Volume 656, December 2021
Solar Orbiter First Results (Cruise Phase)
|Number of page(s)||10|
|Section||The Sun and the Heliosphere|
|Published online||14 December 2021|
The angular-momentum flux in the solar wind observed during Solar Orbiter’s first orbit
Mullard Space Science Laboratory, University College London, Holmbury House, Holmbury St. Mary, Dorking RH5 6NT, UK
2 Space Science Center, University of New Hampshire, Durham, NH 03824, UK
3 Department of Astrophysics-AIM, University of Paris-Saclay and University of Paris, CEA, CNRS, 91191 Gif-sur-Yvette Cedex, France
4 Department of Physics, The Blackett Laboratory, Imperial College London, London SW7 2AZ, UK
5 LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
6 Department of Earth, Planetary, and Space Sciences, University of California Los Angeles, Los Angeles, CA 90095, USA
7 Physics Department, University of California, Berkeley, CA 94720, USA
8 Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
9 School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
10 Institut de Recherche en Astrophysique et Planétologie, 31028 Toulouse Cedex 4, France
11 INAF-Istituto di Astrofisica e Planetologia Spaziali, 00133 Roma, Italy
12 Southwest Research Institute, San Antonio, TX 78238, USA
13 Swedish Institute of Space Physics (IRF), Box 537 751 21 Uppsala, Sweden
14 Radboud Radio Lab., Department of Astrophysics/IMAPP-Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
Accepted: 31 May 2021
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.
Key words: magnetohydrodynamics (MHD) / plasmas / Sun: magnetic fields / solar wind / stars: rotation
© ESO 2021
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