PSD comparison of the solar ¹⁰Be data (large dots) and a DRW model (smaller dots) with τ = 1000 years (indicated by the dotted line). A DRW process has a spectral index of  − 2 on short time scales (shown by the dashed line), which is shallower than that for the ¹⁰Be data on short time scales. The dotted-dashed line shows the average PSD for many DRW models with τ = 1000 years.