  The fast solar wind is a collisionless plasma permeated by plasma waves on
many different scales. A plasma wave represents the natural interplay between
the periodic changes of the electromagnetic field and the associated coherent
motions of the plasma particles. In this paper, a model velocity distribution
function is derived for a plasma in a single, coherent, large-amplitude wave.
This model allows one to study the kinetic effects of wave motions on particle
distributions. They are by in-situ spacecraft measured by counting, over a
certain sampling time, the particles coming from various directions and having
different energies. We compare our results with the measurements by the Helios
spacecraft, and thus find that by assuming high wave activity we are able to
explain key observed features of the measured distributions within the
framework of our model. We also address the recent discussions on nonresonant
wave--particle interactions and apparent heating. The applied time-averaging
procedure leads to an apparent ion temperature anisotropy which is connected
but not identical to the intrinsic temperature of the underlying distribution
function.
