Evolution of the Alfvén radius extrapolated from PSP over solar cycle 25. The horizontal black ticks show PSP’s closest approach in each encounter. The distribution of hourly extrapolations using α = 0.1 is shown with coloured histograms. The longitudinally averaged value for each encounter is indicated with a red marker. The associated red bars show the variation in the average due to different wind accelerations (from α = 0 to α = 0.2). The thin and thick grey lines represent the monthly and monthly smoothed sunspot number from WDC-SILSO.