Solar cycle modulation of solar wind collisionality and mass-proportional alpha particle heating across different wind speed regimes. Panel (a) presents the monthly sunspot number (black) and F10.7 solar radio flux (blue) as indicators of solar activity from 1995 to 2024. Panel (b) shows the percentage of collisionless solar wind intervals (A_c < 1) for various speed bins, with the corresponding Spearman rank correlation coefficients (ρ) and Pearson correlation coefficients (r) with solar activity separately for SC23, SC24, and the full interval (“All”) indicated in the legend. Panel (c) displays the percentage occurrence of strong alpha particle heating (T_α/T_p > 4) in each speed bin, again with the corresponding Spearman rank correlation coefficients (ρ) and Pearson correlation coefficients (r) with solar activity separately for SC23, SC24, and the full interval (“All”) shown in the legend. Panel (d) summarizes the Spearman rank correlation coefficients (ρ) and Pearson correlation coefficients (r) between the occurrence percentages shown in panels (b) and (c) and solar activity as a function of solar wind speed bin. The gray vertical dashed line marks the transition between SC23 and SC24, while the black vertical dashed line marks the transition between SC24 and SC25.