Proton temperature as a function of the distance to the Sun. The three groups of points represent PSP E8 data, OMNI data (2018–2019–2020), and Ulysses data (2007). Points are colored as a function of the wind speed, with slow wind (300 km/s) in dark purple and fast wind (800 km/s) in yellow. Polytropic decay profiles are shown for the fast and slow winds and for γ = 1.1, 1.2, and base temperatures of 7 × 10⁵ K (plain) or 1.1 × 10⁵ K (dashed) at 0.1 au.