The effect of turbulence on the angular momentum of the solar wind

TL;DR

This paper investigates how kinetic and magnetic turbulence influence the angular momentum of the solar wind, using a theoretical model and observational data to quantify the turbulence's effect, which is found to be a small but significant negative contribution.

Contribution

The study introduces a theoretical framework that incorporates turbulence effects into the classical Weber & Davis model, providing analytical expressions for turbulence-induced modifications to solar wind angular momentum.

Findings

Turbulence modifies the solar wind's angular momentum by approximately 3-10%.

The turbulent effect tends to be negative, reducing the angular momentum.

The model aligns with Parker Solar Probe observations at the Alfvén surface.

Abstract

The transfer of a star's angular momentum to its atmosphere is a topic of considerable and wide-ranging interest in astrophysics. This letter considers the effect of kinetic and magnetic turbulence on the solar wind's angular momentum. The effects are quantified in a theoretical framework that employs Reynolds-averaged mean field magnetohydrodynamics, allowing for fluctuations of arbitrary amplitude. The model is restricted to the solar equatorial (\(r-\phi\)) plane with axial symmetry, which permits the effect of turbulence to be expressed in analytical form as a modification to the classic Weber & Davis (1967) theory, dependent on the \(r,\phi\) shear component of the Reynolds stress tensor. A solar wind simulation with turbulence transport modeling and Parker Solar Probe observations at the Alfv\'en surface are employed to quantify this turbulent modification to the solar wind's angular momentum, which is found to be ~ 3% - 10% and tends to be negative. Implications for solar and stellar rotational evolution are discussed.