Alfvén Speed Transition Zone in the Solar Corona

The Alfvén radius, rA, at which solar wind radial outflow speed exceeds the Alfvén wave speed, is an important parameter in understanding solar wind evolution in the extended corona. The mean solar wind angular momentum scales with ${r}_{A}^{2}$ in the axisymmetric steady-state approximation, so the Alfvén radius is often referenced in the study of solar wind corotation and dynamics. Alfvén wave speed is derived from the magnetic-field intensity and plasma mass density. In the inner coronal regions, these parameters were previously estimated using empirical models based on remote sensing observations or from inverse-square scaling of measurements at 1 au. Parker Solar Probe (PSP) orbital encounters now provide in situ coronal plasma measurements to determine Alfvén speeds within 30 solar radii of the heliocenter. We combined the PSP solar-wind speed measurements and calculated Alfvén speeds with an inner corona wind speed profile from remote sensing studies. The zone of super-Alfvénic speed cross over is estimated to occur at mean heliocentric distance of 17.9 ± 2.1 R⊙ for slow solar winds of the low heliolatitude corona in a near-minimum solar activity state. Our rA values constrain the angular momentum flux to a mean of 3.5 ± 1.01 × 1022 N m sr−1, reinforcing the recent PSP results by direct measurements of particle flows.