We extend previous theories of stochastic ion heating to account for the
motion of ions along the magnetic field. We derive an analytic expression for
the ion-to-proton perpendicular temperature ratio in the solar wind for any ion
species, assuming that stochastic heating is the dominant ion heating
mechanism. This expression describes how this temperature ratio depends upon
the average velocity of the ions along the magnetic field direction and the
ratio of the parallel proton pressure to the magnetic pressure. We compare our
model with previously published measurements of alpha particles and protons
from the WIND spacecraft. We find that stochastic heating offers a promising
explanation for these measurements when the fractional cross helicity and
Alfven ratio at the proton-gyroradius scale have values that are broadly
consistent with solar-wind measurements.
