  We utilize observations from the Parker Solar Probe (PSP) to study the radial
evolution of the solar wind in the inner heliosphere. We analyze electron
velocity distribution functions observed by the Solar Wind Electrons, Alphas,
and Protons suite to estimate the coronal electron temperature and the local
electric potential in the solar wind. From the latter value and the local flow
speed, we compute the asymptotic solar wind speed. We group the PSP
observations by asymptotic speed, and characterize the radial evolution of the
wind speed, electron temperature, and electric potential within each group. In
agreement with previous work, we find that the electron temperature (both local
and coronal) and the electric potential are anti-correlated with wind speed.
This implies that the electron thermal pressure and the associated electric
field 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 exospheric
models and the equivalent thermally driven hydrodynamic models can explain the
entire 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 wind
streams, which thus require additional acceleration mechanisms.
