Determination of Solar Wind Angular Momentum and Alfvén Radius from Parker Solar Probe Observations

Abstract

As fundamental parameters of the Sun, the Alfvén radius and angular momentum loss determine how the solar wind changes from sub-Alfvénic to super-Alfvénic and how the Sun spins down. We present an approach to determining the solar wind angular momentum flux based on observations from Parker Solar Probe (PSP). A flux of about $0.15\times 10^{30}$ dyn cm sr${}^{-1}$ near the ecliptic plane and 0.7:1 partition of that flux between the particles and magnetic field are obtained by averaging data from the first four encounters within 0.3 au from the Sun. The angular momentum flux and its particle component decrease with the solar wind speed, while the flux in the field is remarkably constant. A speed dependence in the Alfvén radius is also observed, which suggests a "rugged" Alfvén surface around the Sun. Substantial diving below the Alfvén surface seems plausible only for relatively slow solar wind given the orbital design of PSP. Uncertainties are evaluated based on the acceleration profiles of the same solar wind streams observed at PSP and a radially aligned spacecraft near 1 au. We illustrate that the "angular momentum paradox" raised by Réville et al. can be removed by taking into account the contribution of the alpha particles. The large proton transverse velocity observed by PSP is perhaps inherent in the solar wind acceleration process, where an opposite transverse velocity is produced for the alphas with the angular momentum conserved. Preliminary analysis of some recovered alpha parameters tends to agree with the results.

arXiv commentAccepted for publication in The Astrophysical Journal Letters

References in corpus (5)

• The Solar Probe Cup on Parker Solar Probe
• The Solar Probe ANalyzers -- Electrons on Parker Solar Probe
• First in-situ Measurements of Electron Density and Temperature from Quasi-Thermal Noise Spectroscopy with Parker Solar Probe/FIELDS
• The role of Alfvén wave dynamics on the large scale properties of the solar wind: comparing an MHD simulation with PSP E1 data
• Direct Detection of Solar Angular Momentum Loss with the Wind Spacecraft

Cited by in corpus (8)

• Prediction and Verification of Parker Solar Probe Solar Wind Sources at 13.3 R${}_{\odot }$
• An Extended and Fragmented Alfvén Zone in the Young Solar Wind
• On the Generation and Evolution of Switchbacks and the Morphology of Alfvénic Transition: Low Mach-number Boundary Layers
• Total Electron Temperature Derived from Quasi-Thermal Noise Spectroscopy In the Pristine Solar Wind: Parker Solar Probe Observations
• In Situ Measurements of Dark Photon Dark Matter Using Parker Solar Probe: Going beyond the Radio Window
• Dominance of 2-Minute Oscillations near the Alfvén Surface
• The solar-wind angular-momentum flux observed during Solar Orbiter's first orbit
• Inward Propagating Plasma Parcels in the Solar Corona: Models with Aerodynamic Drag, Ablation, and Snowplow Accretion