Alfvénicity of fluctuations associated with the Kelvin–Helmholtz instability

Abstract

The velocity shear which exists between three layers in an ideal plasma is studied. This configuration is modeled as a jet (or, strictly speaking, a wake) embedded in a uniform medium using a magnetohydrodynamics (MHD) code developed for astrophysical jet simulations. Weak and strong magnetic fields are considered both inside and outside the jet with a shear Mach number of 6. The shear can be Kelvin–Helmholtz (KH) unstable and evolve into a new less sheared pattern. There exists extensive literature on the KH instability which is extended by quantitatively describing the MHD properties of the fluctuations associated with the instability. To do so, a time series analysis of the fluctuations at various points inside and adjacent to the jet is performed. Specifically, points either in the center of the jet or just outside the transition layer—the initial location of the shear layer are considered. In the nonlinear stage, the perturbation is found to be a sum of the fast magnetosonic mode, slow magnetosonic mode, and the Alfvén component. To quantitatively evaluate the fluctuations, the normalized cross-helicity and Elsässer ratios are calculated, which in turn measure the degree of Alfvénicity. Fully nonlinear fluctuations are found to be more Alfvénic than magnetosonic in the low β case (β≈0.833) as compared with high β case (β≈13.3). This is in contrast to linear modes generated by the KH instability, which are magnetosonic modes.