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Nonlinear Interaction of Minor Heavy Ions with Kinetic Alfvén Waves and Their Anisotropic Energization in Coronal Holes

Some recent observations of the solar corona suggest that minor heavy ions undergo an anisotropic and mass-charge dependent energization in the extended corona. In this paper we investigate the nonlinear interaction of minor heavy ions with kinetic Alfvén waves (KAWs) in the extended corona where plasma is low-β, in particular the ion energization by KAWs. We find that the cross-field ion energization depends on the ion mass and charge in the sense that the velocity is proportional to the ion mass-charge ratio and the field-aligned ion energy is directly proportional to the ion charge. Consequently, the effective ion temperature becomes strongly anisotropic, as well as much higher than that of protons. With an empirical model of plasma parameters in a coronal hole and an assumption of anisotropic spectrum for KAWs, we calculate the variations of the ion temperature anisotropy (δi ≡ T/T) and the ion-proton temperature ratio (γi ≡ T/T) with heliocentric distance. The results show that they both increase rapidly with heliocentric distances of 1.5-3.5 R☉. In the lower corona below 1.3 R☉, one has δi and γi ≃ 1, and in the higher corona above 4 R☉, they approach the saturation values δi ≃ 0.5A and γi ≃ 0.9AZi, where Ai is the ion mass-charge ratio in units of the proton mass-charge ratio mp/e, and Zi is the ion charge in units of the elementary charge e. We suggest that not only can the KAW-based model of the anisotropic and mass-charge dependent energization reasonably explain the recent observations of minor heavy ions in polar coronal holes, but KAWs may also be potentially important for understanding the microphysics of the wave-particle interaction in the extended solar corona, where the coronal plasma is heated and accelerated into the solar wind.