The radial temperature profile of the solar wind

The Voyager data show a decrease in temperature in the inner heliosphere, an increase in temperature from 30–50 AU, a decrease from 50–63 AU, followed by another increase from 63–68 AU. Models of pickup proton heating predict a monotonic temperature rise beyond about 30 AU but do not account for the smaller scale (few AU) temperature variations. At 1 AU, the solar wind temperature is a strong function of the solar wind speed. We find that incorporating a temperature dependence on speed into the pickup proton heating results can reproduce much of the smaller‐scale temperature variation observed out to 68 AU. The same speed‐temperature dependence provides good fits to data from both the outer heliosphere and from near Earth. Since a large fraction of the proton energy results from heating, this work implies that the heating rate is a function of the speed.