Extrapolation of the solar wind along Parker spiral magnetic field lines for PSP encounters E04 and E06 (left), and E16 and E18 (right). The path of PSP is coloured red or blue according to the radial magnetic field polarity. Parker spiral magnetic field lines are coloured by solar wind speed using an acceleration of α = 0.1. The solid black lines show the Alfvén surface when averaged into bins of two degrees width in Carrington longitude. The dashed semi-circles represents the longitudinally averaged Alfvén radii.