  Orbits six through nine of the Parker Solar Probe have been studied to show
that solar wind core electrons emerged from 15 solar radii with temperatures
that were constant to within ~10% although the solar wind speed varied from 300
to 800 km/sec. After leaving 15 solar radii, the core electrons were
isotropically heated as much as a factor of two below 30 solar radii by
triggered and natural ion acoustic waves. To distinguish this wave heating from
processes occurring inside 15 solar radii, the electron temperature is modeled
as having two components, the base temperature observed at 15 solar radii and
the temperature resulting from wave interactions between 15 solar radii and the
observing point. As further justification of this temperature model, there were
about a dozen intervals during the four orbits, summing to about 250 hours of
total time, during which there were few waves, TE/TI was small, and the core
electron temperature was close to or slightly greater than the base
temperature. Both this base temperature and the temperature resulting from wave
heating decreased with radius due to the adiabatic expansion of the solar wind.
The waves that produced the electron core heating were triggered [Mozer et al,
2021] or normal {Mozer et al, 2020A] ion acoustic waves. They are the dominant
wave modes at frequencies greater than 100 Hz at solar distances between 15 and
30 solar radii.
