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S. Parhi (NASA/ Marshall Space Flight Center), S.T. Suess (NASA/Marshall Space Flight Center)
The velocity shear which exists between three layers in an ideal plasma is studied. This configuration is very similar to a polar plume embedded in solar wind and can be modelled as a jet (or, strictly speaking a wake) using a MHD code developed for astrophysical jet simulations. Weak and strong magnetic fields are considered both inside and outside the jet with varying shear Mach numbers of the jet. The shear can be Kelvin-Helmholtz unstable and evolve into a new less sheared pattern as observed in plumes down a few solar radii. This numerical result is compared with the recently derived analytical result obtained from the dispersion relations which govern the MHD shear instabilities. These analytical results suggest that there exist two more dispersion relations apart from the two known relations which have been used over the past few years for compressible fluid or plasma studies. These dispersion relations in their incompressible limit exactly reduce to the relations studied elsewhere. The necessary condition for stability is now that both the Alfven Mach number of the jet and of the ambient should be less than unity, in contrast to the earlier observation that only one of them should be less than unity. This clearly elucidates the competition between plasma and magnetic shear confirming our numerical observations and is a consequence of finding the two extra modes.
The author(s) of this abstract have provided an email address for comments about the abstract: shyam.parhi@msfc.nasa.gov