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The decay of the solar wind helium to hydrogen temperature ratio due toCoulomb thermalization can be used to measure how far from the Sun strongpreferential ion heating occurs. Previous work has shown that a zone ofpreferential ion heating, resulting in mass-proportional temperatures, extendsabout $20-40 R_\odot$ from the Sun on average. Here we look at the motion ofthe outer boundary of this zone with time and compare it to other physicallymeaningful distances. We report that the boundary moves in lockstep with theAlfv\'en point over the solar cycle, contracting and expanding with solaractivity with a correlation coefficient of better than 0.95 and with an RMSdifference of $4.23 R_\odot$. Strong preferential ion heating apparently ispredominatly active below the Alfv\'en point. To definitively identify theunderlying preferential heating mechanisms, it will be necessary to make insitu measurements of the local plasma conditions below the Alfv\'en surface. Wepredict Parker Solar Probe (PSP) will be the first spacecraft to directlyobserve this heating in action, but only a couple of years after launch asactivity increases, the zone expands, and PSP's perihelion drops.

Strong Preferential Ion Heating is Limited to within the Solar Alfven Surface