Direct Detection of Solar Angular Momentum Loss with the Wind Spacecraft

TL;DR

This study provides the first direct in-situ measurement of the Sun's angular momentum loss via the solar wind, revealing a rate consistent with models but weaker than needed for Skumanich-like stellar spin-down.

Contribution

It offers a new estimate of the solar angular momentum flux from 1994-2019 using Wind spacecraft data, separating contributions from particles and magnetic stresses.

Findings

Protons carry about three times more angular momentum flux than magnetic stresses.

Fast and slow wind streams tend to have opposite angular momentum flux signs.

The estimated solar angular momentum loss rate is 3.3×10^{30} erg, about half of what is needed for Skumanich-like spin-down.

Abstract

The rate at which the solar wind extracts angular momentum from the Sun has been predicted by theoretical models for many decades, and yet we lack a conclusive measurement from in-situ observations. In this letter we present a new estimate of the time-varying angular momentum flux in the equatorial solar wind, as observed by the \textit{Wind} spacecraft from 1994-2019. We separate the angular momentum flux into contributions from the protons, alpha particles, and magnetic stresses, showing that the mechanical flux in the protons is $\sim$3 times larger than the magnetic field stresses. We observe the tendency for the angular momentum flux of fast wind streams to be oppositely signed to the slow wind streams, as noted by previous authors. From the average total flux, we estimate the global angular momentum loss rate of the Sun to be $3.3\times10^{30}$erg, which lies within the range of various MHD wind models in the literature. This angular momentum loss rate is a factor of $\sim$2 weaker than required for a Skumanich-like rotation period evolution ($\Omega_*\propto $ stellar age$^{-1/2}$), which should be considered in studies of the rotation period evolution of Sun-like stars.