Angular Momentum Transport and Proton-alpha Differential Streaming in the Solar Wind

The effect of solar rotation on the proton-alpha differential flow speed, vαp, and consequently on the angular momentum transport in the solar wind, is explored. Using a 3-fluid model, it is found that the radial component of the momentum equation for ions is modified by the force introduced by the azimuthal components. This force plays an important role in the force balance in interplanetary space, hence impacts the radial flow speed of the species considered, bringing them closer to each other. For the fast solar wind, the model cannot account for the decrease of vαp observed by Helios between 0.3 and 1 AU. However, it is capable of reproducing the profile of vαp measured by Ulysses beyond 2 AU, if the right value for vαp is imposed at that distance. In the slow solar wind, on the other hand, the effect of solar rotation seems to be more pronounced if one starts with the value measured by Helios at 0.3 AU. In this case, solar rotation introduces a relative change of 10-16% in the radial component of the flow speed of the alpha particles between 1 and 4 AU. The model calculations also show that, although alpha particles consume only a small fraction of the energy and linear momentum fluxes of protons, they cannot be neglected when considering the proton angular momentum flux Lp. In most examples, it is found that Lp is determined by vαp for both the fast and the slow wind. In the slow solar wind, it is also found that the proton and alpha angular momentum fluxes Lp and Lα can be several times larger in magnitude than the flux carried by the magnetic stresses LM . While the sum of the angular momentum fluxes LP = Lp +Lα of both species is found to be smaller than the magnetic stress LM , for the fast and slow wind alike, this result is at variance with the Helios measurements. Subject headings: solar wind — Sun: magnetic fields