Authors: Michael F. Zhang (University of Otago), Evan L. Yerger (University of New Hampshire), Matthew W. Kunz (Princeton University), Jonathan Squire (University of Otago)

Alfvénic turbulence is thought to be vital to powering the solar wind and corona, yet has eluded a comprehensive understanding of the kinetic processes by which it dissipates. Minor ions serve as sensitive tracers of these processes, showing extreme perpendicular temperatures relative to the local magnetic field and, perplexingly, mass-weighted temperature trends that sometimes correlate, but sometimes anticorrelate, with mass-to-charge ratio, Aᵢ/Zᵢ. We use a combination of quasilinear theory and 3D hybrid-kinetic particle-in-cell simulations to explain these features and predict further correlations with other properties of the turbulence in the fast solar wind. When Alfvénic turbulence is imbalanced, its cascade to ion-Larmor scales is throttled by the helicity barrier. This barrier ultimately leads to high-frequency proton-cyclotron waves (PCWs), both oblique and parallel, the latter of which produce very flat electric-energy spectra, ℰ_E ~ k_parallel^(-η) with η < 2, over the range of scales that are cyclotron resonant with minor ions. While steeper spectra lead to a positive correlation of heating with Aᵢ/Zᵢ, the shallower spectra cause the dependence to invert, with Qᵢ ∝ Qₚ Aᵢ(Aᵢ/Zᵢ)^(η−2). This result is corroborated by a set of six simulations of both balanced and imbalanced turbulence, spanning βₚ₀ = {1, 0.3, 1/16}, which demonstrate minor-ion heating rates following the power-law scaling (Aᵢ/Zᵢ)^a. We show that minor-ion heating is strongest and most perpendicular in imbalanced turbulence at lower βₚ₀, with extreme temperature ratios T⊥,O⁵⁺/Tₚ ≈ 40 and anisotropy T⊥,O⁵⁺/T∥,O⁵⁺ ∼ 10 at βₚ₀ = 1/16, in agreement with low-coronal observations of extreme temperature ratios and anisotropies. Future minor-ion measurements should test whether intervals in which minor-ion thermal speeds decrease with increasing mass-to-charge ratio are associated with a history of large cross helicity, enhanced power in parallel PCWs, and a steep transition-range spectrum.