Does Turbulence Turn off at the Alfvén Critical Surface?

Abstract

Abstract The Parker Solar Probe (PSP) will eventually reach and cross the Alfvén point or surface as it provides us with direct in situ measurements of the solar atmosphere. The Alfvén surface is the location at which the large-scale bulk solar wind speed and the Alfvén speed A are equal, and thus it separates sub-Aflvénic coronal flow from super-Alfvénic solar wind flow . The nature of turbulence at the Alfvén surface is not fully understood, and the PSP measurements at the Alfvén surface will be revealing. We investigate turbulence at the Alfvén 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 β p ≫ 1 regime, whereas the 2017 Zank et al. model applies to a plasma beta regime of order of β p ∼ 1 or ≪1. The distinction in the β p ≫ 1 and β p ≪ 1 or ∼1 turbulence description is in a sense geometric, in that the β p ≫ 1 description yields a fully 3D description of magnetohydrodynamic turbulence whereas β p ≪ 1 or ∼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 Alfvén 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.