenon and its relevance to our own Sun. I will provide an overview of the current evidence for WMB, and I will summarize what the resulting constraints tell us about the evolution of stellar dynamos beyond the middle of main-sequence lifetimes.
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