  The decay of the solar wind helium to hydrogen temperature ratio due to
Coulomb thermalization can be used to measure how far from the Sun strong
preferential ion heating occurs. Previous work has shown that a zone of
preferential ion heating, resulting in mass-proportional temperatures, extends
about $20-40 R_\odot$ from the Sun on average. Here we look at the motion of
the outer boundary of this zone with time and compare it to other physically
meaningful distances. We report that the boundary moves in lockstep with the
Alfv\'en point over the solar cycle, contracting and expanding with solar
activity with a correlation coefficient of better than 0.95 and with an RMS
difference of $4.23 R_\odot$. Strong preferential ion heating apparently is
predominatly active below the Alfv\'en point. To definitively identify the
underlying preferential heating mechanisms, it will be necessary to make in
situ measurements of the local plasma conditions below the Alfv\'en surface. We
predict Parker Solar Probe (PSP) will be the first spacecraft to directly
observe this heating in action, but only a couple of years after launch as
activity increases, the zone expands, and PSP's perihelion drops.
