Kinetic effects on the Kelvin–Helmholtz instability in ion-to-magnetohydrodynamic scale transverse velocity shear layers: Particle simulations

Abstract

Ion-to-magnetohydrodynamic scale physics of the transverse velocity shear layer and associated Kelvin-Helmholtz instability (KHI) in a homogeneous, collisionless plasma are investigated by means of full particle simulations. The shear layer is broadened to reach a kinetic equilibrium when its initial thickness is close to the gyrodiameter of ions crossing the layer, namely, of ion-kinetic scale. The broadened thickness is larger in B⋅Ω<0 case than in B⋅Ω>0 case, where Ω is the vorticity at the layer. This is because the convective electric field, which points out of (into) the layer for B⋅Ω<0 (B⋅Ω>0), extends (reduces) the gyrodiameters. Since the kinetic equilibrium is established before the KHI onset, the KHI growth rate depends on the broadened thickness. In the saturation phase of the KHI, the ion vortex flow is strengthened (weakened) for B⋅Ω<0 (B⋅Ω>0), due to ion centrifugal drift along the rotational plasma flow. In ion inertial scale vortices, this drift effect is crucial in altering the ion vortex size. These results indicate that the KHI at Mercury-like ion-scale magnetospheric boundaries could show clear dawn-dusk asymmetries in both its linear and nonlinear growth.

Dawn–dusk asymmetry in the Kelvin–Helmholtz instability at Mercury

“…This is important because the simulations reveal that the finite gyroradius of magnetosheath ions and the convection electric field parallel to the velocity shear gradient at dawn broaden the shear layer in which vortices start to form. This lowers the growth rate and is consistent with results of 2.5 D particle-in-cell simulations by Nakamura et al 13 The opposite conditions apply at dusk, which is a factor in explaining why the KHI preferentially occurs at dusk. Figure 1 shows two instances of time (ten gyroperiods apart) in the simulated evolution of plasma density around Mercury.…”

“…The kinetic approach satisfies the requirement to resolve the finite gyroradius 13 of solar wind particles at Mercury, and provides synthetic particle and magnetic field data that can be compared directly with spacecraft observations. An important aspect of this comparison is that synthetic data contains statistical fluctuations related to how many particles are included in the simulation, whereas spacecraft data is limited by the characteristics of the instrument used to collect data, and the random fluctuations in the quantities being sampled.…”

Nonlinear evolution of the magnetized Kelvin-Helmholtz instability: From fluid to kinetic modeling

“…It is unclear whether this behavior affects significantly the evolution of the instability. Note also that the ion tracer show a different thickness in the two different shear layer (central panel), due to the cumulative effect of the (fluid) velocity shear and (particle) ion gyromotion 21 .…”

“…Even if numerical studies of the nonlinear evolution of magnetized shear flows have been carried mainly by the means of fluid models (ideal/resistive MHD, Hall MHD, two-fluid), the increase of computational power has recently enabled to adress the problem of the kinetic modelling of shear flows in collisionless plasmas through hybrid PIC 18,19 and full PIC simulations [20][21][22] . Low resolution simulations of the KHI have also been computed as a test problem to benchmark Vlasov codes 23 .…”

ARTEMIS Science Objectives

“…The structural properties of the KH waves are similar on the dawn and dusk flanks of the near-Earth magnetosphere (Hasegawa et al 2006;Nishino et al 2011). However, non-MHD processes that depend on the polarity of the field-aligned vorticity could lead to dawn-dusk asymmetries further downstream (Nakamura et al 2010).…”