Dispersion relations for parallel‐propagating Alfvén waves are shown with two examples of the resonance condition for helium. The wave polarization and direction is indicated in each quadrant, and outgoing (incoming) waves are marked with squares (circles). The frequency is expressed in terms of the proton gyrofrequency, and the wavenumber in terms of the gyrofrequency over the Alfvén speed. Line 1 corresponds to ﹩v_{\parallel }=-0\mathrm{.}\,1V_{\mathrm{A}\,}﹩, which exhibits resonance with both incoming and outgoing waves, while line 2 corresponds to ﹩v_{\parallel }=0\mathrm{.}\,5V_{\mathrm{A}\,}﹩ and has no resonance with outgoing waves.