  Perihelion passes on Parker Solar Probe orbits six through nine have been
studied to show that solar wind core electrons emerged from 15 solar radii with
a temperature of 55 plus or minus 5 eV, independent of the solar wind speed
which varied from 300 to 800 km/sec. After leaving 15 solar radii and in the
absence of triggered ion acoustic waves at greater distances, the core electron
temperature varied with radial distance, R, in solar radii, as 1900R-4/3
electron volts because of cooling produced by the adiabatic expansion. The
coefficient, 1900, reproduces the minimum core electron perpendicular
temperature observed during the 25 days of observation. In the presence of
triggered ion acoustic waves, the core electrons were isotropically heated as
much as a factor of two above the minimum temperature, 1900R-4/3 eV. Triggered
ion acoustic waves were the only waves observed in coincidence with the
electron core heating. They are the dominant wave mode at frequencies greater
than 100 Hz at solar distances between 15 and 30 solar radii.
