Dependence of extreme minor-ion heating on β and imbalance in solar-wind turbulence

Minor ions in the solar wind can have extremely enhanced temperatures relative to the bulk protons, with preferentially perpendicular temperature anisotropies. The two main dissipation mechanisms of Alfvénic turbulence considered capable of such preferential, perpendicular heating of ions -- ion-cyclotron-wave (ICW) heating and stochastic heating -- differentially heat species depending on mass and charge. Their potency depends too on plasma β, and the level of imbalance between outward- and inward-propagating Alfvénic fluctuations. In particular, the elevated inertial-range amplitudes (and frequencies) that occur above the 'helicity barrier', which forms for imbalanced turbulence in low-β plasmas, promotes heating by exciting high-frequency ICWs. We present results from four Pegasus++ hybrid-kinetic simulations of both imbalanced and balanced turbulence at proton β=[0.3,1/16], each with multiple minor-ion species. Clear phase-space signatures of ICW and stochastic heating are seen in the imbalanced and balanced cases, respectively. Minor-ion heating is more extreme for the former, with temperatures consistent with empirical scalings from spacecraft measurements. We uncover scalings for ion temperatures with charge and mass, and how they are influenced by imbalance and β. Through comparison with measurements from PSP or SO, these physically informed scalings and corresponding phase-space signatures could be used to identify different turbulent heating regimes in the solar wind.