Does Turbulence Turn off at the Alfven Critical Surface?

The Parker Solar Probe (PSP) will eventually reach and cross the Alfven point or surface as it provides us with direct in situ measurements of the solar atmosphere. The Alfven surface is the location at which the large-scale bulk solar wind speed U and the Alfven speed V-A are equal, and thus it separates sub-Aflvenic coronal flow vertical bar U vertical bar << V-A from super-Alfvenic solar wind flow vertical bar U vertical bar >> V-A. The nature of turbulence at the Alfven surface is not fully understood, and the PSP measurements at the Alfven surface will be revealing. We investigate turbulence at the Alfven surface from a theoretical perspective by using the 2012 and 2017 Zank et al. turbulence transport model equations. The 2012 Zank et al. description is applicable to a large plasma beta beta(p) >> 1 regime, whereas the 2017 Zank et al. model applies to a plasma beta regime of order of beta(p) similar to 1 or << 1. The distinction in the beta(p) >> 1 and beta(p) << 1 or similar to 1 turbulence description is in a sense geometric, in that the beta(p) >> 1 description yields a fully 3D description of magnetohydrodynamic turbulence whereas beta(p) >> 1 or similar to 1 describes predominantly quasi-2D (with respect to the large-scale or mean magnetic field) turbulence and a minority slab turbulence component. Our analyses suggest that turbulence at the Alfven surface (i) turns off if the higher order plasma beta turbulence transport model equations are used and (ii) does not turn off if the lower order plasma beta, nearly incompressible turbulence transport model equations are used.