Effects of Non-WKB Alfvén Waves on a Multicomponent Solar Wind with Differential Ion Flow

We present multicomponent solar wind models self-consistently incorporating the contribution from dissipationless, monochromatic, hydromagnetic (with angular frequencies ω well below ion gyrofrequencies), toroidal Alfvén waves, which are coupled to the flow through the wave-induced ponderomotive forces. Protons and alpha particles are treated on an equal footing, and the wavelength is not assumed to be small compared to the spatial scales at which the solar wind parameters vary. We find that the non-WKB effects are significant, for the fast and slow solar wind solutions alike. In comparison with their non-WKB counterparts, the WKB ones are more effective in accelerating the solar wind inside the Alfvén point, producing significantly enhanced ion fluxes and considerably reduced alpha abundance in the inner corona. Only when \omega \gtrsim 3.5\times 10^{-3} (1.5\times 10^{-3} ) rad s–1 can the waves in the fast (slow) winds be adequately described by the WKB limit. Moreover, while the Alfvén waves tend to reduce the magnitude of the proton-alpha speed difference \vert U_{\alpha p}\vert in general, different mechanisms operate in two different regimes separated by an \omega _{c}\sim \mathrm{several}\,\times 10^{-5} rad s–1. This \omega _{c} , defined by equation (15), is closely related to the time required by a solar wind parcel to traverse an Alfvén radius with the speed of center of mass evaluated at the Alfvén point. When \omega > \omega _{c} , the fluctuations are wavelike and tend to accelerate both ion species, thereby losing most of their energy by doing work on ion flows; whereas when \omega <\omega _{c} , a quasi-static behavior results: the fluctuations may act to accelerate the slower flowing ion species but decelerate the faster moving one in a large portion of the computational domain, and only a minor fraction of the wave energy flux injected at the base is lost. The fluctuations with the lowest frequency are no less effective in reducing \vert U_{\alpha p}\vert than the WKB waves: in the slow solar wind solutions, they may be able to quench a significant \vert U_{\alpha p}\vert with base amplitudes as small as 4 km s–1. The consequences of \omega _{c} on the velocity fluctuation spectra of protons and alpha particles, which are likely to be obtained by future missions like the Solar Orbiter and Solar Probe, are discussed.