Summed 1-dimensional histograms of the proton-proton collisional age for different selected time frames. Top: DoY 1−164.44 in 2001. Middle: 1−164.44 in 2005. Bottom: DoY 1−164.44 in 2009. In each subplot, the upper panel gives the non-normalized histograms of all observations for the three solar wind types (coronal hole wind in dark blue, helmet-streamer plasma in orange, and sector-reversal plasma in dark green, following the Xu & Borovsky 2015 scheme), the lower panel shows the three histograms normalized according to solar wind type. The black solid vertical lines indicate the respective minimum between coronal hole wind and helmet-streamer plasma, and the black dashed line indicates the minimum between helmet-streamer and sector-reversal plasma. (To make all three histograms visible at the same time, the alpha channel is set to 0.5). The sample size is indicated in the legend in each subplot.