The Kelvin–Helmholtz instability (KHI) is a ubiquitous physical process in ordinary fluids and plasmas, frequently observed also in space environments. In this paper, kinetic effects at proton scales in the nonlinear and turbulent stage of the KHI have been studied in magnetized collisionless plasmas by means of hybrid Vlasov–Maxwell simulations. The main goal of this work is to point out the back-reaction on particles triggered by the evolution of such instability, as energy reaches kinetic scales along the turbulent cascade. Interestingly, turbulence is inhibited when KHI develops over an initial state that is not an exact equilibrium state. On the other hand, when an initial equilibrium condition is considered, energy can be efficiently transferred toward short scales, reaches the typical proton wavelengths, and drives the dynamics of particles. As a consequence of the interaction of particles with the turbulent fluctuating fields, the proton velocity distribution deviates significantly from the local thermodynamic equilibrium, the degree of deviation increasing with the level of turbulence in the system and being located near regions of strong magnetic stresses. These numerical results support recent space observations from the Magnetospheric MultiScale mission of ion kinetic effects driven by the turbulent dynamics at Earth’s magnetosheath and by the KHI in Earth’s magnetosphere.

The Kelvin-Helmholtz (KH) instability is a physical phenomenon that can develop in both fluids and plasmas in correspondence with velocity shears when a threshold condition is satisfied (Chandrasekhar, 1961;Drazin, 2002;Miura, 1982). In particular, magnetized plasmas are unstable when the jump flow is locally super Alfvénic, due to the stabilizing effect of the magnetic field. The KH instability, after initial exponential growth, generates a train of large-scale vortices that, through nonlinear interactions, can eventually evolve and merge. In the context of space plasmas, the KH instability has been observed in several environments, for example, at the interface of coronal mass ejections (Foullon et al.

The evolution of a linearly-polarized, long-wavelength Alfvén wave -propagating in a collisionless magnetized plasma with a sheared parallel-directed velocity flow-is here studied by means of two-dimensional hybrid Vlasov-Maxwell (HVM) simulations. The unperturbed sheared flow has been represented by an exact solution of the HVM set of equations (Malara et al., Phys. Rev. E 97, 053212), this avoiding spurious oscillations that would arise from the non-stationary initial state and inevitably affect the dynamics of the system. We have considered the evolution of both a small and a moderate amplitude Alfvén wave, in order to separate linear wave-shear flow couplings from kinetic effects, the latter being more relevant for larger wave amplitudes. The phase-mixing generated by the shear flow modifies the initial perturbation, leading to the formation of small-scale transverse fluctuations at scales comparable with the proton inertial length. By analyzing both the polarization and group velocity of perturbations in the shear regions, we identify them as Kinetic Alfvén Waves (KAWs). In the moderate amplitude run, kinetic effects distort the proton distribution function in the shear region. This leads to the formation of a proton beam, at the Alfvén speed and parallel to the magnetic field. Such a feature, due to the parallel electric field associated with KAWs, positively compares with solarwind observations of suprathermal ions' populations, suggesting that it may be related to the presence of ion-scales KAW-like fluctuations. †

Collisionless shocks and plasma turbulence are crucial ingredients for a broad range of astrophysical systems. The shock–turbulence interaction, and in particular the transmission of fully developed turbulence across the quasi-perpendicular Earth’s bow shock, is here addressed using a combination of spacecraft observations and local numerical simulations. An alignment between the Wind (upstream) and Magnetospheric Multiscale (downstream) spacecraft is used to study the transmission of turbulent structures across the shock, revealing an increase of their magnetic helicity content in its downstream. Local kinetic simulations, in which the dynamics of turbulent structures are followed through their transmission across a perpendicular shock, confirm this scenario, revealing that the observed magnetic helicity increase is associated with the compression of turbulent structures at the shock front.