Damping of fast and ion cyclotron oblique waves in the multi-ion fast solar wind

A detailed linear analysis of the behavior of oblique fast waves in a multi-ion low-beta Vlasov plasma such as the solar wind is presented. It is shown that at very oblique propagation angles, proton fundamental and second harmonic cyclotron damping become dominant. The property of oblique fast waves can also be significantly changed in the presence of minor ions. Depending on their abundance and relative speed with respect to protons, minor ions can change the wave polarization from right-handed at small wave numbers to left-handed at larger wave numbers for some propagation angles. Consequently, a power law spectrum of oblique fast waves originating at the Sun may preferentially heat and accelerate alpha particles rather than protons and electrons in the fast solar wind. Furthermore, it is shown that oblique fast waves can heat minor ions more than oblique Alfvénic ion cyclotron waves thus making them a potentially more important candidate for the preferential heating of minor ions in the solar wind than ion cyclotron waves.