Authors: Alaa Fayad (Department of Physics, The University of Texas at Austin, Austin, TX 78712, USA) , Anna Tenerani (Department of Physics, The University of Texas at Austin, Austin, TX 78712, USA), Chadi Salem (Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA)

Solar wind protons exhibit temperature anisotropies that depart from double-adiabatic expectations, implying the presence of additional heating processes. To explore the mechanisms responsible for solar wind heating, we analyze data from the Helios 1 and 2 spacecraft to investigate the radial evolution of adiabatic invariants and anisotropic heating rates of fast solar wind core and beam protons. In contrast to previous studies, our analysis incorporates departures from the Parker spiral arising from large-amplitude magnetic fluctuations, resulting in substantial differences in the inferred parallel heating rates relative to earlier estimates. While previous work has questioned the reliability of power-law fits for determining heating rates and suggested tracking the radial evolution of adiabatic invariants as a more robust alternative, we demonstrate that both approaches employed here yield consistent conclusions. Our results show that core protons require parallel heating to reproduce the observed temperature evolution, whereas beam protons experience parallel cooling. Moreover, all solar wind components examined in this study require perpendicular heating, consistent with earlier findings. We propose that core–beam kinetic instabilities may play an important role in regulating fast solar wind thermodynamics by driving preferential perpendicular heating and parallel cooling.