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

Daniel Verscharen , David Stansby , Adam J. Finley (1) , Christopher J. Owen , Timothy Horbury , Milan Maksimovic (2) , Marco Velli , Stuart D. Bale , Philippe Louarn (3) , Andrei Fedorov (3) , Roberto Bruno , Stefano Livi , Yuri V. Khotyaintsev , Antonio Vecchio (2) , Gethyn R. Lewis , Chandrasekhar Anekallu , Christopher W. Kelly , Gillian Watson , Dhiren O. Kataria , Helen O'Brien , Vincent Evans , Virginia Angelini

1 AIM (UMR_7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
2 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics
3 IRAP - Institut de recherche en astrophysique et planétologie

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.

Mots clés: magnetohydrodynamics (MHD); plasmas; Sun: magnetic fields; solar wind; stars: rotation; Physics - Space Physics; Astrophysics - Solar and Stellar Astrophysics; Physics - Plasma Physics

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

Daniel Verscharen, David Stansby, Adam J. Finley, Christopher J. Owen, Timothy Horbury, et al.. The angular-momentum flux in the solar wind observed during Solar Orbiter's first orbit. Astronomy and Astrophysics - A&A, 2021, 656, ⟨10.1051/0004-6361/202140956⟩. ⟨insu-03672346⟩