Angular momentum transport in a multicomponent solar wind with differentially flowing, thermally anisotropic ions

Institute of Mathematics and Physics, Aberystwyth University, SY23 3BZ, UK e-mail: [bbl;xxl]@aber.ac.uk

Received: 19 September 2008
Accepted: 27 November 2008

Abstract

Context. The Helios measurements of the angular momentum flux L of the fast solar wind lead to a tendency for the fluxes associated with individual ion angular momenta of protons and alpha particles, L_p and , to be negative (i.e., in the sense of counter-rotation with the Sun). However, the opposite holds for the slow wind, and the overall particle contribution tends to exceed the magnetic contribution L_M. These two aspects are at variance with previous models.

Aims. We examine whether introducing realistic ion temperature anisotropies can resolve this discrepancy.

Methods. From a general set of multifluid transport equations with gyrotropic species pressure tensors, we derive the equations governing both the meridional and azimuthal dynamics of outflows from magnetized, rotating stars. The equations are not restricted to radial flows in the equatorial plane but valid for general axisymmetric winds that include two major ion species. The azimuthal dynamics are examined in detail, using the empirical meridional flow profiles for the solar wind, constructed mainly according to measurements made in situ.

Results. The angular momentum flux L is determined by the requirement that the solution to the total angular momentum conservation law is unique and smooth in the vicinity of the Alfvén point, defined as where the combined Alfvénic Mach number . M_T has to consider the contributions from both protons and alpha particles. Introducing realistic ion temperature anisotropies may introduce a change of up to in L and up to ~1.8  in azimuthal speeds of individual ions between 0.3 and 1 AU, compared with the isotropic case. The latter has strong consequences on the relative importance of L_P and L_M in the angular momentum budget.

Conclusions. However, introducing ion temperature anisotropies cannot resolve the discrepancy between in situ measurements and model computations. For the fast-wind solutions, while in extreme cases L_P may become negative, L_p never does. On the other hand, for the slow solar wind solutions examined, L_P never exceeds L_M, even though L_M may be less than the individual ion contribution, since L_p and always have opposite signs for the slow and fast wind alike.