  Plasma processes occurring in the corona and solar wind can be probed by
studying the thermodynamic properties of different ion species. However, most
in-situ observations of positive ions in the solar wind are taken at 1 AU,
where information on their solar source properties may have been irreversibly
erased. In this study we aimed to use the properties of alpha particles at
heliocentric distances between 0.3 and 1 AU to study plasma processes occurring
at the points of observation, and to infer processes occurring inside 0.3 AU by
comparing our results to previous remote sensing observations of the plasma
closer to the Sun. We reprocessed the original Helios positive ion distribution
functions, isolated the alpha particle population, and computed the alpha
particle number density, velocity, and magnetic field perpendicular and
parallel temperatures. We then investigated the radial variation of alpha
particle temperatures in fast solar wind observed between 0.3 and 1 AU. Between
0.3 and 1 AU alpha particles are heated in the magnetic field perpendicular
direction, and cooled in the magnetic field parallel direction. Alpha particle
evolution is bounded by the alpha firehose instability threshold, which
provides one possible mechanism to explain the observed parallel cooling and
perpendicular heating. Closer to the Sun our observations suggest that the
alpha particles undergo heating the perpendicular direction, whilst the large
magnetic field parallel temperatures observed at 0.3 AU may be due the combined
effect of double adiabatic expansion and alpha particle deceleration inside 0.3
AU.
