Enhanced minor ion heating in imbalanced, Alfvénic turbulence

Recent theory and simulations of imbalanced, Alfvénic turbulence in low-beta plasma have shown that the conservation of generalized helicity can lead to a build up of inertial-range turbulent energy above the proton gyroscale. This 'helicity barrier' eventually excites a spectrum of ion-cyclotron-wave fluctuations, which heat protons and thereby dissipate a majority of the energy flux. In doing so, the helicity barrier provides a compelling explanation for how low-frequency Alfvénic turbulence in the fast solar wind can self-consistently produce the observed high-frequency fluctuations and enhanced perpendicular heating of protons. Spacecraft observations also indicate that minor ion species, such as O$^{+5}$ and He$^{++}$, can be substantially hotter than protons. We add a minor-ion module to the hybrid-kinetic particle-in-cell code, Pegasus++, and investigate the differential heating of minor ions in strong, imbalanced turbulence. We demonstrate that the enhanced fluctuation amplitudes and frequencies that occur above the helicity barrier can promote stochastic and cyclotron heating, respectively, in a manner explicable by simple scaling arguments. We also provide various phase-space signatures of this heating that may be sought in data taken from the near-Sun solar wind by Parker Solar Probe.