Heating and cooling of protons in the fast solar wind between 0.3 and 1 AU: Helios revisited

The proton thermal energetics in the fast solar wind between 0.3 and 1 AU is reinvestigated using the Helios 1 and 2 data. Closer to the Sun, it is estimated that, to account for the observed radial profiles of the proton parallel and perpendicular temperature, nonnegligible parallel cooling and perpendicular heating are necessary. Around 1 AU heating is needed in both directions. We also calculate the corresponding rates and find that in total significant interplanetary heating is necessary, in agreement with previous results. The possible influence that deceleration of fast solar wind streams due to interaction with slow ones has on the proton thermodynamics is evaluated.