Locating the Alfvén Surface: A Computed Crossing Radius and Its Sensitivity to Field and Density

The Alfvén surface, where the solar wind outflow speed first exceeds the Alfvén speed, divides the corona that rotates with the Sun from the wind that does not, and Parker Solar Probe first crossed it in 2021. This study locates the crossing with a spiral field model anchored at 3.0 nanotesla radial field at 1 AU, an inverse-square density anchored at 6.0 per cubic centimetre, and a 400 kilometre per second outflow. The Alfvén speed falls from 269 kilometres per second at 0.1 AU to 39.2 kilometres per second at 1 AU, and equals the outflow speed at 0.067 AU, or 14.3 solar radii. Varying the field anchor between 2 and 4 nanotesla and the density anchor between 4 and 10 per cubic centimetre moves the crossing between 7.4 and 23.6 solar radii, a range that brackets the roughly 19 solar radii crossing reported from the first Parker Solar Probe entry into magnetically dominated corona. The study is simulated and computed, not measured.