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

TL;DR

This study investigates how ion temperature anisotropies influence angular momentum transport in the solar wind, revealing that such anisotropies slightly alter fluxes but do not resolve existing measurement-model discrepancies.

Contribution

It introduces a multifluid model with anisotropic ion pressures to analyze angular momentum fluxes in the solar wind, highlighting the limited impact of anisotropies on resolving observational discrepancies.

Findings

Ion anisotropies cause up to 10% change in angular momentum flux.

Anisotropies lead to up to 1.8 km/s difference in ion azimuthal speeds.

Discrepancies between measurements and models remain unresolved.

Abstract

The Helios measurements of the angular momentum flux $L$ for the fast solar wind show that the individual ion contributions, $L_p$ and $L_\alpha$, tend 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 $L_P = L_p + L_\alpha$ tends to exceed the magnetic one $L_M$. These aspects are at variance with previous models. We examine whether introducing realistic ion temperature anisotropies can resolve this discrepancy. From the general multifluid transport equations with gyrotropic species pressure tensors, we derive the equations governing both the meridional and azimuthal dynamics of general axisymmetrical, rotating stellar winds that include two major ion species. The azimuthal dynamics are examined in detail, using the empirically constructed meridional flow profiles for the solar wind. We find that $L$ is determined by requiring that the solution to the total angular momentum conservation law is unique and smooth close to the Alfven point, where the combined Alfvenic Mach number $M_T=1$. Introducing realistic ion temperature anisotropies may introduce a change of up to 10% in $L$ and up to 1.8 km/s 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 $L_M$. However, introducing ion temperature anisotropies cannot resolve the discrepancy between measurements and models. For the fast-wind solutions, while in extreme cases $L_P$ becomes negative, $L_p$ never does. On the other hand, for the slow-wind solutions, $L_P$ never exceeds $L_M$, even though $L_M$ may be less than the individual ion contribution, since $L_p$ and $L_\alpha$ always have opposite signs for the slow and fast wind alike.