Equipartition model derives parameter-free v^{-5} spectrum for suprathermal ions from Alfvénic turbulence

Boldyrev et al. derive the observed f(v)∝v^{-5} tail by requiring that the turbulent cascade transfers energy to ions at each scale until the ion kinetic energy density equals the magnetic fluctuation energy density. This saturation occurs locally inside strong turbulence regions, setting both slope and normalization without adjustable parameters. Quiet-time spectra fall below the predicted level, implying nonlocal acceleration elsewhere. The calculation reproduces the absolute intensity across three decades in magnetic energy when Voyager 1 and ACE magnetic-field strengths are inserted. The common spectrum has been documented for decades yet lacked a first-principles amplitude prediction. Earlier stochastic-acceleration models required fine-tuning or assumed specific wave spectra; the equipartition closure removes those freedoms. It also supplies a physical reason why the same index appears from 1 AU to the heliosheath. Parker Solar Probe and Solar Orbiter turbulence measurements can now test whether the spectrum reaches the predicted ceiling precisely where the turbulence is strong and balanced. Discrepancies would indicate missing dissipation channels or nonlocal transport effects. Confirmation would tighten constraints on particle injection into the anomalous cosmic-ray population.