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The solar wind is the extension of the Sun's hot and ionized corona, and itexists in a state of continuous expansion into interplanetary space. The radialdistance at which the wind's outflow speed exceeds the phase speed of Alfvenicand fast-mode magnetohydrodynamic (MHD) waves is called the Alfven radius. Inone-dimensional models, this is a singular point beyond which most fluctuationsin the plasma and magnetic field cannot propagate back down to the Sun. In themulti-dimensional solar wind, this point can occur at different distances alongan irregularly shaped "Alfven surface." In this article, we review theproperties of this surface and discuss its importance in models of solar-windacceleration, angular-momentum transport, MHD waves and turbulence, and thegeometry of magnetically closed coronal loops. We also review the results ofsimulations and data analysis techniques that aim to determine the location ofthe Alfven surface. Combined with recent perihelia of Parker Solar Probe, thesestudies seem to indicate that the Alfven surface spends most of its time atheliocentric distances between about 10 and 20 solar radii. It is becomingapparent that this region of the heliosphere is sufficiently turbulent thatthere often exist multiple (stochastic and time-dependent) crossings of theAlfven surface along any radial ray. Thus, in many contexts, it is more usefulto make use of the concept of a topologically complex "Alfven zone" rather thanone closed surface. This article also reviews how the Polarimeter to Unify theCorona and Heliosphere (PUNCH) mission will measure the properties of theAlfven surface and provide key constraints on theories of solar-windacceleration.

The Sun's Alfven Surface: Recent Insights and Prospects for the Polarimeter to Unify the Corona and Heliosphere (PUNCH)