Angular Momentum Transport and Proton–Alpha‐Particle Differential Streaming in the Solar Wind

The effect of solar rotation on the proton-alpha differential flow speed,$v_{\alpha p}$, and consequently on the angular momentum transport in the solarwind, is explored. It is found that the force introduced by the azimuthalcomponents plays an important role in the force balance in interplanetaryspace, bringing the radial flow speeds of the species considered closer to eachother. For the fast solar wind, the model cannot account for the decrease of$v_{\alpha p}$ observed by Helios between 0.3 and 1 AU. However, it canreproduce the profile of $v_{\alpha p}$ measured by Ulysses beyond 2 AU, if theright value for $v_{\alpha p}$ is imposed at that distance. In the slow solarwind, the effect of solar rotation is more pronounced if one starts with thevalue measured by Helios at 0.3 AU. In this case, solar rotation introduces arelative change of 10-16% in the radial flow speed of the alpha particlesbetween 1 and 4 AU. The model calculations also show that, although alphaparticles consume only a small fraction of the energy and linear momentumfluxes of protons, they cannot be neglected when considering the proton angularmomentum flux ${\cal L}_p$. In most examples, it is found that ${\cal L}_p$ isdetermined by $v_{\alpha p}$ for both the fast and the slow wind. In the slowsolar wind, it is also found that the proton and alpha angular momentum fluxes${\cal L}_p$ and ${\cal L}_\alpha$ can be several times larger in magnitudethan the flux carried by the magnetic stresses ${\cal L}_M$. While the sum ofthe angular momentum fluxes ${\cal L}_P={\cal L}_p+{\cal L}_\alpha$ of bothspecies is found to be smaller than the magnetic stress ${\cal L}_M$, for thefast and slow wind alike, this result is at variance with the Heliosmeasurements.Comment: 10 pages, 6 figures, ApJ in press (v661n2, June 1 2007 issue), this version differs the previous one in that now it uses the emulateapj packag