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The extreme temperatures and non-thermal nature of the solar corona and solarwind arise from an unidentified physical mechanism that preferentially heatscertain ion species relative to others. Spectroscopic indicators of unequaltemperatures commence within a fraction of a solar radius above the surface ofthe Sun, but the outer reach of this mechanism has yet to be determined. Herewe present an empirical procedure for combining interplanetary solar windmeasurements and a modeled energy equation including Coulomb relaxation tosolve for the typical outer boundary of this zone of preferential heating.Applied to two decades of observations by the Wind spacecraft, our results areconsistent with preferential heating being active in a zone extending from thetransition region in the lower corona to an outer boundary 20-40 solar radiifrom the Sun, producing a steady state super-mass-proportional$\alpha$-to-proton temperature ratio of $5.2-5.3$. Preferential ion heatingcontinues far beyond the transition region and is important for the evolutionof both the outer corona and the solar wind. The outer boundary of this zone iswell below the orbits of spacecraft at 1 AU and even closer missions such asHelios and MESSENGER, meaning it is likely that no existing mission hasdirectly observed intense preferential heating, just residual signatures. Wepredict that {Parker Solar Probe} will be the first spacecraft with a periheliasufficiently close to the Sun to pass through the outer boundary, enter thezone of preferential heating, and directly observe the physical mechanism inaction.

A zone of preferential ion heating extends tens of solar radii from Sun