The sun's Alfvén surface: recent insights and prospects for the Polarimeter to Unify the Corona and Heliosphere (PUNCH)

The radial distance at which the wind's outflow speed exceeds the phase speed of Alfv eacute;nic and fast-mode magnetohydrodynamic (MHD) waves is called the Alfv eacute;n radius. In one-dimensional models, this is a singular point beyond which most fluctuations in the plasma and magnetic field cannot propagate back down to the Sun.

In the multi-dimensional solar wind, this point can occur at different distances along an irregularly shaped "Alfv eacute;n surface." In this article, we review the properties of this surface and discuss its importance in models of solar-wind acceleration, angular-momentum transport, MHD waves and turbulence, and the geometry of magnetically closed coronal loops. We also review the results of simulations and data-analysis techniques that aim to determine the location of the Alfv eacute;n surface.