Abstract

The solar wind is the extension of the Sun's hot and ionized corona, and it exists in a state of continuous expansion into interplanetary space. 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... Show moreThe solar wind is the extension of the Sun's hot and ionized corona, and it exists in a state of continuous expansion into interplanetary space. 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. Combined with recent perihelia of Parker Solar Probe, these studies seem to indicate that the Alfv eacute;n surface spends most of its time at heliocentric distances between about 10 and 20 solar radii. It is becoming apparent that this region of the heliosphere is sufficiently turbulent that there often exist multiple (stochastic and time-dependent) crossings of the Alfv eacute;n surface along any radial ray. Thus, in many contexts, it is more appropriate to use the concept of a topologically complex "Alfv eacute;n zone" rather than one closed surface. This article also reviews how the Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission will measure the properties of the Alfv eacute;n surface and provide key constraints on theories of solar-wind acceleration. Show less