# PSP Bibliography

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Alfv\ en Speed Transition Zone in the Solar Corona

 Author Wexler, David; Stevens, Michael; Case, Anthony; Song, Paul; Keywords Parker Data Used; Solar wind; Solar corona; 1534; 1483 Abstract The Alfv\ en radius, r$_A$, at which solar wind radial outflow speed exceeds the Alfv\ en 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 Alfv\ en radius is often referenced in the study of solar wind corotation and dynamics. Alfv\ en 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 Alfv\ en speeds within 30 solar radii of the heliocenter. We combined the PSP solar-wind speed measurements and calculated Alfv\ en speeds with an inner corona wind speed profile from remote sensing studies. The zone of super-Alfv\ enic speed cross over is estimated to occur at mean heliocentric distance of 17.9 \ensuremath\pm 2.1 R$_\ensuremath\odot$ for slow solar winds of the low heliolatitude corona in a near-minimum solar activity state. Our r$_A$ values constrain the angular momentum flux to a mean of 3.5 \ensuremath\pm 1.01 \texttimes 10$^22$ N m sr$^-1$, reinforcing the recent PSP results by direct measurements of particle flows. Year of Publication 2021 Journal \apjl Volume 919 Number of Pages L33 Section Date Published oct ISBN URL DOI 10.3847/2041-8213/ac25fa