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The angular-momentum flux in the solar wind observed during Solar Orbiter s first orbit

AuthorVerscharen, Daniel; Stansby, David; Finley, Adam; Owen, Christopher; Horbury, Timothy; Maksimovic, Milan; Velli, Marco; Bale, Stuart; Louarn, Philippe; Fedorov, Andrei; Bruno, Roberto; Livi, Stefano; Khotyaintsev, Yuri; Vecchio, Antonio; Lewis, Gethyn; Anekallu, Chandrasekhar; Kelly, Christopher; Watson, Gillian; Kataria, Dhiren; Brien, Helen; Evans, Vincent; Angelini, Virginia; SWA, MAG; Teams, RPW;
Keywordsmagnetohydrodynamics (MHD); plasmas; Sun: magnetic fields; Solar wind; stars: rotation; Physics - Space Physics; Astrophysics - Solar and Stellar Astrophysics; Physics - Plasma Physics
Abstract\ Aims: We present the first measurements of the angular- momentum flux in the solar wind recorded by the Solar Orbiter spacecraft. Our aim is to validate these measurements to support future studies of the Sun s angular-momentum loss. \ Methods: We combined 60-min averages of the proton bulk moments and the magnetic field measured by the Solar Wind Analyser and the magnetometer onboard Solar Orbiter. We calculated the angular-momentum flux per solid-angle element using data from the first orbit of the mission s cruise phase in 2020. We separated the contributions from protons and from magnetic stresses to the total angular-momentum flux. \ Results: The angular-momentum flux varies significantly over time. The particle contribution typically dominates over the magnetic-field contribution during our measurement interval. The total angular-momentum flux shows the largest variation and is typically anti-correlated with the radial solar-wind speed. We identify a compression region, potentially associated with a co-rotating interaction region or a coronal mass ejection, which leads to a significant localised increase in the angular-momentum flux, albeit without a significant increase in the angular momentum per unit mass. We repeated our analysis using the density estimate from the Radio and Plasma Waves instrument. Using this independent method, we find a decrease in the peaks of positive angular-momentum flux, but otherwise, our results remain consistent. \ Conclusions: Our results largely agree with previous measurements of the solar wind s angular-momentum flux in terms of amplitude, variability, and dependence on radial solar-wind bulk speed. Our analysis highlights the potential for more detailed future studies of the solar wind s angular momentum and its other large-scale properties with data from Solar Orbiter. We emphasise the need for studying the radial evolution and latitudinal dependence of the angular-momentum flux in combination with data from Parker Solar Probe and other assets at heliocentric distances of 1 au and beyond.
Year of Publication2021
Number of PagesA28
Date Publisheddec