Anomalous transport by Kelvin-Helmholtz instabilities

Abstract

Simulation of magnetohydrodynamic Kelvin-Helmholtz instabilities has been performed for parallel (B ̰o ∥ V ̰o) and transverse (B ̰o ⊥ V ̰o) configurations, modeling high latitude (or downstream flanks) and dayside low latitude magnetospheric boundaries. In the parallel configuration, a super-Alfvénic and trans-sonic shear flow develops into small eddies, which strongly compresses, twists, and hence amplifies the magnetic field by the dynamo action with an amplification factor MA/2 . In the nonlinear stage, however large the initial Alfvén mach number MA may be, the magnetic field amplified and twisted by the hydromagnetic flow vortices reacts back upon the flow evolution, and the flow vortices cascade into smaller structures. In the transverse configuration the instability leads to the formation of a fast shock discontinuity from an initially sub-fast shear flow. Anomalous tangential stress by the instability in the transverse configuration reaches 1% of the magnetosheath momentum flux, but for the parallel configuration, the anomalous transport is 2-3 times larger than the anomalous transport in the transverse configuration. The anomalous transport for both configurations satisfies the requirement of the viscous-like interaction at the magnetospheric boundary.