Bulk speed radial variation between PSP and SO for all source alignments. Left panel: bulk speed as a function of radial distance for source alignments. The alignments are color-coded by the radial spacecraft separation between the two probes. Right panel: distribution of the relative solar wind speed increase Δv per radial decade, derived from PSP and SO source alignments. Each bin is color-coded by the average wind speed observed at PSP. The vertical black dashed line indicates the average value.