Simplified schematic of the propagation method for a purely radial plasma speed (not to scale). The PSP position at a considered time t_in is indicated by the blue dot. We represent with green dots and at subsequent times (t₁, t₂, and t₃) the position of the plasma parcel that passed PSP at t_in and that propagated outward with V (green vector). We also represent the position of Solar Orbiter (orange dots) and the distance between the parcel and Solar Orbiter (double purple arrows) at the same times. The simplified trajectories of PSP and Solar Orbiter are shown by dashed blue and orange lines, respectively. The trajectory of the plasma parcel is shown by the dashed black line.