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

Aims. We present the first measurements of the solar-wind angular-momentum flux recorded by the Solar Orbiter spacecraft. Our aim is the validation of these measurements to support future studies of the Sun’s angular-momentum 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 angular-momentum 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 angular-momentum flux. Results. The angular-momentum flux varies significantly over time. The particle contribution typically dominates over the magneticfield contribution during our measurement interval. The total angular-momentum flux shows the largest variation and is typically anticorrelated 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 angular-momentum flux, yet without a significant increase in the angular momentum 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 angular-momentum flux but otherwise consistent results. Conclusions. Our results largely agree with previous measurements of the solar-wind angular-momentum 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 angular momentum 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 angular-momentum flux in combination with data from Parker Solar Probe and assets at heliocentric distances of 1 au and beyond.