Kinetic effects on the velocity-shear-driven instability

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Abstract
A comparison is made between the properties of the low‐frequency, long‐wavelength velocity‐shear‐driven instability in kinetic theory and magnetohydrodynamics (MHD). The results show that the removal of adiabaticity along the magnetic field line in kinetic theory leads to modifications in the nature of the instability. Although the threshold for the instability in the two formalisms is the same, i.e., VA∥<V0<Cs, the kinetic growth rate and the unstable range in wave‐number space can be larger or smaller than the MHD values depending on the ratio between the thermal speed, Alfvén speed, and flow speed. When the thermal speed is much larger than the flow speed and the flow speed is larger than the Alfvén speed, the kinetic formalism gives a larger maximum growth rate and broader unstable range in wave‐number space. In this regime, the normalized wave number (kL) for instability can be larger than unity, while in MHD it is always less than unity. The normal mode profile in the kinetic case has a wider spatial extent across the shear layer.