(From top to bottom) The plasma bulk speed V, the Alfvén speed ﹩{V}_{{\rm{A}}}=21.8\,\tfrac{B}{\sqrt{n}}﹩ (B in units of nT, n in units of cm−3), and the PSP radial distance to the Sun R. Dark and light blue lines in the top panel refer to slow (V < 400 km s−1) and fast (V > 400 km s−1) solar wind intervals, respectively.