Abstract
The flow shear-driven Kelvin-Helmholtz (KH) instability is ubiquitous in planetary magnetospheres. At Earth these surface waves are important along the dawn and dusk flanks of the magnetopause boundary while at Jupiter and Saturn the entire dayside magnetopause boundary can exhibit KH activity due to corotational flows in the magnetosphere. Kelvin-Helmholtz waves can be a major ingredient in the so-called viscous-like interaction with the solar wind. In this paper, we review the KH instability from the perspective of hybrid (kinetic ions, fluid electrons) simulations. Many of the simulations are based on parameters typically found at Saturn’s magnetopause boundary, but the results can be generally applied to any KH-unstable situation. The focus of the discussion is on the ion kinetic scale and implications for mass, momentum, and energy transport at the magnetopause boundary.
Highlights
From the perspective of hydrodynamics, any flow shear with a deformed interface develops pressure gradients following from the Bernoulli principle
The inverse cascade results in smaller scale structures being subsumed by larger scale structure, resulting in complex magnetic field topologies with a patchy network of strong guide field reconnection sites
Maxwell stresses associated with magnetic field line threading of the magnetopause boundary lead to substantial momentum transport and contribute to viscous-like tangential drag at the boundary
Summary
From the perspective of hydrodynamics, any flow shear with a deformed interface develops pressure gradients following from the Bernoulli principle. The planetary magnetopause boundary is a prime example of a potentially KHunstable interface, provided that there is sufficient kinetic energy in the flow to overcome magnetic tension forces. While magnetopause boundary processes garner much of the attention, it should be noted that a discontinuous flow shear with sufficient kinetic energy density to overcome magnetic tension will be KH unstable (Chandrasekhar, 1961). The flow shear on the dawn and dusk flanks of the terrestrial magnetopause boundary are inherently KH unstable, leading to global-scale KH vortices advecting down the magnetotail (Figure 1). KH vortices could be considered as a microcosm for kinetic-scale plasma physics. This papers explores the wealth of KH-related plasma processes with specific application to planetary magnetospheres. We discuss KH growth characteristics, magnetic field topology, heavy ion effects, momentum transport, particle transport, energy tranport and heating, and electron energization due to parallel electric fields associated with driven KH reconnection
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