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

Abstract

Aims: We present the first measurements of the solar-wind angular-momentum (AM) flux recorded by the Solar Orbiter spacecraft. Our aim is the validation of these measurements to support future studies of the Sun's AM loss. Methods: We combine 60-minute averages of the proton bulk moments and the magnetic field measured by the Solar Wind Analyser (SWA) and the magnetometer (MAG) onboard Solar Orbiter. We calculate the AM flux per solid-angle element using data from the first orbit of the mission's cruise phase during 2020. We separate the contributions from protons and from magnetic stresses to the total AM flux. Results: The AM flux varies significantly over time. The particle contribution typically dominates over the magnetic-field contribution during our measurement interval. The total AM 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, that leads to a significant localised increase in the AM flux, yet without a significant increase in the AM per unit mass. We repeat our analysis using the density estimate from the Radio and Plasma Waves (RPW) instrument. Using this independent method, we find a decrease in the peaks of positive AM flux but otherwise consistent results. Conclusions: Our results largely agree with previous measurements of the solar-wind AM flux in terms of amplitude, variability, and dependence on radial solar-wind bulk speed. Our analysis highlights the potential for future, more detailed, studies of the solar wind's AM and its other large-scale properties with data from Solar Orbiter. We emphasise the need to study the radial evolution and latitudinal dependence of the AM flux in combination with data from Parker Solar Probe and assets at heliocentric distances of 1 au and beyond.

10 pages, 9 figures. Accepted for publication in A&A

References in corpus (16)

• The Solar Orbiter mission -- Science overview
• The Mass-Dependence of Angular Momentum Evolution in Sun-Like Stars
• The Effect of Combined Magnetic Geometries on Thermally Driven Winds I: Interaction of Dipolar and Quadrupolar Fields
• Constraining the evolution of stellar rotation using solar twins
• Coronal Magnetic Field Topology From Total Solar Eclipse Observations
• Turbulence characteristics of switchbacks and non-switchbacks intervals observed by \emph{Parker Solar Probe}
• Density Fluctuations Associated with Turbulence and Waves: First Observations by Solar Orbiter
• Global solar magnetic field organization in the outer corona: influence on the solar wind speed and mass flux over the cycle
• Magnetic braking of Sun-like and low-mass stars: Dependence on coronal temperature
• Deceleration of Alpha Particles in the Solar Wind by Instabilities and the Rotational Force: Implications for Heating, Azimuthal Flow, and the Parker Spiral Magnetic Field
• Direct Detection of Solar Angular Momentum Loss with the Wind Spacecraft
• Determination of Solar Wind Angular Momentum and Alfvén Radius from Parker Solar Probe Observations
• Flux conservation, radial scalings, Mach numbers, and critical distances in the solar wind: magnetohydrodynamics and Ulysses observations
• The Solar Wind Angular Momentum Flux as Observed by Parker Solar Probe
• Rotation of solar analogs cross-matching Kepler and Gaia DR2
• The Contribution of Alpha Particles to the Solar Wind Angular Momentum Flux in the Inner Heliosphere

Cited by in corpus (3)

• Accounting for Differential Rotation in Calculations of the Sun's Angular Momentum-loss Rate
• Reconstructing the Sun's Alfvén surface and wind braking torque with Parker Solar Probe
• Transport of electrons in tangled magnetic fields