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We utilize observations from the Parker Solar Probe (PSP) to study the radialevolution of the solar wind in the inner heliosphere. We analyze electronvelocity distribution functions observed by the Solar Wind Electrons, Alphas,and Protons suite to estimate the coronal electron temperature and the localelectric potential in the solar wind. From the latter value and the local flowspeed, we compute the asymptotic solar wind speed. We group the PSPobservations by asymptotic speed, and characterize the radial evolution of thewind speed, electron temperature, and electric potential within each group. Inagreement with previous work, we find that the electron temperature (both localand coronal) and the electric potential are anti-correlated with wind speed.This implies that the electron thermal pressure and the associated electricfield can provide more net acceleration in the slow wind than in the fast wind.We then utilize the inferred coronal temperature and the extrapolated electric+ gravitational potential to show that both electric field driven exosphericmodels and the equivalent thermally driven hydrodynamic models can explain theentire observed speed of the slowest solar wind streams. On the other hand,neither class of model can explain the observed speed of the faster solar windstreams, which thus require additional acceleration mechanisms.

The Radial Evolution of the Solar Wind as Organized by Electron Distribution Parameters