  The fast solar wind's high speeds and nonthermal features require that
significant heating occurs well above the Sun's surface. Two leading theories
seem incompatible: low-frequency "Alfv\'enic" turbulence, which transports
energy outwards and is observed ubiquitously by spacecraft but struggles to
explain the observed dominance of ion over electron heating; and high-frequency
ion-cyclotron waves (ICWs), which explain the nonthermal heating of ions but
lack an obvious source. Here, we argue that the recently proposed "helicity
barrier" effect, which limits electron heating by inhibiting the turbulent
cascade of energy to the smallest scales, can unify these two paradigms. Our
six-dimensional simulations show how the helicity barrier causes the
large-scale energy to grow in time, generating small parallel scales and
high-frequency ICW heating from low-frequency turbulence. The resulting
turbulence and ion distribution function also closely match in-situ
measurements from Parker Solar Probe and other spacecraft, explaining, among
other features, the decades-long puzzle of the steep "transition range"
observed in magnetic fluctuation spectra. The theory predicts a causal link
between plasma expansion and the ion-to-electron heating ratio. Given the
observational association between wind speed and expansion, we argue that the
helicity barrier could play a key role in regulating the bimodal speed
distribution of the solar wind.
