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S. Parhi, S. Suess, M. Sulkanen (NASA/Marshall Space Flight Center)
We study the velocity shear between plumes and the interplume flow in coronal holes. We model these plumes as jets (or, strictly speaking, wakes). Weak and strong magnetic fields are considered both inside and outside the jet for a shear Mach number 6. The shear can be unstable and evolve into a new less sheared pattern. As the instability sets in, the jet first develops a cocoon of intermediate speed flow and slowly a bridge develops between upstream and downstream flows. This marks the onset of jet disruption via what appears to be mass entrainment and fluid instability. This could also be induced by the jet's passage through the accompanying fast shock formation. The jet bends upon crossing the oblique shocks because all streamlines bend away from the shock normal. In a short time the downstream flow just ahead of the bending suffers a change in speed but still maintains or reestablishes supersonic conditions somehow. The transverse velocity here is very low because the instability generated in the disturbed region reduces the shear ahead. The shear ultimately must dissipate. The generation of this instability depends both temporally and spatially on the amount of shear and the time needed for nonlinear growth. To analyse the fluctuations quantitatively we perform a time series analysis at various points inside and adjacent to the jet. Specifically we consider points either in the center of the jet or just outside the transition layer- the initial location of the shear layer. We find the fully developed nonlinear fluctuations are more Alfvénic than magnetosonic in the high \beta case than in low \beta case.
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