Source alignment occurrences between PSP and SO. Panel (a): bulk solar wind speed; panel (b): Spacecraft radial distance; panel (c): coronal magnetic field expansion factor derived from the PFSS model; panel (d): Carrington latitude of the photospheric footpoints. The bulk speed in panel (a) is averaged over the injection time τ_inj (see Appendix A.1 for the details). Panels (a) and (b) show PSP and SO observations using horizontal crosses and dots, respectively. Mean trends are represented by running averages over 10 occurrences, shown with solid and dashed lines for PSP and SO, respectively, in panel (a). The corresponding PSP encounters for each alignment are indicated in panel (b) and separated by vertical dashed lines.