Does Turbulence Turn off at the Alfv\ en Critical Surface?

<p>The\&nbsp;<em>Parker Solar Probe</em>\&nbsp;(<em>PSP</em>) will eventually reach and cross the Alfv\ en point or surface as it provides us with direct in situ measurements of the solar atmosphere. The Alfv\ en surface is the location at which the large-scale bulk solar wind speed\&nbsp;<img alt="$\boldsymbolU$" src="" />\&nbsp;and the Alfv\ en speed\&nbsp;<img alt="$\boldsymbolV$" src="" />\&nbsp;<sub>A</sub>\&nbsp;are equal, and thus it separates sub-Aflv\ enic coronal flow\&nbsp;<img alt="$| \boldsymbolU| \ll | \boldsymbolV_\rmA| $" src="" />\&nbsp;from super-Alfv\ enic solar wind flow\&nbsp;<img alt="$| \boldsymbolU| \gg | \boldsymbolV_\rmA| $" src="" />. The nature of turbulence at the Alfv\ en surface is not fully understood, and the\&nbsp;<em>PSP</em>\&nbsp;measurements at the Alfv\ en surface will be revealing. We investigate turbulence at the Alfv\ en 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\&nbsp;<em>β</em>\&nbsp;<sub><em>p</em>\&nbsp;</sub><img alt="Gt" src="" />\&nbsp;1 regime, whereas the 2017 Zank et al. model applies to a plasma beta regime of order of\&nbsp;<em>β</em>\&nbsp;<sub><em>p</em>\&nbsp;</sub>\&nbsp;~\&nbsp;1 or\&nbsp;<img alt="Lt" src="" />1. The distinction in the\&nbsp;<em>β</em>\&nbsp;<sub><em>p</em>\&nbsp;</sub><img alt="Gt" src="" />\&nbsp;1 and\&nbsp;<em>β</em>\&nbsp;<sub><em>p</em>\&nbsp;</sub><img alt="Lt" src="" />\&nbsp;1 or ~1 turbulence description is in a sense geometric, in that the\&nbsp;<em>β</em>\&nbsp;<sub><em>p</em>\&nbsp;</sub><img alt="Gt" src="" />\&nbsp;1 description yields a fully 3D description of magnetohydrodynamic turbulence whereas\&nbsp;<em>β</em>\&nbsp;<sub><em>p</em>\&nbsp;</sub><img alt="Lt" src="" />\&nbsp;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 Alfv\ en 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.</p>
Year of Publication
The Astrophysical Journal
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Date Published