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Minor ion heating in spectra of linearly and circularly polarized Alfvén waves: Thermal and non-thermal motions associated with perpendicular heating

Minor ion (such as $He^{2+}$) heating via nonresonant interaction with spectra of linearly and circularly polarized Alfvén waves (LPAWs and CPAWs hereafter) is studied. The obtained analytic solutions are in good agreement with the simulation results, indicating that newborn ions are heated by low-frequency Alfvén waves with finite amplitude in low-beta plasmas such as the solar corona. The analytic solutions also reproduce the preferential heating of heavy ions in the solar wind. In the presence of parallel propagating Alfvén waves, turbulence-induced particle motion is clearly observed in the wave (magnetic field) polarized directions. After the waves diminish, the newborn ions are heated, which is caused by the phase difference (randomization) between ions due to their different parallel thermal motions. The heating is dominant in the direction perpendicular to the ambient magnetic field. The perpendicular heating, $η=(T_{i\perp}^R-T_{i0\perp}^R)/T_{i0\perp}^R$ (where $T_{i0\perp}^R$ and $T_{i\perp}^R$ are the perpendicular temperature of species $i$ before and after genuine heating, respectively), in the spectrum of CPAWs is a factor of two stronger than that of LPAWs. Moreover, we also study the effect of field-aligned differential flow speed of species $i$ relative to $H^+$, $δv_{ip} = (\mathbf{v_i}-\mathbf{v_p}) \cdot \mathbf{B} / |\mathbf{B}|$ (where $\mathbf{v_i}$ and $\mathbf{v_p}$ denote vector velocities of the $H^+$ and species $i$, respectively), on the perpendicular heating. It reveals that large drift speed, $v_d=δv_{ip}$, has an effect on reducing the efficiency of perpendicular heating, which is consistent with observations.