Snapshots of the coronal magnetic field at; (1) sunspot minimum (blue label), (2) after ∼1.5 yr (purple), (3) after ∼3 yr (red), (4) after ∼5 yr (orange), and (5) after ∼7.5 yr (green), for solar cycles 21–25 (cycle 25 is in progress). Each PFSS field line extrapolation is coloured by the photospheric rotation rate at its footpoint. The surface of each model is coloured by the corresponding radial magnetic field strength. The source of each magnetogram, along with the Carrington rotation number is given in the labels. The monthly sunspot number is shown below, with the time of each snapshot identified by a coloured marker. The bottom row displays the effective rotation rate of the open magnetic field at the source surface (weighted by sin θ), derived from a timeseries of PFSS models using the WSO (black) or SOHO/MDI & SDO/HMI (magenta) Carrington magnetograms.