  The effect of solar rotation on the proton-alpha differential flow speed,
$v_{\alpha p}$, and consequently on the angular momentum transport in the solar
wind, is explored. It is found that the force introduced by the azimuthal
components plays an important role in the force balance in interplanetary
space, bringing the radial flow speeds of the species considered closer to each
other. 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 can
reproduce the profile of $v_{\alpha p}$ measured by Ulysses beyond 2 AU, if the
right value for $v_{\alpha p}$ is imposed at that distance. In the slow solar
wind, the effect of solar rotation is 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 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 ${\cal L}_p$. In most examples, it is found that ${\cal L}_p$ is
determined by $v_{\alpha p}$ 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
${\cal L}_p$ and ${\cal L}_\alpha$ can be several times larger in magnitude
than the flux carried by the magnetic stresses ${\cal L}_M$. While the sum of
the angular momentum fluxes ${\cal L}_P={\cal L}_p+{\cal L}_\alpha$ of both
species is found to be smaller than the magnetic stress ${\cal L}_M$, for the
fast and slow wind alike, this result is at variance with the Helios
measurements.

Astrophysics