Relationship between the measured velocity, v_wind, and the final expansion factor value, f_ss, computed with PFSS from the back-mapping applied to the data in Fig. 1. Panel a: The wind speed, v_wind, measured by Solar Orbiter as a function of the expansion factor, f_ss, calculated using PFSS at the source surface (located at r_ss). The mapping results cover from 1/08/2020 to 17/03/2022. The photospheric magnetic field is displayed with the color coding shown in the color bar, and with partial transparency to limit the masking effect. The distribution has a Pearson correlation coefficient of −0.27. Panel b: Same as panel a but colored in black for low unsigned latitudes (< 45°) and in orange for high latitude footpoints (> 45°) for each mapped observation. The typical range of values from Wang & Sheeley (1990) study, mapping observation at 1 au, are displayed by the horizontal green bars. The high and low latitudes distributions have a Pearson correlation coefficient of −0.51 and −0.24, respectively.